1 Chapter 2 JAVA FUNDAMENTALS. 2 THE PARTS OF A JAVA PROGRAM // Program by: L. Thompson // A simple...

1

Chapter 2JAVA FUNDAMENTALS

2

THE PARTS OF A JAVA PROGRAM

// Program by: L. Thompson

// A simple Java program that displays the message

// Have a good semester! on a line on the computer screen

public class DisplayMessage

{

public static void main(String[ ] args)

{

System.out.println("Have a good semester!");

System.exit(0);

}

}

3

THE PARTS OF A JAVA PROGRAM Comments





{


{


System.exit(0);

}

}

These are comments. They

are ignored by the compiler.

4


• Comments are notes of explanation used to document programs, sections of programs, or program statements for the humans who must read programs.

• Every program should start with a comment that supplies the name of the author of the program and a description of the program.

DisplayMessage.java begins with the following comments:




5


• The compiler, the program that translates Java programs to byte code, ignores everything from the two forward slashes to the end of the line.

• Comments do not end with a semicolon.

6

THE PARTS OF A JAVA PROGRAM





{


{


System.exit(0);

}

}

The compiler ignores blank lines. It is a good idea to use

blank lines to make your program easier to read.

7

THE PARTS OF A JAVA PROGRAMClass Definition





{


{


System.exit(0);

}

}

This is the class header for the class named DisplayMessage.

This is a class

definition.

8


• Every Java program must have at least one class definition.

• We will use a class as a container for the statements that comprise our program.

Notice that the class named DisplayMessage contains the statements that make up our program.


{


{


System.exit(0);

}

}

9



{


{


System.exit(0);

}

}

• The word public is an access specifier that controls where the class can be accessed from. The key word public in the class header specifies that the class is unrestricted (i.e. the class can be accessed from any other class).

10



{


{


System.exit(0);

}

}

• The key word class marks the beginning of the class.• The class name, DisplayMessage, was chosen by the programmer. It is

a user-defined identifier.• The class header does not end with a semicolon. It is only the beginning

of a class definition; it is not a complete statement.

No semicolon here! The class header is only the beginning of the class

definition.

11



{


{


System.exit(0);

}

}

• A file can contain more that one class, but there cannot be more than one public class in a file.

• When a Java file contains a public class, the name of the public class and the filename (without the .java extension) must be the same. The source file containing the class shown above must be named DisplayMessage.java

12



{


{


System.exit(0);

}

}

• The { is an opening brace.

• The } is the closing brace.

• All the statements in a class definition are enclosed in a set of braces {}. We say that the braces delimit the body of the class.

• It is considered good programming style to indent the statements inside a set of braces one level.

13

THE PARTS OF A JAVA PROGRAMMethod Definition





{


{


System.exit(0);

}

}

This is the method header for the method named main.

This is a method

definition.

14


The method definition shown below is from DisplayMessage.java:


{


System.exit(0);

}

• A method is a named block of statements that perform a specific task when executed.

• Every Java application program must have a method called main. Java programs begin executing at the main method.

15


The method definition shown below is from DisplayMessage.java:


{


System.exit(0);

}

• The method header is not a complete statement, so it does not end with a semicolon.

• The statements in a method must be enclosed in a set of braces {}.

• Again, indent the statements inside the braces.

No semicolon here! The method header is only the beginning of the method

definition.

16

THE PARTS OF A JAVA PROGRAMThe println Method of the out Object of the System Class





{


{


System.exit(0);

}

}

This statement tells the computer to display the message “Have a good

semester!” on a line on the computer screen.

17



• A sequence of characters between the pair of quotation marks is called a string literal.

• The quotation marks delimit the string literal, they will not be displayed on the screen when the println method is executed.

18



• The println statement ends with a semicolon. It is a complete Java statement that instructs the computer to display a message on the computer screen.

Notice the semicolon here!

19

THE PARTS OF A JAVA PROGRAMThe exit Method of the System Class





{


{


System.exit(0);

}

}

This statement tells the computer to end the

execution of the process/program.

20

THE PARTS OF A JAVA PROGRAMThe exit Method of the System Class

System.exit(0);

• The System classes exit method ends the execution of a process/program.

• The use of the exit method is optional here. When we display the output of our program in a dialog box, it will be necessary to use this method.

21

THE print AND println METHODS OF THE System.out OBJECT AND THE JAVA API

• Many of the programs we write will display console output. Console output is plain text.

• The standard output device is a console window.

• We will send strings of text to a console window using an object from the Java API.

• The Java API (Application Programmer Interface) is a standard library of prewritten classes available to Java programs for performing common operations.

22


The program named DisplayMessage.java contains the statement:


• This statement uses the System class from the Java API.

• The System class contains objects and methods that perform system level tasks.

• The out object is a member of the System class that contains the methods named print and println. These methods perform the task of writing strings of characters in a console window on the computer screen.

23



• The beginning of the line above is read System dot out dot print line.

• The dot is the membership operator in Java. It is used to specify that println is a member method of the out object which is a member of the System class.

*** See Figure 2-3 from the text

Membership operators

24



• We place the value that is to be displayed on the computer screen inside parentheses to pass it to the println method.

• A value passed to a method is called an argument.

• The argument passed to the println method in this example is the string literal "Have a good semester!".

25


• The println method displays a stream of characters and then advances the cursor to the beginning of the next line.

• The print method displays a stream of characters, but does not advance the cursor to the next line.

26


***See GettysburgA.java on webct

27


***See GettysburgB.java on webct

28


• Notice the difference in the output produced by GettysburgA.java and GettysburgB.java.

• The characters sent to the console are displayed in a continuous stream.

• A subsequent insertion begins where the previous insertion left off.

• Even if the output is broken into several print statements, one on each line, the output is displayed as one long continuous stream, unless we specify otherwise.

• Spaces we want included in the output must be included in the literals we send to the method.

29


***See GettysburgC.java on webct

30


• It is possible to mix the use of the print and println methods to get the desired output.

31


***See GettysburgD.java on webct

32

THE print AND println METHODS OF THE System.out OBJECT AND THE JAVA API Escape Sequences• Another way to tell the computer to move the cursor to the

beginning of a new line is to embed the newline escape sequence in a string literal at the point you want the new line to begin.

• The newline escape sequence is \n.

• In general, an escape sequence is a backslash, \, followed by one or more characters.

• Escape sequences are used to control the way output is displayed.

33

THE print AND println METHODS OF THE System.out OBJECT AND THE JAVA API Escape Sequences

Causes a double quotation mark to be printeddouble quote\”

Causes a single quotation mark to be printedsingle quote\’

Causes a backslash to be printedbackslash\\

Causes the cursor to go to the beginning of the current line, notthe next line

carriage return\r

Causes the cursor to back up, or move left, one positionbackspace\b

Causes the cursor to skip over to the next tab stoptab\t

Advances the cursor to the next line for subsequent printingnewline\n

34


***See GettysburgE.java on webct

35

VARIABLES

• A variable is a named location in memory that can store a data value that may be changed as the program executes.

• Part of the job of programming is determining what values need to be stored. The values stored in variables are often the inputs of the program and the results of calculations/processing.

36

VARIABLES

• In Java, variables must be declared before they are used. You declare a variable by writing a statement called a variable declaration.

• A variable declaration tells the compiler that storage is needed for a value of a particular data type.

• A variable declaration consists of the data type of the variable (the kind of data it can store) followed by the name of the variable.

• A variable declaration ends with a semicolon.

37

VARIABLES

The statement below declares a variable named radius that can be used to store a double value. Variables of type double can store real numbers (numbers that may include a decimal point and a fractional portion).

double radius;

The declaration begins with the

data type. The key word double is a data type for real

numbers.

38

VARIABLES

*** See the program AreaOf2Circles.java on webct

39

VARIABLES

• You may declare several variables of the same data type in a single statement by separating the variable names with commas.

40

VARIABLES

The statement below declares four separate variables of data type int. Integer variables like these can store whole numbers.

int payments, month, day, year;

The statement above is equivalent to the group of statements below:

int payments;

int month;

int day;

int year;

41

VARIABLES

In AreaOf2Circles.java we have the declarations:

double radius;

double area;

These statements could be replaced with the following statement:

double radius, area;

42

VARIABLES

• Values may be assigned to variables using an assignment statement.

• An assignment copies the value of the expression on the right of the assignment operator into the location corresponding to the variable that is on the left of the assignment operator.

43

VARIABLES

Given the declaration:

double radius;

We can assign the value 3.1 to the variable named radius withthe following statement:

radius = 3.1;

Note: The variable must be on the left side of theassignment operator. 3.1 = radius; is erroneous.

44

VARIABLES

• When a numeric variable is used in an arithmetic expression, the value stored in the memory location corresponding to the variable is used in the calculation.

45

VARIABLES

The statement below is from AreaOf2Circles.java. When this statement is executed 3.14159 is multiplied by the value of radius and then the result of this calculation is multiplied by the value of radius and finally that result is stored in the area variable.

area = 3.14159 * radius * radius;

46

VARIABLES

• When you send the name of a variable to the print or println method the contents of the variable are displayed.

47

VARIABLES

The following statement from AreaOf2Circles.java displays the value in the area variable:

System.out.println(area);

48

VARIABLES

• When using a variable, be sure not to enclose the name of the variable in quotation marks.

• Remember, a sequence of characters enclosed in quotation marks is a string literal, not the name of a variable.

49

VARIABLES

The statement:


From AreaOf2Circles.java, could not be written as:

area = 3.14159 * "radius" * "radius"; // This is a syntax error

The "radius" is a string literal. It is not legal to perform a mathematical operation on a string.

50

THE STRING CONCATENATION OPERATOR (+)

• The + symbol represents two operations:

– The addition operation on numeric values

– The concatenation operation when at least one of its operands is a string

• The concatenation operator creates a new string containing the characters representing both of its operands.

51


In the statement below, the concatenation operator creates the new string "Hello World" which is then passed as the argument to the println method for display in a console window.

System.out.println("Hello " + "World");

Note: A space character was placed inside the quotation marks after the word Hello. If this space was not there the string created by the concatenation operator would be “HelloWorld”.

52


• If only one of the operands of the concatenation operator is a string, the other operand is converted to a string and then the two strings are concatenated.

53


For example, the following two statements are from the program AreaOf2Circles.java:

System.out.print("The area of the first circle is ");

System.out.println(area);

These two statements could be replaced with the single statement:

System.out.println("The area of the first circle is " + area);

The resulting output is the same.

54


In this statement, the numeric value stored in the variable area is converted to a string and this string is concatenated to the string "The area of the first circle is ". If the value stored in area is 12.56636, the new string created is "The area of the first circle is 12.56636". This string is the argument sent to the println method for display.

System.out.println("The area of the first circle is " + area);

55


• In Java, a string literal cannot extend over a line.

The following statement will result in an error:

System.out.println("Four score and seven years ago our fathers brought forth on

this continent, a new nation,\nconceived in liberty, and dedicated to the proposition

that all men are created equal.");

It is an error to extend a string literal across a line.

56


We can use the concatenation operator to combine a number of smaller string literals into a single string.

System.out.println("Four score and seven years ago our fathers brought forth " +

"on this continent, a new nation,\n" +

"conceived in liberty, and dedicated to the proposition that" +

" all men are created equal.");

Notice the style used when extending a statement over a line. Indent the second and

subsequent lines.

57

LITERALS

• A literal is a constant value written in the code of a program.

• Literals are frequently used as:

– The values assigned to variables

– The operands in a arithmetic expressions

– Display values

58

LITERALS

Highlighted below are some of the literals used in the program AreaOf2Circles.java:

radius = 2;


System.out.print("The area of the first circle is ");

59

IDENTIFIERS

• An identifier is a programmer-defined name that corresponds to some element of a program.

• In this class the elements we will give names include:

– Variables and named constants

– The class containing our application program

– Methods

60

IDENTIFIERS Rules for Naming Identifiers

There are some rules that must be followed when creating thenames of variables and other identifiers.

1. Do not use any of the Java key words as identifiers.2. The first character of an identifier must be an a-z, A-Z, an

underscore character (_), or the dollar sign ($).3. After the first character you can use letters a-z or A-Z, digits

0-9, underscores, or dollar signs.4. Spaces are not allowed in identifiers.

61

IDENTIFIERS Conventions for Naming Identifiers

• It is good programming style to give your identifiers meaningful names. This means giving them names that give an indication of their usage.

62

IDENTIFIERS Conventions for Naming Variables

• Write variable names using lowercase letters, capitalizing the first letter of the second and subsequent words that make up the variable name. This helps to visually break up the words since spaces cannot be used.

Example:

double accountBalance, interestRate;

63

IDENTIFIERS Conventions for Naming Classes

• It is standard practice to begin a class name with an uppercase letter and capitalize the first letter of subsequent words that make up the class name.

For example, notice the name of the class in the class header from the application program that calculates and displays the area of two circles:

public class AreaOf2Circles

This is the name of the class that is being

defined.

1 Chapter 2 JAVA FUNDAMENTALS. 2 THE PARTS OF A JAVA PROGRAM // Program by: L. Thompson // A simple...

Documents

Transcript of 1 Chapter 2 JAVA FUNDAMENTALS. 2 THE PARTS OF A JAVA PROGRAM // Program by: L. Thompson // A simple...