JAVA FUNDAMENTALS

History JDK and JRE Byte Code and JVM (Java Virtual Machine) Platform Independent Applications and Applets Principle of OOPs First Java Application

History of JavaJava is a General Purpose, Object Oriented Programming Languagedeveloped by Sun Microsystems of USA I 1991. Originally called Oak byGames Gosling, one of the invention of the language, java was designedfor the development of software for consumer electronic devices like TVs, VCRs, toasters and such other electronic Machines.This goal had a strong impact on the development team to make the language simple, portable and highly reliable. The java team which included Patrick Naughton discovered that the existing languages like C and C++ had limitations in terms of both reliability and portability. However, they modeled their new language Java on C and C++ but removed a number of features of of C and C++ that were considered as source of problems and thus made java really simple, reliable,portable and powerful language.

Java Milestones :

1990 Sun Microsystems decided to develop special software that could be used to manipulate consumer electronic devices. A team of Sun Microsystems programmers headed by James Gosling was formed to undertake this task.1991 After exploring the possibility of most Object Oriented Programming Language C++, the team announced a new language named “Oak”.1992 The team, known as a Green Project team by Sun, demonstrated the application of their new language to control a list of home appliances using a hand-held device with a tiny touch sensitive screen.1993 The World Wide Web(WWW) appeared on the internet and transformed the text-based Internet into a Graphical-rich environment. The green Project team came up with the idea of developing Web Applets(tiny programs) using the new language that could run on all types of computers connected to Internet.1994 The team developed a web browser called “Hot Java” to locate and run applet programs on Internet. Hot Java demonstrated the power of the new language, thus making it instantly popular among the Internet users.1995 Oak was named “Java”, due to some legal snags. Java is just a name and is not an acronym. Many popular companies including Netscape and Microsoft announce to their support to Java.

1996 Java established itself not only a leader for Internet Programming but also as a general-purpose, object oriented programming language. Java found its home.

The most striking feature of the language is that it is a platform-neutral language. Java is a first programming language that is not tied to any particular hardware or operating system.

Features of Java :

• Compiled and Interpreted.• Platform-Independent and Portable• Object-Oriented• Robust and Secure• Distributed• Familiar, Simple and Small• Multithreaded and Interactive• High Performance• Dynamic and Extensible

JDK & JREJava Environment includes a large number of development tools and hundreds of classes and

methods. The development tools are part of the system known as Java Development Kit (JDK)

and the classes and methods are part of the Java Standard Library (JSL), also known as the

Application Programming Interface (API).

JDK : Java Development Kit comes with a collection of tools that are used for developing and

running Java Programs. They include :

appletviewer-> Enables us to run Java Applets (Without using java compatible browser)

Java Java Interpreter, which runs applets and applications by reading and interpreting bytecode

files.

Javac The Java compiler, which translates Java source code to byte code files that the

interpreter understad.

Javadoc Creates HTML-format documentation from Java source code files.

Javah Produces header files for use with native methods.

javap Java disassembler, which enables us to convert bytecode files into a program

description.

Jdb Java debugger, which helps us to find errors in our programs.

Byte Code & JVM(Java Virtual Machine)

Since platform-independence is a defining characteristic of Java, it is important to understand how it is achieved. Programs exist in two forms; source code and object code. Source Code is the textual version of the program that you write using a text editor. The programs printed in a book are shown as source code. The executable form of a program is object code. The computer can execute object code. Typically, object code is specific to a particular CPU. Therefore, it cannot be executed on a different platform. Java removes this feature in a very elegant manner. Like all computer languages, a java program begins with its source code. The difference iswhat happens when a Java program is compiled. Instead of producing executable code, the Java Compiler produces an object file that contains bytecode. Bytecodes are instructions that are not for any specific CPU. Instead, they are designed to be interpreted by a Java Virtual Machine (JVM). The key to Java’s platform-independence comes from the fact that the same bytecodes can be executed by any JVM on any platform. As long as there is a JVM implemented for a a given environment, it can run any Java program. For example, Java programs can execute under Windows 98,Solaris,IRIX, or any other platform for which a JVM can be implemented for thatplatform. This would then allow any Java program to execute in that new environment.

Platform IndependentCompilation is the process of converting the code that you type into a languagethat the computer understands-machine language. When you compile a program, the compiler checks for syntactical errors in code and lists all the errors on the screen. You have to rectify the errors and recompile the program to get the machine language code. The Java compiler compiles the code to bytecode that is understood by the Java environment.

The bytecode is the result of compiling a Java program. You can execute this code on any platform. In other words, due to the bytecode compilation processand interpretation by a browser, Java programs can be executed on a variety of hardware and operating systems. The only requirement is that the system should have a java-enabled Internet browser.The java compiler is written in Java, and the interpreter is written in C. The Java Interpreter can execute Java Code directly on any machine on which a Java interpreter has been installed.

Application and AppletsThere are two types of programs that can be built in Java Applications and applets. Applications can be directly executed by a JVM. In fact, Java can be used to develop programs for all kinds of applications, Hot Java itself is a Java Application program. Applets are small Java programs developed for Internet Applications. An applet located on distant computer (Server) can be downloaded via Internet and executed on a local computer (Client) using a Java enabled browser. We can develop applets for doing everything from simple animated graphics to complex games and utilities. Since applets are embedded in an HTML document and run inside a Web Page, creating and running applets are more complex than creating application. Stand alone program can read and write files and perform certain operations that applet can not do. An Applet can only run within a Web Browser. The Web browser includes a JVM that provides an execution environment for the applet. It is also possible to use a tool called the appletviewer to run an applet. This utility is included in the Java Development Kit(JDK) and is used to test applets. In this manner, an applet written by any developer in the world may be dynamically downloaded from the Web Server and executed on a client PC orworkstation.

Java CompilerJava Program

Java Interpreter Machine Code

Virtual Machine

Bytecode

Source Code Bytecode

Virtual Machine Real Machine

Process of Compilation

Process of Converting bytecode into machine code

Java Source Code

Java Compiler

Java Enabled Browser

Java Interpreter

Output Output

Application TypeApplet Type

Principal of OOPsObject Oriented Programming (OOP) attempts to emulate the real world in software systems. The real world consists of objects, categorized in classes. In Object Oriented Programming, classes have attributes, represented by data member. The attributes distinguish an object of the class. Classes have behaviors, which are represented by methods. The methods define how an object acts or reacts.Feature of Object Oriented Programming :Information Encapsulation(Hiding) :- Objects provide the benefit of information hiding. Electrical writing in a television should not be tempered with, and therefore should be hidden from the user. Object Oriented programming allows you to encapsulate data that you do not want users of the object to access. Typically, attributes of a class are encapsulated.Abstraction :- Abstraction allows us to focus on only those parts of an object that concern us. Person operating the television does not need to know the intricacies of how it works. The person just needs to know how to switch it on, change channels, and adjust the volume.All the details that are unnecessary to users are encapsulated, leaving only a simple interface to interact with. Providing users with only what they need to know is known as abstraction. i.e. Abstraction lets us ignore the irrelevant details and concentrate on the essentials.

Inheritance :- Inheritance is the process by which objects of one class acquire the properties of objects of another class. Inheritance supports the concept of hierarchical classification. In OOP, the concept of inheritance provides the idea of reusability. This means that we can add additional features to an existing class without modifying it. This is possible by deriving a new class from the existing one. The new class will have the combined features of both the classes.

Polymorphism :- Polymorphism means “One Interface, multiple implementations.”

Shape

Ellipse Rectangle Triangle

The class Shape defines a method called getArea() that returns the area of a shape. However, this method is not implemented by that class. Therefore, it is an abstract method and Shape is an abstract class.

This means that no objects of class Shape can be created. However, the functionality of that class can be inheritated by its subclass. The various subclasses of Shape like Ellipse,Rectangle,Triangle do implement the getArea() method.

First Java Application Create the File

The first step to create the HelloWorld application is to copy the text from listing 7.1 into a file called HelloWorld.java using your favorite text editor (by choosing Windows, NotePad, or SimpleText on the Macintosh). It is very important to call the file HelloWorld.java, because the compiler expects the file name to match the class identifier

Listing 7.1 The HelloWorld application.

public class HelloWorld {

public static void main(String args[]){System.out.println("Hello World!!");

}}

Class Declaration

The first line public class HelloWorld declares a class, which is an Object-Oriented construct. As stated earlier Java is true Object-Oriented language and therefore, everything must be placed inside a class. Class is a keyword and declares that a new class definition follows.

Opening Bace

Every class definition in Java begins with an opening brace “{“ and ends with a matching closing brace “}”, appearing in the last line in the example.

The main() method

Every java application program must include the main() method. This is starting point for the interpreter to begin the execution of the program. A Java application can have any number of classes but only one of them must include a main method to initiate the execution.

Public : The Keyword public is an access specifier that declares the main method as unprotected and therefore making it to accessible to all other classes.

Static : The keyword static which declares this method as one that belongs to the entire Class and not a part of any Objects of the class.

The main must always be declared be declared as static since the interpreter uses

this method before any objects are created.

Void : The type modifier void states that the main method does not return any value.

All parameters to a method are declared inside a pair of parentheses. Here,

String args[] declares a parameter named args, which contains an array of objects

Of the class type String.

The Output Line

The only executable Statement in the program isSystem.out.println("Hello World!!");Since Java is a true Object Oriented Language, every method must be part of an Object. The println method is a member of the out Object, which is static data Member of the System class. This line printsHello World!!to the screen. The method println always appends a newline character to the end of the string.

Compile the CodeTo compile the program, you need to first install the JDK. Then, use the program javac included with the JDK to convert the text in listing 7.1 to code which the computer can run. To run javac, on a Macintosh drag the source file over the javac icon. On any other computer, type the line:javac HelloWorld.java at a command prompt. The javac program creates a file called HelloWorld.class from the HelloWorld.java file. Inside this file (HelloWorld.class) is text known as bytecodes which can be run by the Java interpreter. Run the ProgramNow that you have compiled the program, you can run it by typing at the command prompt:java HelloWorld After you do this, the computer should print to the screenHello World!!That may not seem very interesting, but then it's a simple program. If you don't see the Hello World!! on the screen, go back and make sure you have typed in the file exactly as shown in listing 7.1, and make sure that you called the file HelloWorld.java.

JAVA CONTROLS Variables and Constants Arithmetic Operator and Expressions Type Conversion in Java Comments in Java(3 Types) Java’s Control Statements

o Ifo If-elseo Do-whileo Whileo foro Increment and Decrement Operatorso Escape Sequences Characterso Relational and Logical Operatorso Ternary Operatorso Switch caseo Breako Bitwise Operatorso Arrays-Single and Multidimensional

Variables and ConstantsConstants : Constants in Java refer to fixed values that do not change during theexecution of a program. Java supports several types of constants given in figure below :

Java Constant

Numeric Constant Character Constant

Integer Real Character String

Integer Constants : Refers to a sequence of digits. There are three types of Integers, namely, decimal,octal and hexadecimal integer.

Decimal Integer consist of of a set of digits, 0 through 9, preceded by an optional minus sign.

An octal integer constant consists of any combination of digits from the set 0 through 7, with a leading 0.

A sequence of digits preceded by ox or OX is considered as hexadecimal integer. They may also include alphabets A through F.

Real Constants : Integer constant are inadequate to represent quantities that vary continuously, such as distance, heights, temperature, prices and so on. These quantities are represented by numbers containing fractional parts like 17.546. Such numbers are called real.

The real number may also be expressed in exponential (or scientific ) notation. For example, the value 215.65 may be written as 2.1565e2 in exponential notation. e2 means multiply by 102. The general form is :

mantissa e exponent

mantissa is either a real number expressed in decimal notation or an integer. The exponent is an integer with an optional plus or minus sign. The letter e separating the mantissa and the exponent can be written in either lowercase or uppercase. Since the exponent causes the decimal point to “float”, this notation is said to represent a real number in floating point form.

Single Character Constants : A single character constant (or simply character constant ) contains a single character enclosed within a pair of single quote marks. Examples of character constants are : ‘5’ ‘X’ ‘;’

String Constant : A string constant is a sequence of characters enclosed between double quotes. The characters may be alphabets,digits,special characters and blank spaces. Example are : “Hello Java” “1997”

Variable : A variable is an Identifier that denotes a storage location used to store a data value. Unlike constants that remain unchanged during the execution of program. Examples of variables : average,height,total_height.

Variable name may consist of alphabets,digits,the underscore(_) and dollor characters.

Rules to write Variable/Identifier in Java :

• They must not begin with digit

• Upper and lowecase are distinct. This means that the variable Total is not the sam as total or TOTAL.

• It should not be a keyword.

• White space is not allowed.

• Variable names can be of any length.

Data TypesData Types in Java

Primitive (Intrinsic) Non-Primitive ( Derived)

Numeric Non-Numeric Classes Arrays

Integer Floating Point

Character BooleanInterface

Integer TypesJava consists of four integer types: byte, short, int, and long, which are defined as 8-, 16-, 32-, and 64-bit signed values as summarized in Table below :

The Java integer primitive types.

Type Bit Size Minimum Value Maximum Value

byte 8 -256 255

short 16 -32,768 32,767

int 32 -2,147,483,648 2,147,483,647

long 64 -9,223,372,036,854,775,808 9,223,372,036,854,775,807

Integer literals can be specified in decimal, hexadecimal, or octal notation. To specify a decimal value, simply use the number as normal. To indicate that a literal value is a long, you can append either "L" or "l" to the end of the number. Hexadecimal values are given in base 16 and include the digits 0-9 and the letters A-F. To specify a hexadecimal value, use 0x followed by the digits and letters that comprise the value. Similarly, an octal value is identified by a leading 0 symbol. For examples of specifying integer literals, see Table 3.7.

Integer Long Octal Hexadecimal

0 0L 0 0x0

1 1L 01 0x1

10 10L 012 0xA

15 15L 017 0XF

16 16L 020 0x10

100 100L 0144 0x64

Table 3.7. Examples of integer literals.

Floating-Point TypesSupport for floating-point numbers in Java is provided through two primitive types-float and double, which are 32- and 64-bit values, respectively.

Similar to integer literals are Java's floating-point literals. Floating-point literals can be specified in either the familiar decimal notation (for example, 3.1415) or exponential notation (for example, 6.02e23).

To indicate that a literal is to be treated as a single precision float, append either "f" or "F". To indicate that

it is to be treated as a double precision value, append either "d" or "D". Java includes predefined constants, POSITIVE_INFINITY, NEGATIVE_INFINITY, and NaN, to represent the infinity and not-a-number values. The following list shows some valid floating-point literals: 43.3F,3.1415d,-12.123f,6.02e+23f,6.02e23d,6.02e-23f,6.02e23d

Boolean :- Java supports two Boolean literals-true and false. Character Literals :-A character literal is a single character or an escape sequence enclosed in single quotes, for example, 'b'. Escape sequences are used to indicate special characters oractions, such as line feed, form feed, or carriage return. The available escape sequences ar shown in Table 3.8.

String LiteralsAlthough there is no string primitive type in Java, you can include string literals in your programs. Most applications and applets will make use of some form of string literal, probably at least for error messages. A string literal consists of zero or more characters (including the escape sequences shown in Table 3.8) enclosed in double quotes. As examples of string literals, consider the following: "A String“,"Column 1\tColumn 2”

Table 3.8. Escape sequences.

Sequence Purpose

\b Backspace

\t Horizontal tab

\n Line feed

\f Form feed

\r Carriage return

\" Double quote

\' Single quote

\\ Backslash

\uxxxx Unicode character

Arithmetic Operators and ExpressionsAn expression is a operators and operands. It follows the rules of algebra and should be familiar. Java allows several types of expressions. The arithmetic operators are given below :

+ addition

-subtraction

*multiplication

/ division

% modulus

+= addition assignment

-=subtraction assignment

*=multiplication assignment

/=division assignment

%=modulus assignment

++increment,--decrement

An expression may appear on the right side of an assignment statement. For example,

Int answer;

Answer=100*31;

Java expression may contain Variables, constants or both. For example assuming that answer and count are variables, this expression is perfectly valid.

Answer=count-100;

Increment/Decrement OperatorsThe increment and decrement operators are used with one variable (they are known as unary operators):++ increment operator -- decrement operator For instance, the increment operator (++) adds one to the operand, as shown in the next line of code:x++; is the same as x+=1;The increment and decrement operators behave slightly differently based on the side of the operand they are placed on. If the operand is placed before the operator (for example, ++x), the increment occurs before the value is taken for the expression. So, in the following code fragment, the result of y is 6:int x=5;int y=++x; // y=6 x=6 If the operator appears after the operand, the addition occurs after the value is taken. So y is 5 as shown in the next code fragment. Notice that in both examples, x is 6 at the end of the fragment.int x=5;int y = x++; //y=5 x=6 Similarly, the decrement operator (--) subtracts one from the operand, and the timing of this is in relation to the evaluation of the expression that it occurs in.

The Relational OperatorsThe most intuitive comparative operators are those that fall into a category known as relational operators. Relational operators include those standard greater-than and less-than symbols you learned about back in third grade. Conveniently enough, they work the same way as they did back in third grade, too. For instance, you know that if you write (3>4), you wrote something wrong (false). On the other hand (3<4) is correct (true). In Java and most other languages, you are not limited to evaluating constants; you are free to use variables, so the statement (Democrats> Republicans) is also valid. The complete list of relational operators is shown here:Operator Boolean Result < Less than <= Less than or equal to > Greater than >= Greater than or equal to The following two assignment statements produce identical results:result1 = a+b < c*d ;result2 = (a+b) < (c*d) ; Logical ExpressionsThe third set of evaluation operators fall into a category known as logical expressions. Logical expressions work a bit differently than the previous operators, and are probably not something you covered in your third grade math class.

Logical expressions operate either on a pair of booleans, or on the individual bits of an object. There are two types of logical operators which are divided roughly along these lines:Boolean operators. Only operate on Boolean values. Bitwise operators. Operate on each bit in a pair of integral operands.

The Conditional-And and Conditional-Or OperatorsThere are two primary Boolean operators:Logical-AND: && Logical-OR: || Oddly, in most computer languages, including Java, there is no Conditional-XOR operator.

When A isAnd when B is (A && B) (A || B)

false false false false

false true false true

true false false true

true true true true

The Conditional OperatorThe conditional operator is the one ternary or triadic operator in Java, and operates as it does in C and C++. It takes the following form:expression1 ? expression2 : expression3In this syntax, expression1 must produce a Boolean value. If this value is true, then expression2 is evaluated, and its result is the value of the conditional. If expression1 is false, then expression3 is evaluated, and its result is the value of the conditional.Consider the following examples. The first is using the conditional operator to determine the maximum of two values; the second is determining the minimum of two values; the third is determining the absolute value of a quantity:BestReturn = Stocks > Bonds ? Stocks : Bonds ;LowSales = JuneSales < JulySales ? JuneSales : JulySales ;Distance = Site1-Site2 > 0 ? Site1-Site2 : Site2 - Site1 ;In reviewing these examples, think about the precedence rules, and convince yourself that none of the three examples requires any brackets in order to be evaluated correctly.

Type Conversion in JavaImplicit Type ConversionsJava performs a number of implicit type conversions when evaluating expressions.For unary operators (such as ++ or --), the situation is very simple: operands of type byte or short are converted to int, and all other types are left as-is.For binary operators, the situation is only slightly more complex. For operations involving only integer operands, if either of the operands is long, then the other is also converted to long; otherwise, both operands are converted to int. The result of the expression is an int, unless the value produced is so large that a long is required. For operations involving at least one floating point operand, if either of the operands is double, then the other is also converted to double and the result of the expression is also a double; otherwise, both operands are converted to float, and the result of the expression is also a float. Fortunately, implicit conversions take place almost always without your wanting or needing to know. The compiler handles all the details of adding bytes and ints together so you don't have to.

short Width;long Length, Area;Area = Length * Width;// In the division below, Area will be converted to a double,// and the result of the calculation will be a double.

Conversions and the Cast Operator :Normally with implicit conversion, the conversion is so natural that you don't even notice. Sometimes though it is important to make sure a conversion occurs between two types. Doing this type of conversion requires an explicit cast, by using the cast operator.The cast operator consists of a type name within round brackets. It is a unary operator with high precedence and comes before its operand, the result of which is a variable of the type specified by the cast, but which has the value of the original object. The following example shows an example of an explicit cast:float x = 2.0;float y = 1.7;x - ( (int)(x/y) * y) When x is divided by y in this example, the type of the result is a floating-point number. However, value of x/y is explicitly converted to type int by the cast operator, resulting in a 1, not 1.2. So the end result of this equation is that x equals 1.7.Not all conversions are legal. For instance, boolean values cannot be cast to any other type, and objects can only be converted to a parent class.Casting and Converting IntegersThe four integer types can be cast to any other type except boolean. However, casting into a smaller type can result in a loss of data, and a cast to a floating-point number (float or double) will probably result in the loss of some precision, unless the integer is a whole power of two (for example, 1, 2, 4, 8...).

Casting and Converting CharactersCharacters can be cast in the same way 16-bit (short) integers are cast; that is, you can cast it to be anything. But, if you cast into a smaller type (byte), you lose some data. In fact, even if you convert between a character and a short you an loose some data.If you are using the Han character set (Chinese, Japanese, or Korean), you can lose data by casting a char into a short (16-bit integer), because the top bit will be lost.Casting and Converting BooleansThere are not any direct ways to cast or convert a Boolean to any other type. However, if you are intent on getting an integer to have a 0 or 1 value based on the current value of a Boolean, use an if-else statement, or imitate the following code:int j;boolean tf;...j = tf?1:0; // integer j gets 1 if tf is true, and 0 otherwise. Conversion the other way can be done with zero to be equal to false, and anything else equal to true as follows:int j;boolean tf;...tf = (j!=0); // Boolean tf is true if j is not 0, false otherwise.

Comments in JavaJava supports three types of comment delimiters-the traditional /* and */ of C, the // of C++, and a new variant that starts with /** and ends with */. The /* and */ delimiters are used to enclose text that is to be treated as a comment by the compiler. These delimiters are useful when you want to designate a lengthy piece of code as a comment, as shown in the following: /* This is a comment that will span multiple source code lines. */ The // comment delimiter is borrowed from C++ and is used to indicate that the rest of the line is to be treated as a comment by the Java compiler. This type of comment delimiter is particularly useful for adding comments adjacent to lines of code, as shown in the following: Date today = new Date();      // create an object with today's dateSystem.out.println(today);    // display the date

Finally, the /** and */ delimiters are new to Java and are used to indicate that the enclosed text is to be treated as a comment by the compiler, but that the text is also part of the automatic class documentation that can be generated using JavaDoc

The Java comment delimiters are summarized in Table 3.1.Table 3.1. Java comment delimiters.

Start End Purpose

/* */ The enclosed text is treated as a comment.

// (none)

The rest of the line is treated as a comment.

/** */ The enclosed text is treated as a comment by the compiler but is used by JavaDoc to automatically generate documentation.

Java Control Statements

if

Selection Statement Iteration Statement Jump Statement

If-else switch

while do for

breakcontinue

return

Control Statement

Selection Statement : These select one of several control flows. There are three types of selection statement in Java : if,if-else, and switch.

If statement :

The if statement is a powerful decision making statement and is used to control the flow of execution of statements. It is a two-way decision statement and is used in conjunction with an expression. The general form is :

If(test expression)

{

Statement-block;

}

Statement-x;

It allows the computer to evaluate the expression first and then, depending on whether the value of the expression (relation or condition) is ‘true’ or ‘false’. It transfers the control to a particular statement.

If the statement is true then the Statement block will be executed;otherwise the statement-block will be skipped and the execution will jump to the statement-x. It should be remember that when the condition is true both the statement-block and statement-x are executed in sequence.

Example :

Class Demo {

public static void main(String args[]) {

If(args.length==0)

System.out.println(“You must have command line arguments”); }}

If-else statement

if(test expression)

{

True-Block Statement(s);

}

Else

{

False-Block statement(s);

}

Statement-x;

If the test expression is true, then the true-block statement(s) executed immediately following to the if statement, are executed; otherwise the false statement(s) will be executed, not both.In both the cases, the control is transferred subsequently to the statement-x.

Nesting of If—else Statement

If(test condition1)

{ if(test condition2) {

Statement-1; }

else {

Statement-2; }

}

else {

Statement-3; }

Statement-x;

If the condition-1 is false, the statement-3 will be executed; otherwise it continues to perform the second test. If the condition-2 is true, the statement-1 will be evaluated;otherwise statement-2 will be evaluated and then control is transferred to the statement-x.

Switch Statement: The Java switch statement is ideal for testing a single expression against a series of possible values and executing the code associated with the matching case statement.

Switch(expression) {

Case value-1:

block-1;

break;

Case value-2:

block-2;

break;

……

……

default:

default-block;

break; }

Statement-x;

Iteration Statement : These specify how and when looping will take place. There are three types of Iteration statements: while, do and for

The for Statement The first line of a for loop enables you to specify a starting value for a loop counter, specify the test condition that will exit the loop, and indicate how the loop counter should be incremented after each pass through the loop. This is definitely a statement that offers a lot of bang for the buck. The syntax of a Java for statement is as follows: for (initialization; testExpression; incremement)    statement For example, a sample for loop may appear as follows: int count;for (count=0; count<100; count++)    System.out.println("Count = " + count); In this example, the initialization statement of the for loop sets count to 0. The test expression, count < 100, indicates that the loop should continue as long as count is less than 100. Finally, the increment statement increments the value of count by one. As long as the test expression is true, the statement following the for loop setup will be executed, as follows: System.out.println("Count = " + count); Of course, you probably need to do more than one thing inside the loop. This is as easy to do as using curly braces to indicate the scope of the for loop.

The while StatementRelated to the for loop is the while loop. The syntax for a while loop is as follows: while (booleanExpression)    statement As you can tell from the simplicity of this, the Java while loop does not have the built-in support for initializing and incrementing variables that its for loop does. Because of this, you need to be careful to initialize loop counters prior to the loop and increment them within the body of the while loop. For example, the following code fragment will display a message five times: int count = 0;while (count < 5) {    System.out.println("Count = " + count);    count++;} The do…while Statement The final looping construct in Java is the do…while loop. The syntax for a do…while loop is as follows: do {    statement} while (booleanExpression);

This is similar to a while loop except that a do…while loop is guaranteed to execute at least once. It is possible that a while loop may not execute at all depending on the test expression used in the loop. For example, consider the following method: public void ShowYears(int year) {    while (year < 2000) {        System.out.println("Year is " + year);        year++;    }} This method is passed a year value, then loops over the year displaying a message as long as the year is less than 2000. If year starts at 1996, then messages will be displayed for the years 1996, 1997, 1998, and 1999. However, what happens if year starts at 2010? Because the initial test, year < 2000, will be false, the while loop will never be entered. Fortunately, a do…while loop can solve this problem. Because a do…while loop performs its expression testing after the body of the loop has executed for each pass, it will always be executed at least once. This is a very valid distinction between the two types of loop, but it can also be a source of potential errors. Whenever you use a do…while loop, you should be careful to consider the first pass through the body of the loop.

JumpingOf course, it is not always easy to write all of your for, while and do…while loops so that they are easy to read and yet the loops terminate on exactly the right pass through the loop. Java makes it easier to jump out of loops and to control other areas of program flow with its break and continue statements. The break StatementEarlier in this chapter, you saw how the break statement is used to exit a switch statement. In a similar manner, break can be used to exit a loop As an example of this, consider the following code: int year = 1909;while (DidCubsWinTheWorldSeries(year) == false) {    System.out.println("Didn't win in " + year);    if (year >= 3000) {        System.out.println("Time to give up. Go White Sox!");        break;    }}System.out.println("Loop exited on year " + year);

This example shows a while loop that will continue to execute until it finds a year that the Chicago Cubs won the World Series. Because they haven't won since 1908 and the loop counter year starts with 1909, it has a lot of looping to do. For each year they didn't win, a message is displayed. However, even die-hard Cubs fans will eventually give up and change allegiances to the Chicago White Sox. In this example, if the year is 3000 or later, a message is displayed and then a break is encountered. The break statement will cause program control to move to the first statement after the end of the while loop. In this case, that will be the following line: System.out.println("Loop exited on year " + year); The continue Statement Just as a break statement can be used to move program control to immediately after the end of a loop, the continue statement can be used to force program control back to the top of a loop

ARRAYSOne Dimensional Array : is a list of variables of the same type that are

accessed through a common name. An Individual variable in the array is called an array element. Arrays from a convenient way to handle groups of related data.

To create an array, you need to perform two steps :

1. Declare Array

2. Allocate space for its elements.

General Form for declaring one dimensional array given below :

type varName[];

Here, type is a valid Java data type and varName is the name of the array. Like int a[];

This creates a variable named a that refers to an integer array. But it does not actually create storage for the array.

Second approach to allocate space for One Dimensional Array is

varName=new type[size];

Here varName is name of the array, type is a valid Java type, and size specifies the number of elements in the array. You can see that the new operator is used to allocate memory for the array.

These two steps combines like

type varName=new type[size];

For example consider this declaration and allocation :

Int ia=new int[10];

Represents the structure of a one-dimensional array,here ia is array variable name that can hold 10 integer values.

Multidimensional Array :

In addition to one dimensional we can create arrays of two or more dimensions. In Java, Multidimensional array are implemented as arrays of arrays. You need to perform two steps to work with multidimensional arrays : 1. Declare the array and 2. allocate space for its elements.

The General form is given below :

Type varname = new type[size1][size2];

float a[][]=new float[2][2];

Here a is two dimensional array having 2 rows and 2 columns. i.e. size is 4, we can store 4 elements in that array.

JAVA Classes and Methods The General Form of a class Constructor and Method Overloading The new Operator Garbage Collection Finalize method Command Line Arguments The System Class

The General Form of ClassA class array contains three types of items : variable,methods, and constructors. Variable represent its state. Method provide the logic that constitutes the behavior defined by a class. The variable and methods can be both static and instance variable. Constructors initialize the state of a new instance of a class.

The Simplified form of a class is :

class clsName {

//instance variable declarations

type1 varName1=value1;

type2 varName2=value2;

…..

//constructor

clsName(cparams1) {

// body of constructor

}

clsName(cparamsN) {

//body of constructor

}

…

//methods

rtype1 mthName(mparams1) {

//body of method

}

…..

}

}

The keyword class indicates that a class named clsName is being declared. This name must follow the Java naming convention for identifiers. The instance Variables named varName1 through varNameN included using the normal variable declaration syntax. Each variable must be assigned a type shown as

type1 through typeN and may be initialized to value shown as value1 through valueN.

Constructors always have the same name as the class. They do not have return values.

Method named mthName1 through mthNameN can be included. The return type of the methods are rType1 through rtypeN, and their Optional Parameter lists are mparams1 through mparamsN.

Constructors

Often an object will require some form of initialization when it is created. To accommodate this, Java allows you to define constructor for your classes. A Constructor is a special method that creates and initializes an object of a particular class. It has the same name as its class and may accept arguments. Constructor does not have a return type. Instead, a constructor returns a reference to the object that it creates.

Constructor Overloading

A class may have several constructors. This feature is called constructor overloading. When constructors are overloaded each is still called by the name of its class. However it must have a different parameter list. i.e. Signature of each constructor must differ.

Method OverloadingLike Constructor, method can also be overloaded. This occurs when two or more methods have the same name but different signature. Recall that the signature of a method is a combination of its name and the sequence of the parameter types.

Method Overloading allows you to use the same name for a group of methods that basically have the same purpose. The println() is an good example of this concept. It has so many Overloaded forms.Each of these accepts one argument of a different type. Another advantage of method overloading is that it provides an easy way to handle default parameter values. The New OperatorObjects are created using the new operator. The new Operator creates an instance of a class. It is invoked as follows :clsName obhRef=new clsName(args);Here, clsName is the name of the class to instantiated. (Instantiated means to create an instance.) A reference to the new object is assigned to a variable named objRef. Notice the expression immediately to the right of the keyword new. This is known as constructor. A constructor creates an instance of the class. It has the same name as the class and may optionally have an argument list args.

Garbage Collection

Garbage collection is the mechanism that reclaims the memory resources of an object when it is no longer referenced by a variable. It prevents programming errors that could otherwise occur by incorrectly deleting or failing to delete objects. An object becomes eligible for garbage collection when it is no longer referenced by any Variable. However, the JVM determines exactly when this task is performed.finalize method :Constructor method used to initialize an object when it is declared. This process is known as initialization. Similarly, Java supports a concept called finalization, which is just opposite to Initialization. We know that java run-time is an automatic garbage collecting system. It automatically frees up the memory resources used by the objects. But objects may hold other non-object resources such as file descriptors or window system fonts. The garbage collector cannot free these resources. In order to free these resources we must use a finalizer method. This is similar to destructor of C++.The finalizer method is simply finalize() and can be added to any class. Java calls that method whenever it is about to reclaim the space for that object. The finalize method should explicitly define the tasks to be performed.

Command Line Arguments

All java application contain a static method named main().This method takes one argument that is an array of String objects. These objects represent any arguments that may have been entered by the user on the command line. The number of command line arguments is obtained via the expression args.length. This is an int type. The individual arguments are accessed as arg[0],args[1],args[2], and so forth.

The System Class

The system class defines several attributes relate to the run-time environment. It has a static variable named out that contains a reference to a PrintStream object. The print() and println() methods of that object display their string arguments on the standard output. The static variable err also holds a reference to PrintStream object. This is the standard error stream. The static variable in contains a reference to an InputStream object. PrintStream and InputStream are classes that provide support for I/O.Another static method of the system class is exit(). It determines the current application.

Using Classes and Methods

Instance Methods and Variables Static keyword Inner Class/Nested Classes String Class Math Class Wrapper Class

Instance Variables and MethodsEach object has its own copy of all the instance variables defined by its class. Thus an Instance variable relates to an instance (object) of its class. Instance variable may be declared by using the following form :type varName1;Here, the name of the variable is varName1 and the required type is type.Instance variables are initialized to default values during the creation of an object.A Instance Method acts upon an object and can be declared, as follows:rtype mthname(mparams) {//body of method}Here mthName is the name of the method and type is its return type.Static Variables and MethodsA static variable is shared by all objects of its class and thus relates to the class itself. A static variable may be declared by using the static keyword as a modifier as follows:static type varName1;Here the name of the variable ios varName1 and its type is type.Static variables are initialized to default values when the class loaded into the memory.

A static method may be declared by using the static keyword as a modifier. A static method acts relative to a class. It deos not act on specific objects of its class. To declare a static method as :static rtype mthName(mparams) {//body of method}}Here mthName is the name of the method and rtype is its return type an optional list of parameter types shown as mparams may be specified.

String ClassStrings are class instantiated object and implemented using String Class.String class expressed as :String stringName=new String(“string”);A string class defines a number of methods that allows you to accomplish a variety of string manipulation tasks like compareTo() method used to sort an array of strings in alphabetical order.

Math Class

Mathematical functions such as cos,sqrt, log etc. are frequently used to analysis of real-life problems. Java supports these basic functions through Math class defined in the java.lang package.The math function used as follows

Math.function_name();

Like double y=Math.sqrt(x);

Wrapper Class There are wrapper classes for each of the eight simple types defined by Java. These are Boolean, Character, Byte,Integer,Float,Long,Short and Double. They encapsulate boolean,char,byte,int,float,long,short and double values.The Benefit of using wrapper classes is that they provide methods to convert strings to simple types. This is a very useful because it gives us a mechanism by which to process user input.

Inheritance

Single and Multilevel Call by value and Call by reference Method Overriding this and super keyword Abstract and Final Classes

Class InheritanceIn Java, every class you declare will be derived from another class. You can specify the class to derive from by using the extends keyword as follows:

public class ClassicCar extends Car {     // member methods and variables}

A derived class is commonly referred to as a subclass, while the class it is derived from is commonly referred to as a superclass. The term immediate superclass is used to describe the class from which a subclass is directly derived. In Figure 3.5, for example, ClassicCar is a subclass of both Car and Object. Car and Object are both superclasses of ClassicCar, but only Car is the immediate superclass of ClassicCar.

Fig 3.5

Call By Value and Call By Reference (Argument Passing)

All methods are passed by value. This means that copies of the arguments are provided to a method. Any changes to those copies are not visible outside method.

The situation when an array of objects are passed as an argument. In this case the entire array or object is not actually copied.Instead, only a copy of the reference is provided. Therefore any changes to the array or object are visible outside the method. However the reference itself is passed by value.

Overriding Member Methods When you create a subclass, you inherit all of the functionality of its superclass, and then you can add or change this functionality as desired. As an example of this, consider the altered declaration of a Car class in the following code:

        

public class Car {    private int year;    private float originalPrice;    // calculate the sale price of a car based on its cost    public double CalculateSalePrice() {        double salePrice;         if (year > 1994)            salePrice = originalPrice * 0.75;        else if (year > 1990)            salePrice = originalPrice * 0.50;else            salePrice = originalPrice * 0.25;        return salePrice;     }    // a public constructor   

 public Car(int year, float originalPrice) {        this.year = year;         this.originalPrice = originalPrice;    }}

ClassicCar is derived from Car, as follows:

public class ClassicCar extends Car {     // calculate the sale price of a car based on its cost    public double CalculateSalePrice() {        return 10000;     }    // a public constructor    public ClassicCar(int year, float originalPrice) {        super(year, originalPrice);     }}

Because ClassicCar is derived from Car, it inherits all of the functionality of Car, including its member variables year and originalPrice. The function CalculateSalePrice appears in both class declarations. This means that the occurrence of this function in ClassicCar overrides the occurrence of it in Car for object instances of ClassicCar. As an example of how this works, consider the following: ClassicCar myClassic = new ClassicCar(1920, 1400);

double classicPrice = myClassic.CalculateSalePrice();Car myCar = new Car(1990, 12000);double price = myCar.CalculateSalePrice();

The variable myClassic is of type ClassicCar and is constructed using that class's constructor

Class ModifiersClasses that are created in Java can be modified by any of three class modifiers. The Java class modifiers are public, final, and abstract. If no class modifier is used, then the class may only be used within the package in which it is declared. A public class is a class that can be accessed from other packages. A class that is declared as final cannot be derived from, meaning it cannot have subclasses.

public class Car {    String manufacturer;    String model;   int year;    int passengers;   // a public constructor    public Car(String madeBy, String name, int yr, int pass,float cst) {        manufacturer = madeBy;        model = name;         year = yr;        passengers = pass;         cost = cst;} }

The this VariableAll Java classes contain a hidden member variable named this. The this member can be used at runtime to reference the object itself. One excellent use of this is in constructors. It is very common to have a set of instance variables in a class that must be set to values that are passed to a constructor. When you are doing this, it would be nice to have code that was similar to the following: year = year; Ideally the variable on the left could be the instance variable, and the variable on the right could be the parameter passed to the constructor.

 

Here, we've had to come up with two names for each concept: the best variable names (manufacturer, model, and so on) are used as the instance variables in the class declaration. The less satisfactory names are passed as parameters so as to distinguish them from the instance variables. The assignment statements are then very readable by Java but seem a little contrived to human readers. Java's this keyword provides a very effective solution to this problem in that the constructor can be written as follows: public class Car {    String manufacturer;    String model;    int year;    int passengers;    float cost;    // calculate the sale price of a car based on its cost    public double CalculateSalePrice() {        return cost * 1.5;    }   // a public constructor    public Car(String manufacturer, String model, int year,int passengers, float cost) {       this.manufacturer = manufacturer;        this.model = model;

In this case, the variables like this.year refer to the instance variables, whereas the unqualified variables like year refer to the constructor's parameters.

this.year = year;         this.passengers = passengers;        this.cost = cost;   } }

       

The super VariableIn the preceding declaration for ClassicCar, you may have noticed that the constructor made use of a variable named super. Just as each object has a this variable that references itself, each object (other than those of type Object itself) has a super variable that represents the parent class. In this case, super(year, originalPrice) invokes the constructor of the superclass Car.

Use of Super

• Super is used to invoke the constructor method of the superclass. • Super may be only used within a subclass constructor method.• The call to superclass constructor must appear as the first statement within the subclass constructor.• The parameters in the super call must match the order and type of the inheritance variable declared in the subclass.

Abstract ClassesSometimes you may want to declare a class and yet not know how to define all of the methods that belong to that class. For example, you may want to declare a class called Mammal and include in it a member method called MarkTerritory. However, you don't know how to write MarkTerritory because it is different for each type of Mammal. Of course, you plan to handle this by deriving subclasses of Mammal, such as Dog and Human. But what code do you put in the MarkTerritory function of Mammal itself? In Java you can declare the MarkTerritory function of Mammal as an abstract method. Doing so allows you to declare the method without writing any code for it in that class. However, you can write code for the method in the subclass. If a method is declared abstract, then the class must also be declared as abstract. For Mammal and its subclasses, this means they would appear as follows:

abstract class Mammal {    abstract void MarkTerritory();}public class Human extends Mammal {    public void MarkTerritory() {        // mark territory by building a fence    }}

public class GangMember extends Mammal {    public void MarkTerritory() {        // mark territory with graffiti    }}public class Dog extends Mammal {    public void MarkTerritory() {        // mark territory by doing what dogs do    }}

With the preceding declarations, the Mammal class contains no code for MarkTerritory. The Human class could contain code that would mark territory by building a fence around it, while the GangMember class could contain code that would mark territory by spray-painting graffiti. The Dog class would mark territory by raising the dog's leg and doing what dogs do to mark territory.

Final Variables, Methods and ClassAll methods and variables can be overridden by default in subclasses. If we wish to prevent the subclasses from overriding the members of the super class, we can declare them as final using the keyword final as a modifier. For example : final int SIZE=100;final void showStatus(…)Making a method final ensures that the functionality defined in this method will never be altered in any way. Similarly the value of a final variable can never be change.

Sometimes we may like to prevent a class being further subclassed for security reasons. A class that can not be subclassed is called a final class. This is achieved in Java using the keyword final as follows:Final class AClass ( ………….)Any attempt to inherit these classes will cause an error and the compiler will not allow it.

STRINGSStrings represent a sequence of characters. The easiest way to represent a sequence of characters in Java is by using a character array. Example : char charArray[]=new char[4];

We can declare string using Characters but there is limitation that we can not use functions related with strings using array like copy of an array is difficult. For that Java provides String class using objects.

for that two classes are used String and StringBuffer. A Java String is an instantiated object of the String class.

String stringName;

stringName=new String ("string");

String Array :

String itemArray[]=new String[3];//will create itemArray of size 3 to hold three string constants.

String Methods :

The String Class defines a number of methods that allow us to accomplish a veriety of string manipulation tasks.

s2=s1.toLowerCase; //converts the string s1 to all lowercase

s2=s1.toUpperCase; //converts the string s1 to all Uppercase

s2=s1.replace('x','y'); //Replace all occurance of x with y

s2=s1.trim() //Removes the white spaces at the beginning and end of the String s1.

s1.equals(s2) //Returns true if s1 is equal to s2

s1.equalsIgnoreCase(s2) //Returns true if s1=s2, ignoring the case of characters.

s1.length() //Gives the Length of s1

s1.charAt(n) //Gives nth character of s1

s1.compareTo(s2) //Returns negative if s1<s2,positive if s1>s2, and zero if s1 is equal to s2

s1.concat(s2) //concatenates s1 and s2

s1.substring(n) //Gives substring starting from nth character

s1.subsctring(n,m) //Gives substring starting from nth charater upto mth (not including mth)

String.ValueOf(p) //Creates a string object of the parameter p (simple type or object)

p.toString(); //creates a string representation of the object p

s1.indexOf('x') //Gives the position of the first occurence of 'x' in the string s1.

s1.indexOf('x','n') // Gives the position of the 'x' that occurs after nth position in the string s1.

String.valueOf(variable) //Converts the parameter value to string representation.

String Buffer Class :

StringBuffer is a peer class of String. While String creates strings of fixed length, StringBuffer

creates strings of flexible length that can be modified in terms of both length and content. We can

insert characters and substrings in the middle of a string, or append another string to the end.

s1.setCharAt(n,'x')//Modifies the nth character to x

s1.append(s2)//Appends the string s2 to s1 at the end

s1.insert(n,s2)//Inserts the string s2 at the position n of the string s1.

s1.setLength(n)//Sets the length of the string s1 to n. If n<s1.length() s1 is truncated. If n>s1.length()

zeros are added to s1.

Vectors :

Java does not support the concept of variable arguments to a function. This feature can be achieved in Java through the use of the Vector class contained in the java.util package. This class can be used to create a generic dynamic array known as vector that can hold objects of any type and any number.

The objects do not have to be homogenious . Arrays can be eaisly implemented as vectors. Vectors are created like arrays as follows :

Vector intVect = new Vector();//declaring without size

Vector intVect = new Vector(3);//declaring with size

Vectors possess a number of advantages over arrays :

1. It is convenient to use vectors to store objects.

2. A vector can be used to stroe a list of objects that may vary in size.

3. We can add and delete objects from the list as the list as and when required.

A major constraints in using vectors is that we can not directly store simple data types in a vector.

we can only store objects. Therefore, we need to convert simple types to objects. This can be done using the wrapper classes discussed in the next section.The vector class supports a number of methods that can be used to manipulate the vectors created.

list.addElement(item)//Adds the item specified to the list at the end

list.elementAt(10)//Gives the name of the 10th object

list.size()//Gives the number of objects present

list.removeElement(item)//Removes the specified item from the list

list.removeElementAt(n)//Removes the item stored in the nth position of the list.

list.removeAllElements()//Removes all the element in the list

list.copyInto(array)//Copies all items from list to array

list.insertElementAt(item,n)//Inserts the item at nth position.

Wrapper Classes

Vectors not handled primitive data types like int,long,char and double. Primitive data types may

be converted object types by using the wrapper classes contained in the java.lang package.

Wrapper class for convering primitive data types are given below

Simple type Wrapper Class

boolean Boolean

char Character

double Double

float Float

int Integer

long Long

The Wrapper classes have number of unique methods for handling primitive data types and objects.

Converting Primitive Numbers to Object Numbers Using Constructor Methods :

Integer IntVal=new Integer(i);//Primitive integer to Integer Objects here i as primitive data values.

Converting Object Numbers to primitive Numbers using typeValue() method

int i=IntVal.intValue();//Object to primitive integer

Converting Numbers to Strings Using toString() method

str=Integer.toString(i);//Primitive integer to string

Converting String Objects to Numeric Objects using the Static Method valueOf()

IntVal=Integer.valueOf(str);//converts string to Integer objects

Converting Numeric Strings to Primitive Numbers using Parsing Method

int i=Integer.parseInt(str);//converts string to primitive integer.

Input and Output in JavaFiles in Java

The Path Name

While dealing with the files, a source of confusion for a beginner in Java programming is the path name. For example, consider the following path name on a Unix/Linux machine:

/java/scjp/temp

The first forward slash represents the root directory. This path name in Windows machines may be written as

C:\java\scjp\temp

Constructors for the File Class

File(String pathname)

Creates an instance of the File class by converting the path name

String to an abstract path name.

File(String parent, String child)

Creates an instance of the File class by concatenating the child String to the parent String, and converting the combined String to an

abstract path name.

File(File parent, String child)

Creates an instance of the File class by constructing an abstract path name from the abstract path name of the parent File, and the String path name of child.

Example:

File f1 = new File("java/scjp");

File f2 = new File("java/scjp", "temp/myProg.java");

File f3 = new File(f1, "temp/myProg.java");

An instance of the File class is immutable. This means that once you have created a File object by providing a path name, the abstract path name represented by this object will never change.

When you create an instance of the File class, no real file is created in the file system.

Methods Of File Class:

boolean canRead():boolean canWrite():boolean createNewFile():boolean delete():boolean exists():String getAbsolutePath():String getCanonicalPath():String getName():String getParent():boolean isAbsolute():boolean isDirectory():boolean isFile():String[] list():String[] listFiles():boolean mkDir():boolean mkDirs():

Understanding Streams

Whether the source or destination is a file or a socket, any read or write is performed in three simple steps:

1. Open the stream.2. Until there is more data, keep reading in a read, or writing in a write.3. Close the stream.

The Low-Level StreamsA low-level input stream reads data and returns it in bytes, and a low-level output stream accepts data as bytes and writes the output in bytes. Two examples of low-level streams are represented by the classes FileInputStream and FileOutputStream, which are subclasses of InputStream and OutputStream, respectively.

The FileInputStream Class

The FileInputStream class is designed for reading image files as it reads a stream of raw bytes.

Constructors:

FileInputStream(File file)

FileInputStream (String name)

Methods of FileInputStream

int read() throws IOException: Returns the next byte of data, or -1 if the end of the file is reached

int read(byte[] bytes) throws IOException: Reads bytes.length number of bytes from the stream into an array, and returns the number of bytes read, or -1 if the end of the file is reached

int read(byte[] bytes, int offset, int len) throws IOException: Reads up to a total of len bytes (starting from offset) into an array, and returns the number of bytes read, or –1 if the end of the file is reached

void close(): Closes the input stream and releases any system resources assigned to the stream

The FileOutputStream Class

The FileOutputStream class is meant for writing streams of raw bytes into files, such as image files.Constructors:

FileOutputStream(File file)FileOutputStream (String name)

Methods of FileOutputStreamvoid write(int b) throws IOException: Writes the passed-in byte to the streamvoid write(byte[] bytes) throws IOException: Writes bytes.length number of bytes from the passed-in array to the streamvoid write(byte[] bytes, int offset, int len) throws IOException: Writes up to a total of len bytes (starting from offset) from the passed-in array to the streamvoid close(): Closes the output stream and releases any system resources assigned to the stream

Example code:

1. import java.io.*; 2. public class FileByteCopier { 3. public static void main(String[] args) throws IOException { 4. File inputFile = new File("scjp.txt"); 5. File outputFile = new File("scjpcopy.txt"); 6. FileInputStream in = new FileInputStream(inputFile); 7. FileOutputStream out = new FileOutputStream(outputFile); 8. int c; 9. while ((c = in.read()) != -1)out.write(c); 10. in.close(); 11. out.close(); 12. } 13.}

The High-Level StreamsWhen the unit of information you are interested in is a high-level data type such as a float, an int, or a String, and you don’t want to deal with bytes directly, you can work with high-level streams.

Two examples of high-level streams are DataInputStream and DataOutputStream.

DataInputStream class

constructor for the DataInputStream class is:

public DataInputStream(InputStream in)

Methods of DataInputStream class

boolean readBoolean() throws IOException byte readByte() throws IOException char readChar()throws IOException double readDouble() throws IOException float readFloat() throws IOException int readInt() throws IOException long readLong() throws IOException short readShort() throws IOException

The DataOutputStream Class

The constructor for the DataOutputStream class ispublic DataOutputStream(OutputStream out)

Methods of DataOutputStream class void writeBoolean(boolean b) throws IOException void writeByte(byte b) throws IOException void writeBytes(String s) throws IOException void writeChar(int c) throws IOException void writeChars(String s) throws IOException void writeDouble(double d) throws IOException void writeFloat(float f) throws IOException void writeInt(int i) throws IOException void writeLong(long l) throws IOException void writeShort(short s) throws IOException

Readers and Writers

To read data in text format,Java offers so-called reader and writer streams. Note that some authors do not refer to readers and writers as streams. All the classes corresponding to reader and writer streams are subclasses of the Reader and Writer classes and are used to read character streams.

Low-Level Readers and Writers

The low-level reader streams read data and return it in characters, and low-level output streams accept data as characters and write the output in characters. Two examples of low-level reader and writer streams are FileReader and FileReader.

The FileReader Class

Constructors for the FileReader ClassFileReader(File file)FileReader(String name)

Methods of the FileReader Classint read() throws IOException: Returns the next character of data, or -1 if the end of the file is reached

int read(char[] cbuf) throws IOException: Reads characters from the stream into the array cbuf, and returns the number of characters read, or -1 if the end of the file is reachedint read(char[] cbuf, int offset, int len) throws IOException: Reads up to a total of len characters (starting from offset) into the array cbuf, and returns the number of chars read, or –1 if the end of file is reachedvoid close(): Closes the input stream and releases any system resources assigned to the stream

The FileWriter Class

Constructors for the FileWriter ClassFileWriter(File file)FileWriter(String name)

Methods of the FileWriter Class

void write(int c) throws IOException: Writes the passed-in single character to the stream void write(char[] ch) throws IOException: Writes ch.length number of characters from the passed-in array to the streamvoid write(String str) throws IOException: Writes the passed-in string to the streamvoid write(char[] cbuf, int offset, int len) throws IOException: Writes up to a total of len characters (starting from offset) from the passed in array to the stream

void write(String str, int offset, int len) throws IOException: Writes up to a total of len characters (starting from offset) from the passed-in string to the stream

void flush() throws IOException: Flushes the stream, which means the remaining (buffered) data that you have written to the stream is sent out to the file before closing

void close(): Closes the output stream and releases any system resources assigned to the stream

Example Code: 1. import java.io.*; 2. public class FileByteCopier { 3. public static void main(String[] args) throws IOException { 4. File inputFile = new File("scjp.txt"); 5. File outputFile = new File("scjpcopy.txt"); 6. FileReader in = new FileReader(inputFile); 7. FileWriter out = new FileWriter(outputFile); 8. int c; 9. while ((c = in.read()) != -1)out.write(c); 10. in.close(); 11. out.flush(); 12. out.close(); 13. } 14.}

High-Level Readers and WritersAs you know, you can use DataInputStream and DataOutputStream to read and write the primitive types in binary format. Similarly, you can read and write characters in character streams in big chunks (buffers) and in text format by using the BufferedReader and BufferedWriter classes, respectively.

BufferedReader and BufferedWriter Classesconstructors:

BufferedReader(Reader in);

BufferedReader(Reader in, int size);

BufferedWriter(Writer out);

BufferedWriter(Writer out, int size);

Example Code: 1. import java.io.*; 2. public class FileBufferCopier { 3. public static void main(String[] args) throws IOException { 4. File inputFile = new File("scjp.txt"); 5. File outputFile = new File("scjpcopy.txt"); 6. BufferedReader in = new BufferedReader(new

FileReader(inputFile)); 7. BufferedWriter out = new BufferedWriter(new

FileWriter(outputFile)); 8. String line; 9. while ((line = in.readLine()) != null){ 10. out.write(line); 11. out.newLine(); 12. } 13. in.close(); 14. out.close(); 15. } 16.}

The PrintWriter Class

The PrintWriter class, a subclass of the Writer class.

Constructor for the PrintWriter class:

PrintWriter (Writer out, true)

The most commonly used methods of the PrintWriter class are

public void print(…) and public void println(…)

Exceptions During I/O Operations

Object Streams and SerializationJava offers high-level streams ObjectInputStream and ObjectOutputStream, which, when chained to low-level streams such as FileInputStream and FileOutputStream, can be used by programs to read and write objects. The process of writing an object to somewhere is called object serialization, and the process of reading a serialized object back into the program is called deserialization.

The goal here is to save the state of an object.

To make the objects of a class serializable, the class must implement the interface Serializable:

class MySerialClass implements Serializable

{

// body of the class

}

The Serializable interface is an empty interface (i.e. no methods are declared inside it) and is used to just tag a class for possible serialization.

Writing with ObjectOutputStream

To write an object to a file, you use the ObjectOutputStream to write it to a low-level stream, which in turn will write it to the file. For example, consider the following code fragment:

FileOutputStream out = new FileOutputStream("objectStore.ser"); ObjectOutputStream os = new ObjectOutputStream(out); os.writeObject("serialOut"); os.writeObject(new MySerialClass()); os.writeObject("End of storage!"); os.flush();

following are saved in serialization:

The values of the instance variables of the serialized object.

The class description of the object, which includes the class name, the serial version unique ID, a set of flags describing the serialization method, and a description of the data fields.

All the objects that a serialized object refers to through object reference variables. That means those objects must be serializable; otherwise, you will get a compiler error.

Reading with ObjectInputStream

The objects can be read back in the same order in which they were stored.

Here is a code fragment that reads inthe String and the Date objects that were written to the file named objectStore.ser in the previous example:

FileInputStream in = new FileInputStream("objectStore.ser"); ObjectInputStream is = new ObjectInputStream(in); String note = (String)is.readObject(); MySerialClass serialIn1 = (MyClassSerial)is.readObject(); MySerialClass serialIn2 = (MyClassSerial)is.readObject();

Note the following points about serialization:

If a class is serializable, then all the subclasses of this superclass are implicitly serializable even if they don’t explicitly implement the Serializable interface.

If you want to serialize an array (or some other collection), each of its elements must be serializable.

Static variables are not saved as part of serialization. Recall that the purpose of serialization is to save the state of an object, and a static variable belongs to the class and not to an object of the class.

MultithreadingMultitheading is specialized form of multitasking. like windows and other os uses multitasking technique i.e. they handled more than one process at a time, the same thing we can do in Java using multithreading technique.

A thread is similar to a program that has a single flow of control. It has a beginning, a body, and an end,and executes commands sequentially. Java enables us to use multiple flows of control in developing programs.

Each flow of control is represented by thread that runs parallel to others. i.e. a program that contains multiple flows of control is known as multithreaded program.Threads in Java are subprograms of a main application program and share the same memory space, known as lightweight threads or lightweight process.

Threds running in parallel does not mean that they actually run at the same time. Since all the threads are running on the same processor, the flow of execution is shared between the threads. The Java interpreter handles the switching of control between the threads in such a way that it appears they are running concurrently.

Creating Threads :

Threads are implemented in the form of objects that contain a method called run(). The run() method

is the heart and soul of any thread.

public void run()

{

....

(statements for implementing threads)

}

The run() method should be invoked by an object of the concerned thread. This can be achieved by

creating the thread and initiating it with the help of another thread method called start().

A new thread can be created in two ways.

1. By creating a thread class : Define a class that extends Thread class and override its run() method

with the code required by the thread.

2. By converting a class to a thread :Define a class that implements Runnable interface. The Runnable

interface has only one method, run(), that is to be defined in the method with the code to be executed by the thread.

Extending the Thread Class :

We can make our class as runnable as a thread by extending the class java.lang.Thread. This gives us access

to all the thread methods directly. steps

1. Declare a class as extending the Thread class

2. implement the run() method that is responsible for executing the sequence of code that the thread will execute.

3. Create a thread object and call the start() method to initiate the thread execution.

Stopping and Blocking a Thread

Stopping a Thread

Whenever we want to stop a thread from running further, we may do so by calling its stop() method,like

aThread.stop();

This statement causes the thread to move to the dead state. A thread will also move to the dead state automatically when it reaches the end of its method.

Blocking a Thread

A thread can also be temporarily suspended or blocked from entering into the runnable and subsequently

running state by using either of the following thread methods:

sleep();//blocked for a specified time

suspend();//blocked until further orders

wait();//blocked until certain condition occurs

These methods causes the thread to go into the blocked (or non- runnable) state.the thread will return to the runnable state when the specified time is elapsed in the case of sleep(), the resume() method is invoked in the case of suspend(), and the notify() method is called in the case of wait().

Life Cycle of a Thread

During the life time of a thread, there are many states it can enter. they include :

1. Newborn state

2. Runnabke state

3. Running state

4. Blocked state

5. Dead State

yield

stop

LIFE CYCLE OF A THREAD

Newborn

Running RunnableActive Thread

Dead

Killed Thread

startstop

stopresume

notify

suspend

sleep

wait

BlockedIdle Thread

(Not Runnable

New Thread

Newborn StateWhen we create a thread object, the thread is in born and is said to be in newborn state. The thread is not yet scheduled for running. At this state we can do only one of the following things with it:

•Schedule it for running using start()

•Kill it using stop()

If scheduled it moves to the runnable state. If we attempt to use any other method at this stage,

an exception will be thrown.

Runnable State

The runnable state means thread are ready for execution and is waiting for the

the availabilty of the processor. i.e. the thread has joined the queue of threads that are waitng

for execution. If all threads have equal priority, then they are given time slots for execution in round robin fashion,i.e.,

first-come,fisrt-server manner. The thread that relinquishes control joins the queue at the end again waits for its turn.

This process of assigning time to threads is known as time-slicing.

However,if we want a thread to relinquish control to another thread of equal priority before its turn comes, we can do so be using the yield() method.

Running State

Running means that the processor has given its time to the thread for its execution.

The thread runs until it relinquishes control on its own or it is preempted by a higher priority thread. A running thread may relinquish its control in one of the following situations.

1. It has been suspended using suspend() method. Suspend() can be revived by using the resume() method.

2. It has made to sleep. We can put a thread a sleep for a specified time period using the method sleep(time) where time is in milliseconds. This means that the thread is out of the queue during this time period.

3. It has been told to wait until some event occurs. This is done by using wait() method. The thread can be scheduled to run again using the notify() method.

Blocked State

A thread is said to be blocked when it is prevented from entering into the runnable state and subsequently the running state. This happens when the thread is suspended,sleeping, or waiting in order to satisfy certain requirements. A blocked thread is considered "not runnable" but not dead and therefore fully qualified to run again.

Dead State

Every thread has a life cycle. A running thread ends its life when it has completed executing its run() method. It is natural death. However, we can kill it by sending the stop message to it at any state thus causing a premature death to it.

Thread Priority

In java each thread is assigned a priority, which affects the order in which it is scheduled for

running. The threads that we have discussed so far are of the same priority. The threads of the same priority are given equal treatment by the java schedular and,therefore,they share the processor on first-come,fist-serve basis.

JAva permits us to set the priority of a thread using the setPriority() method as follows:

ThreadName.setPriority(intNumber);

the intNumber is an integer value to which the thread's priority is set. The Thread class defines

several priority constants :

MIN_PRIORITY=1

NORM_PRIORITY=5

MAX_PRIORITY=10

intNumber may assume one of these constants or any value between 1 and 10.the default setting is NORM_PRIORITY.

By assigning priorities to threads, we can ensure that they are given the attention they deserve.

For example, we may need to answer an input as quickly as possible. Whenever multiple threads are ready for execution, the java system chooses the highest priority thread and executes it. For a thread of lower priority to gain control, one of the following thing should happen :

1. It stops the running of the run()

2. It is made to sleep using sleep()

3. It is told to wait using wait()

However, If another thread of higher priority comes along, the currently running thread will be

preempted by the incomming thread thus forcing the current thread to move to the runnable state. Rememberthat the highest priority thread always preempts any lower priority threads.

Syncronization

When multiple threads access shared data then sometimes Data corruption occurs if Multithreaded programs is not designed correctly that leads to system failure.Unfortunately, such problems can occur at unpredictable times and be very difficult to reproduce.

As an example, consider A bank account that is shared by multiple customers. Each of these customers can make deposits to or withdrawals from this account. That application might have a separate thread to process the actions of each user.

Time

t0

t1

t2

t3

t4

t5

t6

Thread A Thread B

Read Balance

Context Switch

Read Balance

Add $10 to Balance

Context Switch

Add $10 to Balance

Balance

$0

$0

$0

$0

$10

$10

$10

Fig 1 Thread SchedulingFig 1 depicts one possible scheduling of these threads. At time t0, the account balance is zero. Thread A is executing and wants to deposit $10 to the account. The current value of the account is read at time t1. However, a context switch from thread A to thread B then occurs at time t2. Thread B then reads the value of the account at time t3. It increments this value by $10 at time t4. Another context switch occurs at time t5. This returns control to Thread A. At time t6, it sets account balance to $10.

The net effect of this sequencing is that the final account balance is only $10. It should be $20. Data corruption has resulted.

The solution to this problem is to synchronize the access to this common data. This can be done in two common ways. First a method can be synchronized by using the synchronized keyword as a modifier in the method declaration.

When a thread begins executing a synchronized instance method, it automatically acquires a lock on that object. The lock is automatically relinquished.

When the method completes. Only one method may have this lock at any time. Therefore, only one method may execute any of the synchronized instance method for that same object, the JVM automatically causes the second thread to wait until the first thread relinquishes the lock.

Another way to synchronize access to common data is via a syncronized statement block. The syntax is :

Synchronize(obj) {

//statement block

}

Here, obj is the object to be blocked. If you wish to protect the instance data, you should lock against that object. If you wish to protect against the class data, you should lock the appropriate Class object. One important benefit of Java class libraries is that they are designed to be thread-safe. In other words, multiple threads may access their methods.

Deadlock

Deadlock is an error that can be encountered in multithreaded programs. It occurs when two or more thread wait indefinitely for each other to relinquish locks. Assume that thread 1 holds a lock on object 1 and waits for a lock on object 2. Thread 2 holds a lock object 2 and waits for a lock on object 1. Neither of these method may proceed. Each waits forever for the other to relinquish the lock it needs.

Thread Communication

In deadlock you have seen that how thread acquires a lock and does not relinquish it. In Thread communication threads can communicate with each other. Thread can temporarily release a lock so other threads can have an opportunity to execute a synchronized method or statement block. That lock can be acquired again at a later time.

A class objects defines three methods that allow threads to communicate with each other. The wait() method allows a thread that is executing a synchronized method or statement block on that object to release the lock and wait for a notification from another thread. It has these three forms :

Void wait() throws InterruptedException

Void wait(long msec) throws InterruptedException

Void wait(long msec,int nsec) throws InterruptedException

The first form causes the current thread to wait indefinitely. The second form causes the thread to wait for a msec milliseconds. The last form causes the current thread to wait for msec milliseconds plus nanoseconds.

notify() method allows a thread that is executing a synchronized method or statement block to notify another thread that is waiting for a lock on this object. If several threads are waiting, only one of these is selected. The selection criteria are determined by the implementer of the JVM. The signature is : void notify()

It is important to understand that when a thread executes the notify() or notifyAll() method it does not relinquish its lock at that moment. This occurs only when it leaves the synchronized method or statement block.

APPLETAn applet is a program that can be referenced by the html source code of web page. It is dynamically downloaded from a Web Server to a browser. The applet then executes within the environment provided by the browser. Alternatively you may use a tool such as the appletviewer to run it.

It is important to recognize that downloading code from the Internet and executing it on your computer is inherently dangerous. Therefore, applet do not have the same capabilities as Java applications. They are restricted to operating within the confines of a “sandbox”. In other words code that is “untrusted” is not allowed to operate outside certain boundaries.

For Example, applets are normally not allowed to read or write to your local disk. This would obviously be risky because they could accidentally or maliciously destroy any data stored on that device.Applet can not execute any native code.

An applet may open a socket connection back to the host from which it was downloaded, but not to any other host. The reason for this restriction can be understood if you imagine a configuration in which a firewall protects a corporate Intranet from computer hackers. Assume that an employee has downloaded an applet from internet to an PC or workstation. If that applet is allowed to open sockets to any machine, it would then have the potential to steal proprietary information and send back to the hacker’s machine. This must be prevented. Therefore, an applet is not allowed to contact any of those private machines.

Difference between Applet and Application

Applet are not full-featured application programs. They are usually written to accomplish a small task or a component of a task. Since they are usually designed for use on the Internet, they impose certain limitations and restrictions in their design.

• Applet do not use main() method for initiating the execution of the code. Applets, when loaded, automatically call certain methods of Applet class to start and execute the applet code.

• Unlike stand-alone applications, applet can not be run independently. They are

run from inside a web page using a special feature known as HTML tag.

• Applets cannot red from or write to the files in the local computer.

• Applets cannot run any program the local computer.

Writing Applet Program

1. Building an applet code (.java file).

2. Creating an executable applet(.class file).

3. Designing a Web Page using HTML tags.

4. Preparing <Applet> tag, Incorporating <applet> tag into the Web Page.

5. Creating HTML file. Testing the applet code

Building Applet Code

It is essential that our applet code uses the services of two classes, namely, Applet and Graphics from the Java Class Library. The Applet class which is contained in the java.applet package provides life and behavior to the applet through its methods such as init(),start() and paint() . Unlike with applications, where Java calls main() method directly to initiate the execution of the program, when an applet is loaded, Java automatically calls a series of Applet class methods for starting, running, and stopping the applet code. The applet class therefore maintains the lifecycle of an applet.

The paint() method of the Applet class, when it is called, actually displays the result of the applet code on the screen. The output may be text, graphics, or sound. The paint() method, which requires a Graphics object as an argument, is defined as follows :

public void paint(Graphics g)

This requires that the applet code imports java.awt package that contains the Graphics class. All output operations of an applet are performed using the methods defined in the Graphics class.

import java,awt.*;

import java.applet.*;

public class appletclassname extends Applet {public void paint(Graphics g) {g.dreawString(“Hello Java”,10,100);}}

Here Applet class itself a subclass of the Panel class, which is again subclass of the Container class and so on given below :

java.lang.Object

java.awt.Component

java.awt.container

java.awt.Panel

java.applet.AppletChain of classes inherited by Applet class

Applet Life CycleEvery Java applet inherits a set of default behaviors from the applet class. The applet state include :

1. Born or Initialization state 2. Idle State

3. Running state 4. Dead or Destroyed State

Born

Running Idle

Dead

Begin Applet

(Load Applet)

Initialization

start() stop()

start()paint()

Display Stopped

destroy()

EndDestroyed

Initialization State : Applet enters the initialization state when it is first loaded. This is achieved by calling the init() method of Applet Class.The applet is born. We required following at this stage : Create objects needed by the applet. Set up initial values Load images or fonts Set up colors

The initialization occurs only once in the applet’s life cycle. To provide any of the behavior we must override the init() method.public void init() {-----}

Running State : Applet enters in the running state when the system calls the start() method of Applet class. This occurs automatically after the applet is initialized. Starting can also occur if the applet is already in “stopped”(idle) state. For example, we may leave the web page containing the applet temporarily to another page and return back to the page. This again starts applet running. Note that, unlike init() method, the start() method may be called more than once. We may override the start() method to create a thread to control a thread to control the applet.

Idle or Stopped State: An applet becomes idle when it is stopped from running. Stopping occurs automatically when we leave the page containing the currently running applet. We can also do so by calling the stop() method explicitly.If we use a thread to run the applet, then we must use stop() method to terminate the thread. We can achieve by overriding the stop() method.

Dead State : An applet is said to be dead when it is removed from memory. This occurs automatically by invoking the destroy() method when we quit the browser. Like Initialization, destroying stage occurs only once in the applets life cycle.

Display State: Applet moves to the display state whenever it has to perform some output operations on the screen. This happens immediately after the applet enters into the running state. The paint() method is called to accomplish this task.

The Graphics Class : A Graphics object encapsulates a set of methods that can perform graphics output. Specifically it allows you to draw lines,ovals,rectangles, strings, images, characters, and arcs, Some of the commonly used methods of the Graphics class are summarized below :Method Description

void drawArc(int x,int y,int w, Draws an arc between degrees0 and degrees1. The center

int h, int degrees0,int degrees1) of the arc is the center of a rectangle with upper-left corner at coordinates x and y, width w, and height h. Zero degrees is at position 3pm on a watch. The angle increases in a counter clockwise direction.

Void drawImage(Image img, Draws the image img so its upper-left corner is at x,y.

int x,int y,ImageObserver io) Updates about the progress of this activity are sent to io.

void drawLine(int x0,int y0,int x1, Draws a line between the points at x0,y0 and x1,y1.

Int y1)

Void drawOval(int x,int y, Draws an oval.

int w,int h)

void drawPolygon(int x[],int y[],int n) Draws a polygons with n corners.

Void drawRect(int x,int y,int w,int h) Draws a rectangle.

Void drawString(String str,int x,int y) Draws str at location x,y.

void fillarc(int x,int y,int w,int h, Fills an arc between degrees0 and degrees1.

Int degrees0,int degrees1)

void fillOval(int x,int y,int w,int h) Fills an Oval

void fillPolygon(int x[],int y[],int n) Fills the polygon with n corners.

Void fillRect(int x,int y,int w,int h) Fills a rectangle with upper-left corner at coordinates x and y, width w, and height h.

Color getColor() Gets the color of the current object.

Font getFont() Gets the font of the current object

FontMetrics getFontMetrics() Gets the font metrics of the current object.

Using Colors : the java.awt.Color class is used to work with colors. Each instance of this class represents a particular color. This class has the following three constructors :

Color(int red,int green,int blue)

Color(int rgb)

Color(float r,float g,float b)

Here red,green and blue are int values that range from 0 to 255.

Display Text : Normally drawString() method is used to draw string on the screen well giving additional capabilities for controlling the appearance and placement of a string in an applet. A font determines the size and appearance of characters in a string. That resides in java.awt.Font.

The following is one constructs : Font(String name,int style,int ps)

Here name identifies the font like Arial. The style may be bold,italic or plain and the point size of the font is ps.

To create a Font setFont() method of the Graphics class.

void setFont(Font font)

Here, font is a Font object. After this method is called, any strings that are output via the drawString() method are displayed with that font. The java.awt.FontMetrics class allows you to get several metrics about the size of a font. In addition, you may also determine the size of a string that displayed in that font. These quantities are provided in pixels.

The specific metrics that are available are ascent, descent leading, and height. Characters extend above and below that line. The number of pixels above the baseline is the ascent. The number of pixwls between the descent of one line and the ascent of the next line is the leading. The sum of descent ,ascent and leading is height. The one constructor for this class is :

FontMetrics(Font font)

Here, font indicates the font for which metrics are wanted. Some of commonly methods are given below all returns the value in pixels.

int charWidth(char c) Returns the width of c

int charWidth(int I) Returns the width if one character in lowest 16 bit of I

int getAscent() Returns the ascent

int getDescent() Returns the descent

int getHeight() Returns the height

int getLeading() Returns the leading

Int stringWidth(String str) Returns width of str

Using Applet Dimension : Using dimension the data can be used to calculate the arguments that should be passed to the drawing methods of Graphics class. For example, you can display a circle at the center of an applet. If applet resized, the circle remains at its center.

The getSize() method is used to determine the size of an applet.

Dimension getSize()

A dimension object encapsulate a height and width. The following are some constructor

Dimension(Dimension d)

Dimension(int w,int h)

Here d is a Dimension object.The arguments w and h represents the width and height in pixels. The class has two instance variable width and height in pixels of type int.

The applets extend the java.applet.Applet class. State and behavior are inherited from java.awt.Component,java.awt.Container and java.awt.Panel.The abstract Component class represents many of the different types of elements you can include in a GUI like Buttons,choices,lists and scroll bars. Because applet is also an Component you can receive and process events from it by using the same techniques that apply to other user interface element.The abstract Container class is a component that may contain other components. A concrete subclass of component is Panel.

The applet class extends Panel. It defines the init(), start(), stop() and destroy() methods are used to manager the applet life cycle.

void destroy() Destroy applets

AppletContxt getAppletContext()Returns the applet context

URL getCodeBase() Returns the code base

URL getDocumentBase() Returns the document base

Image getImage(URL url) Returns an Image object for the Image at url

Image getImage(URL url, Returns an Image object for the Image at url

String imgName)

String getParameter(String str) Returns the value of parameter pName

void init() Initializes this applet

Void showStatus(String str) Displays str on the status line

void start() Starts this applet

void stop() Stops this applet

The AppletContext class : The java.applet.AppletContext interface defines methods that allow an applet to interact with context for environment) in which it is executing. This context provided by either a tool such as the appletviewer or a Web Browser.

Applet getApplet Returns the applet named appName

(String appName)

Enumeration getApplets() Returns an enumeration of the applets in the current context

Image getImage(URL url( Returns an Image object for the image ar the url.

Using Images: The getImage() method of the Applet class returns a java.awt.Image object.

Image getImage(Url url)

Image getImage(URL base, String filName)

The first form accepts one argument that is an absolute URL to identify the image resource. The second form accepts two arguments. Its frst argument is a URL to identify the base location from where the image file can be downloaded and its second argument is the name of a specific file.

The drawImage() method of the Graphics class contains the downloading of an Image and displyas subsets of the image data as it is downloaded to a uses machine.

EVENT HANDLINGAn Event is an Object that describes some state change in source. The events generated when a person interacts with an element in a Graphical User Interface like pressing a Button, clicking the Mouse, Double click on a list Box Entry, or closing a window.

A source generates events. It has three main responsibilities.First, it must provide methods that allow listeners to register and unregister for notifications about a specific type of event. Second, it must generate the event. Finally it must send send the event to all registered listeners. The event may be unicast to a single listener or multicast to several listeners. It is possible for a source to generate several types of events. In that case, multiple registration/unregistration methods would be provided.

The methods implemented by a source that allow listeners to register and unregister for events are as follows :

public void addTypeListener(TypeListener el)

public void addTypeListener(TypeListener el) throws TooManyListenersException

public void removeTypeListener(TypeListener el)

Here type is the type of event and el is the event listener. The first form allows several listeners to register for the same type of event. The second form is provided if only one listener may register to receive that type of event. The last form allows a listener to unregister for notifications about a specific type of event.

A listener receives event notifications. It has three main responsibilities. First, it must register to receive notifications about specific events. It does so by calling the appropriate registration method of the source. Second, it must implement an interface to receive events of that type. Finally, it must unregister if it no longer wants to receive those notifications. It does so by calling the appropriate unregistration method of the source.

It does so by calling the appropriate unregistration method of the source.The delegation event model is depicted given below :

Here, a source multicasts an event to a set of listeners. The listeners implement an interface to receive notifications about that type of event. Let us consider how the delegation event model applies to a button in a graphical user interface. This component generates an event when it is pressed. An object of class java.awt.ActionEvent is created to encapsulate information about the event.

The java.awt.Button class provides the following methods sp listeners may register and unregister to receive action events :

Source

Listener

Listener

Listener

Event

void addActionListener(ActionListener al)

void removeActionListenr(ActionListener al)

Here al is a reference to the listener object.

Finally, the java.awt.eventActionListener interface must be implemented by a listener. This defines one method to receive action events as follows :void actionPerformed(ActionEvent ae)

Here ae is a reference to the action event.

Event Classes :

A set of classes are provided to represent the various types of AWT events given below :

The EventObject class extends Object and is part of java.util.package, its constructor has this form : EventObject(Object src) Here src is the object that generates the event.

This class has two methods shown here :

Object getSource()

String toString()

getSource method returns the object that generated the event, and the toString() method returns a string equivalent to the event.

The abstract AWTEvent class extends EventObject and is part of the java.awt.package. All of the AWT event types are subclasses of AWTEvent.

Object

Event Object

AWTEvent

TextEventActionEvent ComponentEvent

ItemEventAdjustmentEvent

MouseEventKeyEvent

ContainerEvent FocusEvent InputEvent WindowEvent

One of its constructors has this form :

AWTEvent(Object source,int id)// Here source is the object that generates the event and id identifies the type of the event.

Two of its method are shown here :

Int getId()// Returns the type of event

String toString()//Returns the string equivalent of the event.

The ComponentEvent class extends AWTEvent. It defines int constants that are used to identify four type of components events.

COMPONENT_HIDDEN The component was hidden

COMPONENT_MOVED The component was moved

COMPONENT_RESIZEDThe component was resized

COMPONENT_SHOWN the component became visible.

Event Description

ActionEvent A button is pressed, a list item is double clicked, or a menu item is selected.

AdjustmentEvent A scrollbar is manipulated

ComponentEvent A component is hidden,moved,resized or becomes visible.

ContainerEvent A component is added or removed from a container.

FocusEvent A component gains or losses the Keyboard focus.

InputEvent A mouse or key event occurs.

ItemEvent A check box or list item is clicked, a choice selection is made, or a checkable menu item is selected or deselected.

KeyEvent Input is retrieved from the Keyboard.

MouseEvent The mouse is dragged or moved, clicked, pressed, or released, or a checkable menu item is selected or deselected.

TextEvent The value of a text area or text field is changed.

WindowEvent A window is activated, closed, deactivated, deiconified, iconified, opened, or quit.

The MouseMotionListener interface defines two methods to receive mouse events, as follows :

void mouseDragged(MouseEvent me)

void mouseMoved(MouseEvent me)//Here me is the MouseEvent object generated by the source.

Components generate mouse events. The appropriate method of all registered listeners is invoked and the event is passed as an argument to that method.

The mouseClicked() method is called when the mouse is clicked. The mouseEntered() and mouseExited() methods are called when the mouse enters or exits the applet. mousePressed() and mouseReleased() methods are called when the mouse pressed or released.

import java.applet.*;import java.awt.*;import java.awt.event.*;

public class MouseEvents extends Applet implements MouseListener {

public void init() { addMouseListener(this); }

public void mouseClicked(MouseEvent me) { setBackground(Color.blue); repaint(); }

public void mouseEntered(MouseEvent me) { setBackground(Color.green); repaint(); }

public void mouseExited(MouseEvent me) { setBackground(Color.red); repaint(); }

public void mousePressed(MouseEvent me) { setBackground(Color.white); repaint(); }

public void mouseReleased(MouseEvent me) { setBackground(Color.yellow); repaint(); } }

Adapter Class : Here you see that MouseListener interface having five methods and in a program these five methods should declared by a component and it is inconvenient sometime.Adapter classes make it easy to deal with this situation. An adapter class provides empty implementation of all methods in a particular listener interface. It can be useful if you want to override only some of the methods defined by that interface.The adapter classes in the java.awt.event package and the listener interfaces that are implemented by each.Here in the given below example an instance of MyMouseAdapter is created and registered to receive mouse events. The MyMouseAdapter class extends MouseAdapter and overrides the mousePressed() and mouseReleased() methods.

import java.applet.*; import java.awt.*; import java.awt.event.*;

public class MouseAdapterDemo extends Applet {

public void init() { setBackground(Color.green);

addMouseListener(new MyMouseAdapter(this)); } }

class MymouseAdapter extends MouseAdapter {

MouseAdapterDemo mad;

public MyMouseAdapter(MouseAdapterDemo mad) { this.mad=mad; }

public void mousePressed(MouseEvent me) { mad.setBackground(Color.red); mad.rapaint(); }

public void mouseReleased(MouseEvent me) { mad.setBackground(Color.green);

mad.rapaint(); } }

Adapter class Listener Interface

ComponentAdapter Component Listener

ContainerAdapter Container Listener

FocusAdapter Focus Listener

KeyAdapter KeyListener

MouseAdapter MouseListener

MouseMotionAdapter MouseMotionListener

WindowAdapter WindowListener

Inner Classes : An Inner class is defined within the scope of an expression or another class. Therefore it has to access to the variables and methods in that scope. The init() method of the applet instantiate the adapter class and registers that object to receive mouse listener events public class.

MouseInnerDemo extends Applet { public void init() { ……

addMouseListener(new MyMouseAdapter()); }

Class MyMouseAdapter extends MouseAdapter {

public void mousePressed(MouseEvent me) { ……..}}}

Ex: Here MyMouseAdapter class extends MouseAdapterDemo and overrides the mousePressed() and mouseReleased() methods.

import java.applet.*; import java.awt.*; import java.awt.event.*;

public class MouseInnerDemo extends Applet {

public void init() { setBackground(Color.green); addMouseListener(new MyMouseAdapter()); }

class MyMouseAdapter extends MouseAdapter {

public void mousePressed(MouseEvent me) { setBackground(Color.red); repaint(); }

public void mouseReleased(MouseEvent me) { setBackground(Color.green); repaint(); } } }

Anonymous Inner Class: An annonymous inner class is an inner class that does not have a name. The init() method of the applet instantiates an anonymous adapter class and registers that

object to receive mouse listener events. The annonymous inner class extends MouseAdapter and

provides an implementation of the mousePressed() method.

public class MouseAnonymouseDemo extends Applet {

public void init() { addMouseListener(new MouseAdapter() {

public void mousePressed(MouseEvent me) { } } ); } }

Ex :

import java.applet.*;import java.awt.*;import java.awt.event.*;

public class MouseAnonymousDemo extends Applet {

public void init() { setbackground(Color.green);

addMouseListener(new MouseAdapter() {

public void mousePressed(MouseEvent me ) { setBackgroundColor(Color.red); repaint(); }

Public void mouseReleased(MouseEvent me) {

setBackground(Color.green); } } ); }

AWTThe container class extends Component. Container as a component that can hold reference to a number of other components. It allows the GUI that nest containers within containers. Each container has an associate LayoutManager object that determines where to position its components. The java.awt.package includes several types of layout managers. The abstract Container class provides only a default constructor. The add() method resides in the container class used to add the components.

Component

Button

Object

CheckBox

Container

Label

List

Scrollbar

Choice

Text

Panel

Window

Applet

Dialog

Frame

FileDialog

Text Area

TextField

Canvas

AWT classes that inherit from Java.lang object.

All the classes that control the placement of objects on the screen inherit from the Java.lang object.class.

The Applet.class inherits from java.awt.Panel,so you can draw directly to an applet.

AWT classes just inherit from Object. In addition, all the interactive elements (except menus) inherit from Component. The only other very important thing to note is that because Applet inherits from Panel (which inherits from Container), applets can directly contain other objects such as buttons, canvases, and so on. This section describes how you can build hierarchies of containers in applets. Graphics ClassThe Graphics class is part of the AWT. It's contained in java.awt.Graphics, and it's the basic class for everything you'll draw on-screen. Applets have associated Graphics instances, as do various components such as buttons. Drawing methods, such as drawLine, work on a Graphics instance, so you'll see many calls in this form in a typical Java applet: public void paint(Graphics g) {    g.drawLine(10,10,20,20);} The Graphics class uses a standard computer coordinate system with the origin in the upper- left corner.

All coordinate measurements in Java are done in pixels. The size of items, therefore, in absolute units such as inches or millimeters, differs on various machines due to differing pixels/inch values. You'll find that whenever your program has to draw something, you'll be using Graphics class methods. The following sections discuss the most useful methods.

The update, paint, and repaint MethodsYou'll encounter three key methods over and over again as you work with the various user interface elements.

repaintRequests a redraw of an item or an entire interface. It then calls update. updateControls what happens when repaint is called; you can override this method. paintDetermines what's done when any item is redrawn. It's called whenever something needs to be redrawn-for example, when a window is uncovered. All displayable entities have paint methods that they inherit from Component.

An example of paint and repaint. import java.awt.*;import java.applet.Applet;public class paint_methods extends Applet{    int y;    public void init() {        y = 1;    }    public void start () {        while(true) {             y += 1;            repaint();             //wait 50 milliseconds and then call repaint again            try {                Thread.sleep(50);                 } catch(InterruptedException e) {}        }    }  public void paint(Graphics g)    {        //draw a string to the screen        g.drawString("Hello, World!", 25, y );    }}

Component Class-Shared Features of All Active GUI Elements

All the active components (other than menus), such as Button, inherit from the Component class. The Component methods provide a wide selection of functionality applicable to any interactive graphical element.boolean, action(Event e, Object o)This method usually is overridden. It's called whenever an ACTION_EVENT occurs on a component.boolean, keyDown(Event e, int key)This is called when a KEY_PRESS or KEY_ACTION event reaches a component. The key parameter specifies which key was involved. You can use this to have components respond to key clicks. boolean, keyUp(Event e, int key)This method is invoked when the component receives a KEY_RELEASE event. boolean, lostFocus(Event e, Object o)This is called when the object receives a LOST_FOCUS event. boolean, mouseDown(Event e, int x, int y)This is invoked when the component receives a MOUSE_DOWN event, caused by the user clicking the mouse inside the component. The x and y coordinates are in the coordinate system of the component, where 0,0 is in the upper-left corner. boolean, mouseDrag(Event e, int x, int y)This is invoked when the user drags the mouse with the mouse button down over the component, generating a MOUSE_DRAG event. boolean, mouseEnter(Event e, int x, int y)This is invoked each time the mouse goes over the component, generating a MOUSE_ENTER event.

boolean, mouseExit(Event e, int x, int y)This is called when the component receives a MOUSE_EXIT event. The x and y values-which are expressed in the component's coordinates-represent the first point outside the component's bounding rectangle that the mouse goes over. Although Component has a large selection of methods, the following are the ones you'll use most often. Rectangle, bounds()Returns the bounding rectangle that contains the component. int, checkImage(Image img, ImageObserver iobs)

Monitors the status of an image as it's being composed. You can use this to wait to display a component, such as a Canvas, that uses an image until the image is ready.

Image, createImage(int width, int height)

Creates a new Image of the specified size.

disable()

Disables the component so that the user can't interact with it. (This is a synchronized method.) The AWT draws a disabled component differently than an enabled one.

enable()

Enables a disabled component. This is a synchronized method.

Color, getBackground()

Returns the color of the background for the component.

Font, getFont()

Returns the current font for the component.

FontMetrics, getFontMetrics()

Gets the FontMetrics, which contains information about the size of text on the current platform, for the component.

Color, getForeground()

Returns the foreground color-the one that will be used to draw lines, fill shapes, and so on.

Graphics, paint(Graphics g)

Redraws the component when it needs to be redrawn. Unless you want some custom behavior, the default method ensures that the component is drawn properly.

boolean, prepareImage(Image img, ImageObserver img_obs)

Enables you to get an image ready for display prior to displaying it on the component. Another version enables you to specify a size for the image so that it can be scaled.

repaint(long time)

Repaints this component by a specified time or cancels the request.

repaint(int x, int y, int width, int height)

Repaints the specified part of the component.

repaint(long time, int x, int y, int width, int height)

Tries to repaint the specified region. If it can't do so before the specified time, it quits.

reshape(int x, int y, int width, int height)

Enables you to specify the position and size of the component. This is a synchronized method.

resize(int width, int height)

Scales the component to fit in the defined bounding rectangle maintaining the same origin. This is the same as the version below except you specify the width and height separately rather than with a Dimension object.

resize(Dimension dim)

Scales the component to fit in the defined bounding rectangle maintaining the same origin. setBackground(Color a_color)

Sets the background color for a component. This is a synchronized method.

setFont(Font a_font)

Specifies the font that will be used for any text drawn in the component. This is a synchronized method.

setForeground(Color a_color)

Sets the color used for drawing lines and filling in shapes. This is a synchronized method.

show()

Makes the component visible if it had been hidden.

Dimension, size()

Returns the height and width of the component.

update(Graphics g)

Erases the contents of the component's graphic area every time it's called.

validate()

Causes the component to see whether it or any of the components it contains is invalid. If any are invalid, the Layout Manager is called to bring things up-to-date

Containers

The AWT containers contain classes that can contain other elements. Windows, panels, dialog boxes, frames, and applets are all containers. Whenever you want to display a component such as a button or pop-up menu, you'll use a container to hold it. The base class for all containers is-surprise! surprise!-the Container class.

The Container class has a number of methods that make it easy to add and remove components as well as to control the relative positioning and layout of those components. Containers can contain other containers, for example, so a window can contain several panels.

Container is an abstract class, and the methods you'll use most often follow.

add(Component a_component)

Adds a component to the container.

add(Component a_component, int pos)

Adds a component at the specified z position. This is a synchronized method. Be warned that the order of clipping based on relative z position may vary between machines. This problem should be fixed eventually, though.

insets, insets()Returns the insets object for the container. Insets define the empty space the Layout Manager reserves around the edge of the container-the minimum distance from the edge of a component to the edge of the container. remove(Component a_component)Removes the component from the container. This is a synchronized method. setLayout(LayoutManager lm)Sets the Layout Manager the container will use. If you supply NULL as the argument, no Layout Manager is used; you can use absolute positioning. PanelsApplet inherits from this class, so this section examines Panel in detail so that you can understand how the various demonstration applets work

Panel inherits from Container. It doesn't create its own window because it's used to group components inside other containers. Panels enable you to group items in a display in a way that might not be allowed by the available Layout Managers. If you have a number of entries in your interface, for example, that have a label and a text field, you can define a panel that contains a label and a text field and add the panel so that the label and the text field always stay together on the same line (which wouldn't be the case if you added the two items separately). Without the panel, the Layout Manager could put the label and the text field on different lines. Panels also are useful in Layout Managers in which only one item is allowed in an area, such as the BorderLayout Manager. By using a panel, you can put several components in a single BorderLayout area, such as North.InsetsAn inset object defines the amount of empty space around the edge of a panel. The creator method for insets follows: Insets, new Insets(int top, int left, int bottom, int right) This defines a new Insets instance, which defines the boundaries specified by the input arguments

FrameA frame is a full-fledged, top-level, resizable window with a menu bar. You can specify the title, an icon, and a cursor. See the "Frames" section for examples. WindowsThis class isn't used very often, but it's a top-level window without borders and a menu bar. LabelsLabels are text items that don't really do much. By using a label instead of drawString, you can use the Layout Managers to control text placement in a platform- and monitor-independent manner.

The label creators and the most useful methods for the Label class follow: new Label(String label)Produces a label with the specified string. new Label(String label,int positioning)Produces a label with the string aligned according to the second value, which should be one of the three constants Label.CENTER, Label.LEFT, or Label.RIGHT. String, getText()Returns the label string. setText(String new_label)Changes the label text

ButtonsJava buttons are just like the buttons in every other GUI. They are text surrounded by a shape, and they generate an ACTION_EVENT event-the argument is a button's label-after the user clicks them

new Button(String the_button_label)Creates a button with the specified label. setLabel(String the_new_label)Sets the button label to the specified string. String getLabel()Returns the current button label as a string. CheckboxesCheckboxes are text items with a checkable icon next to them. They're generally used when you want the user to be able to set several options prior to making a decision. You usually don't do anything when a checkbox is checked or unchecked, you usually just read the values of the checkboxes when some other control, such as a button or menu item, is activated. Just in case you do want the code to do something when a box's state changes, checkboxes generate an ACTION_EVENT with the new Checkbox state as the argument after the user clicks on them.

Radio buttons look just like checkboxes, but they are grouped and only one radio button in a group can be checked at any given time. The next section discusses how to implement radio buttons.new Checkbox()Creates a new checkbox with no label.new Checkbox(String the_label)Creates a new checkbox with a label. new Checkbox(String the_label, CheckboxGroup null, boolean checked?)Creates a new checkbox that is labeled and checked. The middle argument is used with radio buttons.

setLabel(String the_new_label)

Changes the label of a checkbox.

String getLabel()

Returns the current label as a string.

boolean getState()

Gets the current checkbox state (checked = TRUE).

setState(boolean new_state)

Sets the checkbox state.

Radio Buttons

Checkboxes and radio buttons look different. Even though radio buttons are made up of checkboxes, they're called radio buttons because that's what they're called in most current GUIs. The only functional difference is that only one of the items in a radio button group can be selected at one time, like the buttons on your car radio. This is useful when you want your user to select one of a set of options. The AWT creates a radio button group by associating a CheckboxGroup instance with all the checkboxes in the group.

Radio buttons have only one creator method:

new Checkbox(String the_label, CheckboxGroup a_group, boolean checked?) This creates a new Checkbox that is labeled and checked. The middle argument defines which radio button group the checkbox belongs to.

In order to use radio buttons, you also need to create a new checkbox group. Use this code:

new CheckboxGroup() Because radio buttons are implemented as checkboxes, the methods described in the "Checkboxes" section are the ones you'll use to get and set information.

Choice MenusChoice menus-often called pop-up menus-are designed to allow the user to select an option from a menu and see the value chosen at all timesnew Choice()Creates a new Choice item. addItem(String the_item_name)Adds an item to the Choice menu. It can throw a NullPointerException. This is a synchronized method. int countItems()Returns the number of items currently in the menu.String getItem(int menu_item_number)Returns the text of the specified menu item (item 0 is the first item in the menu). int getSelectIndex()Returns the index of the currently selected item (item 0 is the first item in the menu). String getSelectedItem()Returns the text of currently selected menu items. select(int menu_item)Changes the selection to the specified item. This is a synchronized method, and it can throw IllegallArgumentException. select(String menu_item_name)Selects the menu item for which the name is the specified string. Scrolling ListsScrolling lists display multiple lines of text, and each line corresponds to a selection item. Scroll bars are displayed if the text is larger than the available space. The user can select one or more of the lines. Your program can read the user's selections. Lists generate three event types: •ACTION_EVENT: When a list item is double-clicked. The argument is the name of the list item. •LIST_SELECT: When a list item is selected. The argument is the name of the list item selected. •LIST_DESELECT: When a list item is deselected. The argument is the name of the item deselected.

addItem(String item_label)Adds the specified item to the end of the current list of items in the list. This is a synchronized method. addItem(String item_label, int location)Adds the specified item to the list at the specified location. This is a synchronized method. Remember that the first item in the list is numbered 0. For example, addItem("a test", 3) puts "a test" into the fourth position in the list and slides the previous fourth entry and all entries after it down one. int clear()Removes all the entries in the list. This is a synchronized method. int countItems()Returns the number of items currently in the list. String getItem(int location)Returns the label of the list item at the specified location. int getSelectedIndex()Throws an ArrayIndexOutofBoundsException if it's invoked on a list where more than one item is selected. The method returns -1 if no items are selected. This is a synchronized method. int[] getSelectedIndexes()Returns an array of the locations of the selected items. This is a synchronized method. It works with a single selection and with single-selection lists. It returns -1 if no items are selected. String getSelectedItem()Returns the location of the currently selected item. This is a synchronized method. A runtime Exception is thrown if this method is called on a multiple-selection list. For that reason, and the fact that getSelectedItems will work with a single item, it's best to avoid this method. If no item is selected, it returns NULL.

The TextComponent Class-Invisible but UsefulThis class is abstract, but it's extended by both TextFields and TextAreas. All the methods covered here are available in both those GUI elements. TextComponent provides the basic tools for finding out what text is in a Text item (getText), setting the text in an item (setText), and selecting pieces of text (setSelect). When using TextFields or TextAreas, you won't have to worry about managing the cursor location, the insertion point (the vertical cursor that tells the user where newly typed text will be inserted), or the marking of the selected text. All these functions are done for you by the AWT. The most useful TextComponent methods follow.

String getSelectedText()Returns the text currently selected in the text item. The text may have been selected by the user or through the setSelection method. String getText()Returns all the text in the text item. setText(String new_text)Enables you to set the text in the text item. This replaces all the text in the item. If you want to insert or append text, you need to use getText, modify the string, and then use setText to put the modified string back in the text item. Note that TextArea has insert and append methods. TextField Text fields are designed to be used to allow the user to input short pieces of text-usually no more than a few words or a single number. You also can use them to display information to the user, such as a phone number or the current sum of the costs of the items the user is going to order. Because TextField extends TextComponent, you can define whether the user can edit the contents of a TextFieldTextArea Text areas are designed to hold large chunks of text, where large is more than one line. TextArea extends TextComponent by adding a number of additional methods as well as automatic scrolling of the text.

new TextArea()

Defines a default empty TextArea. new TextArea(int rows, int columns)

Defines an empty TextArea with the specified number of rows and columns. new TextArea(String the_contents)

Defines a TextArea that contains the specified string. new TextArea(String the_contents, int rows, int columns)

Defines a TextArea containing the specified string and with a set number of rows and columns. appendText(String new_text)

Appends the specified string to the current contents of the TextArea. int, getColumns()

Returns the current width of the TextArea in columns. int, getRows()Returns the current number of rows in a TextArea. insertText(String the_text, int where_to_add)Inserts the specified string at the specified location. replaceText(String new_text, int start, int stop)Takes the text between start and stop, inclusive, and replaces it with the specified string.

Organizing Your Interface with LayoutsThe traditional method for building a GUI has been to position various interface elements, such as buttons, at specific locations inside a window and then to allow the user to move the windows around. Java has had to explore new approaches to defining the layout of components because of the diversity of standards that it has to support. Although the AWT does let you specify the absolute location of components, it also gives you Layout Managers that let you define the relative placement of components that will look the same on a wide spectrum of display devices. Although you can build your own Layout Manager, it's easiest to use one of the Managers that come with the AWT. In addition, freeware Layout Managers currently are available; these are discussed along with how to build your own Layout Manager, and more will be arriving in the future. FlowLayout This is the default Layout Manager that every panel uses unless you use the setLayout method to change it. It keeps adding components to the right of the preceding one until it runs out of space; then it starts with the next row.GridLayout GridLayout's simple rule is to allow the user to define the number of rows and columns in the layout. GridLayout then sticks one item in each grid cell. The cells are all the same size. The size of the cells is determined by the number of cells and the size of the container.new GridLayout(int rows, int cols)Makes a GridLayout with the specified number of rows and columns. new GridLayout(int rows, int cols, int horizontal_gap, int vertical_gap)Makes a GridLayout with the specified rows and columns and with the specified empty space around each component. GridBagLayout This is the most powerful, complex, and hard-to-use Layout Manager that comes with the AWT. Although it gives you the most flexibility, you should plan to spend some time experimenting with its parameters before you get a layout that you like. The basic principle of GridBagLayout is that you associate a constraint object, an instance of GridBagConstraints, with each component in the layout.

The GridBagLayout Manager uses those constraints to determine how to lay out the components on an invisible grid, where each component can occupy one or more grid cells. The creator methods for GridBagConstraints take no input parameters; you customize the instance by changing the following instance variables.

anchorSpecifies how a component is to be aligned if a component is smaller than the allocated space. The available constants follow: CENTER: Puts the component in the middle of the area.EAST: Aligns it with the right-middle side.NORTH: Aligns it with the top-middle.NORTHEAST: Puts it in the upper-right corner.NORTHWEST: Puts it in the upper-left corner.SOUTH: Aligns it with the bottom-middle.SOUTHEAST: Puts it in the lower-right corner.SOUTHWEST: Puts it in the lower-left corner.WEST: Aligns it with the left-middle side. fillDetermines what happens if the space allotted to a component is larger than its default size. The allowable values follow: BOTH: Tells the component to fill the space in both directions.HORIZONTAL: Tells the component to fill the space in the horizontal direction. NONE: Leaves the component at its default size.VERTICAL: Tells the component to fill the space in the vertical direction. gridheightSpecifies the height of the component in grid cells. The constant REMAINDER specifies that the component is the last one in the column and therefore should get all the remaining cells.

gridwidthSpecifies the width of the component in grid cells. The constant REMAINDER specifies that the component is the last one in the row and therefore should get all the cells remaining in the row. gridxSpecifies the grid position of the left side of a component in the horizontal direction. The constant RELATIVE specifies the position to the right of the previous component.

gridy

Specifies the grid position of the top of a component in the vertical direction. The constant RELATIVE specifies the position below the previous component.

insets

Enables you to set an instance of the Insets class that specifies the whitespace reserved around an object. It provides more flexibility than ipadx and ipady because it allows different whitespace on the left than on the right and different whitespace on the top than on the bottom of the component.

ipadx

Specifies the amount of padding (empty space) to put on either side of a component. This increases the effective size of the component.

ipady

Specifies the amount of padding to put above and below the component.

weightx

Specifies how extra horizontal space (space not needed for the default component sizes) is allocated between components. This is a relative value, normally chosen to be between 0 and 1, and the values of the components are compared when allocating space. If one component has a weight of .7 and another has a weight of .2, for example, the one with weight .7 gets more of the extra space than the one with .2.

weighty

Same as weightx but for the vertical direction.

BorderLayout

The BorderLayout divides the container into five pieces; four form the four borders of the container and the fifth is the center. You can add one component to each of these five areas. Because the component can be a panel, you can add more than one interface element, such as a button, to each of the five areas. BorderLayout makes room for the items in the four border areas (referred to as North, South, East, and West), and then whatever is left over is assigned to the Center area. This layout is nice if you want to place scrollbars around a panel, place the scrollbars in the border regions, use all four scrollbars or just two, and place the panel you want to scroll in the center.

CardLayout

The CardLayout is different from the others because it enables you to create virtual screen real estate by defining multiple Cards, one of which is visible at any time. Each Card contains a panel that can contain any number of interface elements, including other panels. If you've ever used HyperCard on the Mac, you'll be familiar with this Rolodex‰ type of interface. It's also similar to the tabbed dialog boxes that are the rage in Microsoft products, but Cards lack any built-in way to go from Card to Card; you have to provide an interface for that.Images

The developers of Java knew that working with images is a critical part of any modern programming language with a goal of implementing user interfaces that meet the criteria of users. Because Java is platform independent, though, it couldn't use any of the platform-specific formats, such as the Mac's PICT standard. Fortunately, there already are two platform-independent formats: GIF and JPEG.

These formats are especially nice because they are compressed so that transmitting them takes less of the limited network bandwidth. The AWT supports both these compression formats, but it uses neither of them internally. Although you can read in GIF and JPEG files, they are converted into images, which are just bitmaps. All the work you do in Java with images is based on the Image class. For some strange reason, even though there is a special package for image-manipulation-related classes (java.awt.image), the Image class itself resides in the top-level java.awt package.

Image is an abstract class designed to support bitmapped images in a platform-independent manner. Although this class provides just the basics for working with images, it does have several methods you'll find useful.

Java uses a model of image producers and image consumers. Image producers generate pixels from a file or Image object, and image consumers use and/or display those pixels. Both ImageConsumer and ImageProducer are Java interfaces. AWT comes with ImageProducers for reading from local files and URLs, arrays in memory, and Image objects. It also comes with CropImageFilter, RGBImageFilter, and PixelGrabber, which implement the ImageConsumer interface.

Windows

The Window class implements a window with no borders and no menu bar. This generally isn't a useful class on its own, but because Frame and Dialog-which are useful-extend it, it's useful to take a quick look at Window's methods.

dispose()

This gets rid of the window's peer. When the window is destroyed, you need to call this method. This is a synchronized method.

Toolkit, getToolkit()

Returns the Toolkit associated with the window.

show()

Displays the window, making it visible and moving it to the front. This is a synchronized method.

toBack()

Moves the window behind all other windows in the application.

toFront()

Moves the window in front of all other windows. User interactions with a window can cause it to generate the WINDOW_DESTROY, WINDOW_ICONIFY, WINDOW_DEICONIFY, and WINDOW_MOVED events. Frames

A Frame implements a resizable window that supports a menu bar, cursor, icon, and title.

Frames generate the same events as windows, which they extend: WINDOW_DESTROY, WINDOW_ICONIFY, WINDOW_DEICONIFY, and WINDOW_MOVED.

The only parameter you can pass to the Frame constructor is a String, which will be the window title. You generally will create your own class that extends Frame and contains event-handling methods that override Component methods such as action, mouseDown, and keyDown. When you extend the class, you can make creator methods with more input parameters. One useful technique when you're using Frames with applets is to pass the applet, using this-the Java construct that refers to the object in whose scope the program line is in-to the Frame so that the Frame methods can invoke applet methods and read/write applet instance variables.

The most useful Frame methods follow.

dispose()

Enables you to free up windowing resources when you're done with a Frame. This is a synchronized method. int, getCursorType()

Returns the integer constant that defines which cursor currently is displayed.

Image, getIconImage()

Returns the image being used when the window is reduced to an icon.

MenuBar, getMenuBar()

Returns the frame's menu bar.

String, getTitle()

Returns the frame's title.

boolean, isResizable()

Returns TRUE if the frame can be resized. This attribute can be toggled using the setResizable method. remove(MenuComponent mb)

Removes the menu bar associated with the frame. This is a synchronized method.

setCursor(int cursor_constant)

Sets the current cursor to the one specified by the input argument.

setIconImage(Image icon)

Sets the icon to be used when the frame is reduced to an icon to the input image.

setMenuBar(MenuBar mb)

Sets the menu bar for the frame. This is a synchronized method. setResizable(boolean flag)

Changes the frame size if the input parameter is TRUE. If the input is FALSE, the frame is a fixed size. setTitle(String new_title)

Sets the window title.

Menus

You can put a menu bar in a frame or window, but not an applet.

All the other menu classes inherit from MenuComponent. MenuComponent is an abstract class, but you'll use these methods fairly often.

Font, getFont()

Returns the font used for the current item.

setFont(Font a_font)

Sets the font to be used to display the item on which the menu is invoked. The MenuBar class is a container for a set of menus displayed with a frame. The key MenuBar methods follow.

Menu, add(Menu a_menu)

Adds a menu to the menu bar. The return value is a handle to the added menu. Menus are added left to right. This is a synchronized method.

int, countMenus()

Returns the number of menus currently in the menu bar.

Menu, getHelpMenu()

Returns the menu that is defined as the Help menu for the menu bar.

Menu, getMenu(int pos)

Returns the menu item at a given location in the menu bar.

remove(int pos)

Removes the menu at the specified position. This is a synchronized method. remove(MenuComponent menu)

Removes the specified menu from the menu bar. This is a synchronized method.

setHelpMenu(Menu a_menu)

Sets the specified menu to be the Help menu, which always is placed on the right side of the menu bar. This is a synchronized method. The Menu class implements pull-down menus. There are two constructor methods and some useful Menu methods.

new Menu(String a_label)

Creates a new menu with the specified label.

new Menu(String a_label, boolean tear_off)

Creates a new menu with the specified label, which can be torn off from the menu bar.

MenuItem, add(MenuItem an_entry)

Adds the specified menu item to the menu. You make hierarchical menus by adding menus to another menu. This is a synchronized method.

add(String label)

Adds a new entry to the menu.

addSeparator()

Adds a separating line to the menu.

int, countItems()

Returns a count of the number of items in the menu.

MenuItem, getItem(int position)

Returns the menu item at the specified location.

boolean, isTearOff()

Returns TRUE if the menu has tear off enabled.

remove(int position)

Removes the menu item at the specified location. This is a synchronized method. remove(MenuComponent an_item)

Removes the specified menu item. This is a synchronized method. The MenuItem class implements the functionality of a single entry in a pull-down menu. When you create one, you have to supply its label (as a string) as the input parameter. You use the add method to add MenuItems to menus. The most useful MenuItem methods follow.

disable()

Grays out a menu item and prevents the user from selecting it.

enable()

Enables a menu item so the user can select it.

enable(boolean some_statement)

Enables a menu item if the logical statement evaluates to TRUE.

StringgetLabel()

Returns the item's label.

boolean, isEnabled()

Returns TRUE if the user can select the item.

setLabel(String new_label)

Changes the label of the menu item to the specified string. An ACTION_EVENT is generated whenever a menu item is selected by the user. The CheckboxMenuItem class extends MenuItem and implements the functionality of a menu item with an associated checkbox. The two methods of this class that will come in handy follow.

boolean, getState()

Returns TRUE if the menu item is checked.

setState(boolean new_state)

Sets the state of the menu item.

Dialog You can build your own dialog boxes using Frames, but the Dialog class enables you to create modal dialog boxes. A modal dialog box forces the user to deal with the dialog box before doing anything else. Although this generally isn't a good idea, certain types of actions, such as notifying a user of a problem, require user input before the program can do anything else. Dialog boxes are containers, so you can add components to them, but their default Layout Manager is BorderLayout rather than FlowLayout. User interaction with dialog boxes can generate the WINDOW_DESTROY, WINDOW_ICONIFY, WINDOW_DEICONIFY, and WINDOW_MOVED events. As with other window-related classes, you should call the dispose method (inherited from Window) when the window is destroyed in order to free up window system resources. The two dialog creator methods follow.

new Dialog(Frame a_parent, boolean modal_flag)Creates a new dialog box that is modal if the second argument is TRUE. new Dialog(Frame a_parent, String dialog_title, boolean modal_flag)Functions like the preceding method, except that you can specify the name of the dialog box. Unfortunately, Dialogs can be created only as children of Frames, as you can see from the arguments to the creator functions. This might make you think that you can't use Dialogs with your applets but, through the use of a minor subterfuge, you canFileDialog This class enables you to access the user's native file opening and saving dialog boxes. It's a modal dialog box that you can create with the following two creator methods. new FileDialog(Frame a_parent, String title)Creates a file selection modal dialog box with the specified title. new FileDialog(Frame a_parent, String title, int mode_flag)Functions like the preceding method, except that you can specify, using the third parameter, whether this is a file selection or a file saving dialog box. The two constants you should use follow:

FileDialog.LOAD: Open File dialog boxFileDialog.SAVE: Save File dialog box The general approach to using this class is to create the dialog box and then show it when you want the user to select or save a file. When the user finishes with the modal dialog box, you use the getDirectory and getFile methods to get a path to the file that's to be saved or loaded. The methods you'll find most useful for FileDialog follow. String, getDirectory()Returns the directory to the file the user has selected or the directory where the user wants to save a file. The string uses backslashes to separate directories and it doesn't end with a backslash. A file on disk Space inside folder Files, for example, would return /Space/Files. String, getFile()Returns the name of the file to be opened or the name the file is to be saved as. FilenameFilter, getFilenameFilter()Returns the FilenameFilter-an interface specification that enables you to filter which files appear in the dialog box-associated with the File dialog box. nt, getMode()Returns the mode of the dialog box. setDirectory(String default_directory)Enables you to set the directory the user sees when the dialog box opens. Specify the directory with the same string format returned by the getDirectory method.setFile(String a_file)Sets the file in the dialog box. setFilenameFilter(FilenameFilter a_filter)Associates an instance of the FilenameFilter with the dialog box. The one method in the FilenameFilter class is called for every file, and only those that pass the test are displayed.