Java - Declaring Objects

Declaring Objects

Obtaining objects of a class is a two-step process. First, you must declare a variable of the class type. This variable does not define an object. Instead, it is simply a variable that can refer to an object. Second, you must acquire an actual, physical copy of the object and assign it to that variable. You can do this using the new operator. The new operator dynamically allocates (that is, allocates at run time) memory for an object and returns a reference to it.

First Method:

Box mybox = new Box();

Second Method:

Box mybox; // declare reference to object mybox = new Box(); // allocate a Box object

The first line declares mybox as a reference to an object of type Box. After this line executes, mybox contains the value null, which indicates that it does not yet point to an actual object. Any attempt to use mybox at this point will result in a compile-time error. The next line allocates an actual object and assigns a reference to it to mybox. After the second line executes, you can use mybox as if it were a Box object. But in reality, mybox simply holds the memory address of the actual Box object.

Constructor:

A constructor initializes an object immediately upon creation. It has the same name as the class in which it resides and is syntactically similar to a method. Once defined, the constructor is automatically called immediately after the object is created, before the new operator completes. Constructors look a little strange because they have no return type, not even void. This is because the implicit return type of a class’ constructor is the class type itself.

Example for Constructor:

class rect{

int Ilength,Ibreadth;

// Here rect() is an Constructor rect(){

// here the member variables are initialized Ilength=10;

Ibreadth=20;

}

int fun_area()

{

return Ilength*Ibreadth;

}

}

public class cons {

public static void main(String args[])

{

rect op=new rect();

int Iarea;

Iarea=op.fun_area();

System.out.println("Area of rectangle is" + Iarea);

}

}

Parameterized Constructors:

We can pass the initialization values to the constructor.it is know as parameterized constructor.

import java.util.Scanner;

class cube{

int ISide; cube(int x){

// this is constructor ISide=x;

}

int fun_volume()

{

return ISide*ISide*ISide;

}

}

public class paramconst {

public static void main(String args[])

{

Scanner sr= new Scanner(System.in); System.out.println("Enter the side value"); int side=sr.nextInt();

// Parameterised Constructor cube op=new cube(side);

int IVolume; IVolume=op.fun_volume();

System.out.println("Volume of Cube is" + IVolume );

}

}