classes&methods in java

7/27/2019 Classes&Methods in java

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CLASSES& METHODS


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CLASSESThe general form of a class definition is shown here:

class ClassName {

type instance-variable1;

type instance-variable2;

// ...

type instance-variableN;

type methodName1(parameter-list) {// body of method

}

type methodName2(parameter-list) {

// body of method

}// ...

type methodNameN(parameter-list) {

// body of method

}

}2


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The data, or variables, defined within a

class are calledins tance variab les.

The code is contained within methods.

Collectively, the methods and variables

defined within a class are called

membersof the class. Example:class Vehicle {

int passengers; // number of passengersint fuelcap; // fuel capacity in gallons

int mpg;//fuel consumption in miles per gallon

}

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Creating Object

Vehicle minivan = new Vehicle(); /*create a Vehicle object called minivan*/

minivan.fuelcap= 16; /* Accessing themember of the Vehicle class */

(or)

Vehicle minivan; /* declare reference to

object */minivan = new Vehicle(); /* allocate aVehicle object */

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Examples

// This program creates two Vehicle objects.

class Vehicle {

int passengers;

int fuelcap;

int mpg;}

class VehicleDemo {public static void main(String args[]) {

Vehicle minivan = new Vehicle();

Vehicle sportscar = new Vehicle();

int range1, range2;// assign values to fields in minivan

minivan.passengers = 7;

minivan.fuelcap = 16;

minivan.mpg = 21;5


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// assign values to fields in sportscar

sportscar.passengers = 2;

sportscar.fuelcap = 14;

sportscar.mpg = 12;

// compute the ranges assuming a full tank of gas

range1 = minivan.fuelcap * minivan.mpg;

range2 = sportscar.fuelcap * sportscar.mpg;

System.out.println("Minivan can carry " +

minivan.passengers +" with a range of " +range1);

System.out.println("Sportscar can carry " +sportscar.passengers +

" with a range of " + range2);

}

}

Output:

Minivan can carry 7 with a range of 336

Sportscar can carry 2 with a range of 1686


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Adding a Method// Add range to Vehicle.

class Vehicle {

int passengers; // number of passengers

int fuelcap; // fuel capacity in gallons

int mpg; // fuel consumption in miles per gallon

// Display the range.

void range() {

System.out.println("Range is " + fuelcap * mpg);

}

}

class AddMeth {public static void main(String args[]) {

Vehicle minivan = new Vehicle();

Vehicle sportscar = new Vehicle();

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// assign values to fields in minivan

minivan.passengers = 7;

minivan.fuelcap = 16;

minivan.mpg = 21;

// assign values to fields in sportscar

sportscar.passengers = 2;

sportscar.fuelcap = 14;

sportscar.mpg = 12;System.out.print("Minivan can carry " +

minivan.passengers +". ");

minivan.range(); // display range of minivan

System.out.print("Sportscar can carry " +sportscar.passengers +". ");

sportscar.range(); // display range of sportscar.

}

}

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Constructors

A constructor initializes an objectimmediately upon creation.

It has the same name as the class in which it

resides and is syntactically similar to amethod.

Constructors have no return type, not even

void. The constructors job to initialize the internal

state of an object so that the code creating

an instance will have a fully initialized,

usable object immediately. 9


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Example// Add a constructor.

class Vehicle {

int passengers; // number of passengers

int fuelcap; // fuel capacity in gallons

int mpg; // fuel consumption in miles per gallon

// This is a constructor for Vehicle.

Vehicle(int p, int f, int m) {

passengers = p;

fuelcap = f;

mpg = m;

}// Return the range.

int range() {

return mpg * fuelcap;

}10


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// Compute fuel needed for a given distance.

double fuelneeded(int miles) {

return (double) miles / mpg;

}

}class VehConsDemo {

public static void main(String args[]) {

// construct complete vehicles

Vehicle minivan = new Vehicle(7, 16, 21);

Vehicle sportscar = new Vehicle(2, 14, 12);

double gallons;

int dist = 252;

gallons = minivan.fuelneeded(dist);

System.out.println("To go " + dist + " miles minivanneeds " +gallons + " gallons of fuel.");

gallons = sportscar.fuelneeded(dist);

System.out.println("To go " + dist + " miles sportscarneeds " +gallons + " gallons of fuel.");

}

} 11


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The this Keyword

this can be used inside any method to

refer to the current object.

That is, this is always a reference to

the object on which the method was

invoked.Vehicle(int p, int f, int m) {

this.passengers = p;this.fuelcap = f;

this.mpg = m;

}

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Garbage Collection

In some languages, such as C++,dynamically allocated objects must be

manually released by use of a delete

operator.

Java takes a different approach; it

handles de-allocation for you

automatically. The technique that

accomplishes this is called garbage

col lect ion.

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The finalize( ) Method Sometimes an object will need to

perform some action when it isdestroyed.

To handle such situations, Java provides

a mechanism called f inal izat ion. To add a finalizer to a class, you simply

define the finalize( ) method.

The finalize( ) method has this generalform:protected void finalize( )

{

// finalization code here 14


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Overloading Methods

In Java it is possible to define two ormore methods within the same class

that share the same name, as long as

their parameter declarations aredifferent.

When an overloaded method is invoked,

Java uses the type and/or number ofarguments as its guide to determine

which version of the overloaded method

to actually call.15


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Exampleclass OverloadDemo {

void test() {

System.out.println("No parameters");

}

// Overload test for one integer parameter.

void test(int a) {System.out.println("a: " + a);

}

//Overload test for two integer parameters.

void test(int a, int b) {System.out.println("a and b: "+a+" " + b);

}

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// overload test for a double parameter

double test(double a) {

System.out.println("double a: " + a);

return a*a;

}

}

class Overload {


OverloadDemo ob = new OverloadDemo();double result;

// call all versions of test()

ob.test();

ob.test(10);

ob.test(10, 20);

result = ob.test(123.25);

System.out.println("Result of ob.test(123.25): "+ result);

}

} 17


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Overloading Constructorsclass MyClass {

int x;

MyClass() {

System.out.println("Inside MyClass().");

x = 0;

}

MyClass(int i) {System.out.println("Inside MyClass(int).");

x = i;

}

MyClass(double d) {System.out.println("Inside MyClass(double).");

x = (int) d;

}

MyClass(int i, int j) {

System.out.println("Inside MyClass(int, int)."); 18


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x = i * j;

}

}class OverloadConsDemo {


MyClass t1 = new MyClass();

MyClass t2 = new MyClass(88);

MyClass t3 = new MyClass(17.23);

MyClass t4 = new MyClass(2, 4);

System.out.println("t1.x: " + t1.x);




}

}

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P i Obj


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Passing Objects as

Parameters

// Objects may be passed to methods.class Test {

int a, b;

Test(int i, int j) {

a = i;

b = j;

}

/* return true if o is equal to the invokingobject */

boolean equals(Test o) {if(o.a == a && o.b == b) return true;

else return false;

}

} 20


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class PassOb {public static void main(String args[]) {

Test ob1 = new Test(100, 22);

Test ob2 = new Test(100, 22);

Test ob3 = new Test(-1, -1);System.out.println("ob1==ob2:"+ob1.equals(ob2));

System.out.println("ob1==ob3:"+ob1.equals(ob3));

}

}

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Returning Objects

// Returning an object.

class Test {

int a;

Test(int i) {

a = i;

}

Test incrByTen() {

Test temp = new Test(a+10);

return temp;}

}

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class RetOb {public static void main(String args[]) {

Test ob1 = new Test(2);

Test ob2;

ob2 = ob1.incrByTen();System.out.println("ob1.a: " + ob1.a);

System.out.println("ob2.a: " + ob2.a);

ob2 = ob2.incrByTen();

System.out.println("ob2.a after second increase:"+ob2.a);

}

}

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Access Specifiers

/* This program demonstrates the difference

between public and private. */class Test {

int a; // default access

public int b; // public access

private int c; // private access// methods to access c

void setc(int i) { // set c's value

c = i;

}

int getc() { // get c's value

return c;

}

}

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class AccessTest {


Test ob = new Test();

/* These are OK, a and b may be accesseddirectly */

ob.a = 10;

ob.b = 20;// This is not OK and will cause an error

// ob.c = 100; // Error!

// You must access c through its methods

ob.setc(100); // OKSystem.out.println("a, b, and c: " + ob.a + " "+ ob.b + " " + ob.getc());

}

}

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static Keywordclass StaticDemo {

int x; // a normal instance variablestatic int y; // a static variable

}

class SDemo {

public static void main(String args[]) {StaticDemo ob1 = new StaticDemo();

StaticDemo ob2 = new StaticDemo();

ob1.x = 10;

ob2.x = 20;

System.out.println("Of course, ob1.x and ob2.x " +

"are independent.");

System.out.println("ob1.x: " + ob1.x +

"\nob2.x: " + ob2.x);

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/* Each object shares one copy of

a static variable. */

System.out.println("The static variable y isshared.");

ob1.y = 19;

System.out.println("ob1.y: " + ob1.y + "\nob2.y:" + ob2.y);

System.out.println("The static variable y can be"+ " accessed through its class.");

StaticDemo.y = 11; /* Can refer to y throughclass name */

System.out.println("StaticDemo.y: " +

StaticDemo.y + "\nob1.y: " + ob1.y + "\nob2.y: "+ ob2.y);

}

}

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static Block

// Use a static blockclass StaticBlock {

static double rootOf2;

static double rootOf3;

static {System.out.println("Inside static block.");

rootOf2 = Math.sqrt(2.0);

rootOf3 = Math.sqrt(3.0);

}

StaticBlock(String msg) {

System.out.println(msg);

}

}

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class SBDemo {public static void main(String args[]) {

StaticBlock ob = new staticBlock("InsideConstructor");

System.out.println("Square root of 2 is "+ StaticBlock.rootOf2);

System.out.println("Square root of 3 is "+ StaticBlock.rootOf3);

}

}

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final Variables

It prevents modifying the value.

It must be initialized.

Choose all uppercase identifiers.

Do not occupy memory on a per-instance basis.

The keyword final can also be applied

to methods.final int FILE_NEW = 1;

final int FILE_OPEN = 2;

final int FILE_SAVE = 3;

final int FILE_SAVEAS = 4;

final int FILE_QUIT = 5; 30


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Array Variables

Declaration type var-name[ ]; //E.g: int arr[ ];

Memory Allocation

array-var= new type[size]; // arr=new

int[5];

Types:

Single and Multi-Dimensional Array

Jagged Array- Different column sized

array

Alternative Array Declaration

int a[ ] = new int[5]; 31


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class JArray {public static void main(Stringargs[]) {

int twoD[][] = new int[4][];

twoD[0] = new int[1];




int i, j, k = 0;

for(i=0; i


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twoD[i][j] = k;k++;

}

for(i=0; i


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Example-2

// To find the length of array member.class Length {


int a1[] = new int[10];

int a2[] = {3, 5, 7, 1, 8, 99, 44, -10};int a3[] = {4, 3, 2, 1};

System.out.println("length of a1 is " + a1.length);



}

}

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Nested and Inner Classes

It is possible to define a class within another

class; such classes are known as nestedclasses.

There are two types of nested classes: static

and non-static.A static nested class is one which has the static

modifier applied.

An inner class is a non-static nested class.

It has access to all of the variables and methods

of its outer class and may refer to them directly

in the same way that other non-static members

of the outer class do 35

class Outer {


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int outer_x = 100;

void test() {

Inner inner = new Inner();

inner.display();

}

// this is an inner class

class Inner {

void display() {

System.out.println("display: outer_x = " + outer_x);}

}

}

class InnerClassDemo {

public static void main(String args[]) {Outer outer = new Outer();

outer.test();

}

}

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String Class

Stringis probably the most commonly

used class in Javas class library.

Every string that created is actually an

object of type String.

Even string constants are actually

Stringobjects.String myString = "this is a test";

Java defines one operator for String

objects: +.

String myString = "I" + " like " + "Java."; 37


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Example

class StrDemo {


String strOb1 = "First String";

String strOb2 = "Second String";String strOb3 = strOb1+" and "+strOb2;

System.out.println(strOb1);



}

}

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Some methods in String Class

boolean equals(String object)

Compares two given string objects.

int length( )Gives the length of the

given string. char charAt(int index)Gives the

char at the given index.

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class StringDemo2 {


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String strOb1 = "First String";

String strOb2 = "Second String";

String strOb3 = strOb1;

System.out.println("Length of strOb1: " +

strOb1.length());

System.out.println("Char at index 3 instrOb1:" + strOb1.charAt(3));

if(strOb1.equals(strOb2))

System.out.println("strOb1 == strOb2");

else

System.out.println("strOb1 != strOb2");

if(strOb1.equals(strOb3))

System.out.println("strOb1 == strOb3");else

System.out.println("strOb1 != strOb3");

}

}

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Using Command Line Arguments


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Using Command-Line Arguments

A command-line argument is the information that

directly follows the programs name on the commandline when it is executed.

They are stored as strings in the String array passed tomain( ).

Example:

class CommandLine {public static void main(String args[]) {

for(int i=0; i


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Example

class CLDemo{public static void main(String arg[]){

int a[]=new int[arg.length];

int sum=0;

for(int i=0;i

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