Constructor

 Java Constructors

A constructor in Java is a special method that is used to initialize objects. The constructor is called when an object of a class is created. It can be used to set initial values for object attributes:

A constructor initializes an object when it is created. It has the same name as its class and is syntactically similar to a method. However, constructors have no explicit return type.

Typically, you will use a constructor to give initial values to the instance variables defined by the class, or to perform any other start-up procedures required to create a fully formed object.

All classes have constructors, whether you define one or not, because Java automatically provides a default constructor that initializes all member variables to zero. However, once you define your own constructor, the default constructor is no longer used.

Syntax

Following is the syntax of a constructor −

class ClassName {
   ClassName() {
   }
}

Program without using constructors. 

 package com.ritesh;
 class ts{
 private  int n1;
  private  int n2;

    public  int getN1(){
        return  n1;
    }
    public  void setN1(int set1){
        n1=set1;
    }

    public  int getN2(){
        return  n2;
    }
    public  void setN2(int set2){
        n2=set2;
    }

    }
    public class constructor {
    public static void main(String[] args) {
        ts key=new ts();
        key.setN1(3);
        key.setN2(4);
        System.out.println(key.getN1());
        System.out.println(key.getN2());
    }

 }

Program done using constructor..

 package com.ritesh;
 class ts{
    private  int n1;
    private  int n2;

    public  ts(){ //constructor
        n1=20;
        n2=30;
    }

    public  int getN1(){
        return  n1;
    }
    public  void setN1(int set1){
        n1=set1;
    }

    public  int getN2(){
        return  n2;
    }
    public  void setN2(int set2){
        n2=set2;
    }

 }
 public class constructor {
    public static void main(String[] args) {
        ts key=new ts();
        System.out.println(key.getN1());
        System.out.println(key.getN2());
    }

 }

The job of the constructor is to ensure that the new object is in a valid state, usually by giving initial values to the instance variables of the object. So a "constructor" should really be called an "initializer."