Constructors in Java

Constructors : Initializing an Class Object in Java Programming

A constructor in Java is a block of code similar to a method that's called when an instance of an object is created. Here are the key differences between a constructor and a method: 

A constructor doesn't have a return type. 

The name of the constructor must be the same as the name of the class.

Some Rules of Using Constructor :

1. Constructor Initializes an Object.
2. Constructor cannot be called like methods.
3. Constructors are called automatically as soon as object gets created.
4. Constructor don’t have any return Type. (even Void)
5. Constructor name is same as that of “Class Name“.
6. Constructor can accept parameter.

Default Constructor : How Constructor Works ?

class Box {              //class name
  int height;            // variables
  int width;

  Box()                  // constructor same name as class name
  {
  height  = 20;          // initializing variables 
  width = 10;
  }

}

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

  Box b1 = new Box();       // creating object of Box class

  System.out.println("Height of BOX : " + b1.height);
  System.out.println("WIDTH of BOX : " + b1.width);

  }
}

Output :

C:Zubair>java> BoxExample.java

C:Zubair>java BoxExample
Height  of BOX : 20
WIDTH of BOX : 10

Example : Default constructor of a class

Another Example:

Explanation :

1. new Operator will create an object.
2. As soon as Object gets created it will call Constructor-

Box()   //This is Constructor
  {
  height  = 20;
  width = 10;
  }

3. In the above Constructor Instance Variables of Object b1 gets their own values.
4. Thus Constructor Initializes an Object as soon as after creation.
5. It will print Values initialized by Constructor –

System.out.println("Length of Rectangle : " + b1.height);
System.out.println("Breadth of Rectangle : " + b1.width);

Parameterized Constructor: 

An Example of a Box class with a constructor with parameters

//constructor
Box(double w, double h , double d){
width =w; height = h ; depth =d ;

}
// volume() is a function 
double volume(){ return  width * height * depth ;}