Jan. 20041 Java Basics - II Yangjun Chen Dept. Business Computing University of Winnipeg.

Jan. 2004 1

Java Basics - II

Yangjun Chen

Dept. Business ComputingUniversity of Winnipeg

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Outline: Java Basics - IIOutline: Java Basics - II

• Modifiers (Specifiers)• Statements• Array• Control flow

- condition statements:ifswitch

- loop structure:forwhiledo

• others: String, print, new, constructor

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ModifiersModifiers

• Modifiers are special keywords that modify the definition of a class, method, or variables.

modifiers

Modifiersfor methods

Modifiersfor variables

Modifiersfor classes

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Modifiers for Methods and VariablesModifiers for Methods and Variables

• static• final• private• protected• public

access modifiers

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• Static and final Modifiers- The static keyword is used in the declaration of class variables and

class methods.- At times we want to associate a variable or a method with the class

rather than with every instance (every object) of the class.- Within a class, a static variable can be accessed by referring to its

name. - From outside of the class, a static variable can be visited using the dot notation:

className.variableName.- A static method can be visited from outside of the class using the dot notation: className.methodName.

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Class Circle {static double PI=3.141592635;double radius;double circumference;double diameter;… …

}

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• Here, PI shouldn’t be a variable. (It is just a constant.)• To make it not changeable, we add the keyword final to the

declaration. Class Circle {

final static double PI=3.141592635;… …

}- Any attempt to modify PI now will generate an error.- Final modifier can not be used for methods.

• Access modifiers allow you to control the visibility and access to variables and methods inside your class.

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• Difference between instance and class variablesClass Circle {

static double PI=3.141592635;double radius = 2.0;double circumference = 12.56;double diameter =4.0;… …

}

Class ComputationCircles {double x, y, z, w;double computation() {x = Circle.PI;Circle o = new Circle();y = o.radius; z = o.circumference; w = o.diameter;… …}

}

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• Difference between instance and class methodsClass Circle {

… ...static double m1( ) { … }double m2 ( ) { … }

}

Class ComputationCircles {double x, y, z, w;double computation() {… ...x = Circle.m1();Circle o = new Circle();y = o.m2();… …}

}

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• There are three access modifiers in Java:- private

protectedpublic

• private Access Modifier- The private modifier creates a non-accessible member.- A private method or variable restricts access only to those methods in the same class.- The private identifiers are unknown to other classes even if the extend (subclass) from the class.

- To declare a member private, simply put the keyword private in front of its declaration.

private int number;

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• protected Access Modifier- The protected modifier allows access to a member within the same

package and within any subclasses.- We won’t study packages at this point. (They are basically a collection of classes that you can define yourself and used as a library.)- You can declare an identifier protected by placing the protected

keyword in front of its declaration.

protected int number;• public Access Modifier

- The public modifier allows access to a member inside or outside of the class.

public int number;

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Modifiers for ClassesModifiers for Classes

• There are three modifiers for classes in Java.- public

This makes a class accessible to outside of the package.- final

This prevents a class from being extended.- abstract

In an abstract class, some abstract methods (with only method signature; no implementation) are defined. Therefore, an abstract

class can not be instantiated.

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StatementsStatements

• There are several types of statements:- Declaration statements, expression statements, return statements and compound statements.

• Declaration Statements- declare new variables by specifying the type, variable name, properties and optionally initialize the variable. - some example are:

Button clear;

private Point sp=new Point(45, 15), ep;final int Max=100000;int almostMax=Max - 500;

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• Expression Statements- Expression statements cause the expression to be evaluated and then any other actions in the statement are executed. - some example are:

clear = new Button(“Clear”);

i++;p = q;

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• Return Statements- statements in a method with non-void return types that return some

information back to the location where the method was called.- A return statement is of the following form:

return (expression);- The expression can be any expression that is of the same type as the header declaration of the method.- When a return is encountered, it forces the immediate exit from the

method.

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- Example:

double magnitude(double x, double y){

double sqrSum = x*x + y*y;return Math.sqrt(sqrSum);

}• Compound Statements

- a collection of statements enclosed in curly braces.- Example:

{Integer obj = new Integer(8);System.out.println(obj.value);

}

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ArrayArray

• An array is a collection of items.• Each array has some number of slots, each of which can hold

an individual item.- An item can be any object or primitive variable; but an array can be only of one type of items;- You can’t have an array with different types in it.

• Steps to create an array:- 1. Declare a variable to hold the array

int temp[];- 2. Create new array object and assign it:

temp = new int[10];- 3. Store things in the array

temp[0] = 6;

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ArrayArray

• Note that the array index starts from zero, so if the array is of size 10, the index values will run from 0 to 9.

• It is also possible to initialize the array during declaration:- int temp[] = {1, 2, 3, 4, 5};

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Multidimentional ArrayMultidimentional Array

• This is just an array of arrays and is declared in the same manner as regular arrays.- Example: int temp[][] = new int[10][5];- This will create an array that has 10 rows and 5 columns.- This will allow us to store 50 values in temp.- You would access each item of the array by specifying both indices,

for example,temp[6][3] = 12;This will store the value 12 into location with row index 6 and column index 3.

- int temp[][] = {{33, 71}, {-16, 45}, {99, 27}};

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if Statementif Statement

• The if statement- written as:

if (boolean expression)statement; //controlled statement

- If the boolean expression evaluates true, then the controlled statement is executed. If it is false, then the controlled statement is skipped.- The controlled statement can also be a compound statement.

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if Statementif Statement

• The if … else statement- This is another form of the if statement.

- It allows some code to be executed if the boolean expression is

true and some other code if false.

- written as:

if (boolean expression)statement1 //expression is true

elsestatement2; //expression is false

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switch Statementswitch Statement

• The switch statement- Instead of chaining if …else statements, a switch statement can be

used for testing equality.

- This is like a case statement in some other languages.

- The expression must evaluate to a primitive type and the constants

must be literals of final variables.

- The defaul case is evaluated if the expression fails to match any of

the constants.

(The default may be omitted, but it is a good idea to have it

here.)

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Switch (expression) {case constant1:

some statements;

case constant2:some statements;

…default:

some statements:} //end of switch

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• Whenever a label (case constant:) is matched, execution of the program continues from there through the switch statement executing each line. - This is the fall through behavior.

• To avoid this, a statement is used.

- A break statement will force the exit from a switch statement

- In almost all cases, you would write a switch statement with a

break statement separating each case statement.

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Switch (expression) {case constant1:

some statements;break;

case constant2:some statements; break;

…default:

some statements:} //end of switch

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for Statementfor Statement

• The for loop in Java looks as follows:for ( init ; expression ; increment) {

some statements;} //end of the for loop

• The init is an expression that will introduce the start of the for loop.- Example: int i = 0;- declares and initializes the index i for this loop.

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• The expression is a boolean expression that is evaluated before each pass of the loop.- If it is true, then the loop is executed.- Example: i < 10;

• The increment expression is any expression of function call.• The increment is commonly used to bring the state of the loop

closer to returning false and completing.- Example: int i ++;

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• So the for loop looks like this

for (int i = 0; i < 10; i ++){

System.out.println(i);} //end of the for loop

• What is the output?

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while Statementwhile Statement

• while loops like for loops will repeat the execution of a block of code until a specific condition is met.

• A while loop looks like the following:

while (boolean expression){

statements

} //end of the while loop

• As long as the boolean expression evaluates true, the code in the loop is executed.

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do Statementdo Statement

• Another loop that is quite similar to the while loop is called the do loop.

• A do loop takes the form:

do {statements

} while (boolean expression)• The difference between this loop and the while loop is that the

condition is tested at the end of the loop.- do loop always execute the loop at lease once.

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Loop ConstructsLoop Constructs

• For all these loops, you can use the break statement to exit the loop at any time.

• There is also a continue statemment that forces the loop to start over at the next iteration.

Label1:outer-iteration {

inner-iteration {…

break; //break out of the inner iteration…

continue; //move back to the beginning of the inner iteration

continue label1; //move all the way back to Lable1

break lable1; //break out of both iterations}}

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int i = 0;label:for (int j = 0; j < 10; j++) {

while (i < 10) {System.out.println(i);break; i++;

}}

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label:for (int j = 0; j < 10; j++) {

int i = 0;while (i < 10) {

System.out.println(i);continue; i++;

}}

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int k=0;label:for (int j = k; j < 10; j++) {

int i = 0;while (i < 10) {

System.out.println(i);k++;continue label;

}}

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label:for (int j = 0; j < 10; j++) {

int i = 0;while (i < 10) {

System.out.println(i);break label;i++;

}}

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big_loop: while (!done) {if (test(a, b) == 0) continue; //Control goes to point 2

try {for (int j = 0; j < 10; j++) {

if (a[j] == null)continue; //Control goes to point 1

else if b[j] == null)continue big_loop; //Control goes to point 2

doit(a[j], b[j]);//point 1. Increment and start loop again with the test

}}finally {cleanup(a, b);}//point2. Start loop again with the (!done) test;

}

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break_test: if (check(i)) {try {

for (int j =0; j<10; j++) {if (j > i ) break; //Terminate just this loop.if (a[i][j] == null)

break break_test; //Do the finally clause and

//terminate the if statement.

}}

finally {cleanup(a, i, j)}}

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StringsStrings

• Strings are a combination of characters and instances of the class String in Java.- They are not simply an array of characters.

• String literals are a series of characters enclosed in quotation marks. A string declaration would look like:- String myString=“Brand new String”;

• Unlike many other language strings can not be indexed character by character.- There are methods in the String class that will retrieve characters for you.

String s = “This is a string!”;char char_a = s.charAt[0];

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Print StatementsPrint Statements

• The System.out.println method prints massage out to the standard output of your system - to the screen, to a special window, or maybe to a log file.

• It also prints a new line character at the end of a string.• System.err.println() prints on standard err instead.• You can concatenate arguments to println() using the +

operator.• Example:

- System.out.println(“Hello” + “World!”);

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Print StatementsPrint Statements

• Using print() instead of println() does not break the line.• Example:

System.out.print(“Hello”);System.out.print(“World!”);System.out.println();

• This will yield the same output as the previous example.• System.out.println() breaks the line and flushes the output.

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new Operatornew Operator

• The new operator is used for creating new instance of classes or objects.

• To create a new object, use the new operator with the name of the class you want to create an instance of, then parentheses after that.

String str = new String();Random r = new Random();Triple origin = new Triple();

- The parentheses need not be empty; they can contain arguments that determine the initial values of instance variables or other properties.- Example:

Point pt = new Point(0, 0);

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new Operatornew Operator

• The number and type of arguments that can be used in the parentheses with new are defined by the class’ constructor method.- Example:

Date d1 = new Date();Date d2 = new Date(71, 7, 1, 7, 30);

Date d3 = new Date(“April 3 1993 3:24 PM”);

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Class Date {… …Date ( ) { … }Date (int x, int y, int z, int u, int v) { … }Date (String s) { … }… ...

}

ConstructorsConstructors

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• In the above example, we have shown three different constructors for the Date class. This is called method overloading or in this case constructor overloading.

• Overloading is when you have methods with the same name but with different arguments. The compiler will distinguish between the methods by different arguments- different arguments mean

different number of arguments ordifferent type of arguments

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• Constructors are special methods that are used to initialize new object, set its variables, and perform any other functions the object needs to initialize itself.

• These methods are called by Java automatically when you create a new object.

• Constructors always have the same name as class with no return type.

• For example, say we had a class Circle and always wanted the center to start at (100, 100).

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Class Circle {int x, y;Circle( ) {

x = 100;y = 100;

}…

}• So every time we create an object of class Circle, the

constructor will be called automatically setting x and y to 100, respectively.

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Backslash codes:

Code Meaning code Meaning\b Backspace \N Octal constant\n Newline (where N is 0 to 7)\r Carriage return \NN Octal constant\f from feed (where N is 0 to 7)\t Horizental tab \MNN Octal constant\” Double quote (where M is 0 to 3 and\’ Single-quote character N is 0 to 7)\0 Null \uxxxx Unicode character\\ Backslash (where xxxx are four

hexadecimal constants)

Jan. 20041 Java Basics - II Yangjun Chen Dept. Business Computing University of Winnipeg.

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Transcript of Jan. 20041 Java Basics - II Yangjun Chen Dept. Business Computing University of Winnipeg.