Constructors in Java

Constructors are used every time to initialize instance variables. There are some additional rules associated with constructors that are often asked in interviews.Hence revising those here through a blog post.

1.A constructor is used to initialize instance variables.

2.When an object of an class is created,JVM goes to the class and searches for that matching constructor.

If Constructor is NOT PRESENT it gives a compile error.

3.By default every class has a constructor called default no argument constructor

class A{
A(){ //default no arg constructor 
}}

4.A programmer can have multiple constructors in a class provided their signatures are different this is called constructor overloading.

class A{
A(){
//some code
}

A(int x){
//some code
}

A(float x){
//some code
}

A(float x,int y){
//some code
}
A(int x,float y){
}
A(int z){}//THIS WILL GIVE COMPILE ERROR SInce its already defined on top.

}

A a=new A();
new A();//goes to first matching constructor

5.JVM always calls the matching constructor from the class.HOWEVER,a programmer can call other constructors of this class by using the the this() method

class A{
A(){
System.out.println("A");    //I 
A(int x){
this();                     //this will go to constructor A();
System.out.println("AA");   //II
}
}
class App{
public static void main(String[]args){
new A(5);
}}

OUTPUT:
A
AA

6.If a programmer desires it can call the constructor of the super class as well from its own constructor using the super() method.

class A{
A(){
System.out.println("A");    //I 
}
}
class B extends A{
B(){
super();             //this is called implicitly refer next point also 
System.out.println("B");
}}


class HelloWorld {
    public static void main(String[] args) {
        new B();
    }
}
OUTPUT:
A
B

7.Whenever a programmer creates a constructor ,JVM writes super() in every constructor implicitly as its first line.

Note:If a class does not extend any class it by default extends the Object class.
Do Try this code in your ide to see it for yourself

class A{
A(){
//super will be called implicitly at the first line of this constructor and here since it does not extend any class it will extend the Object class
System.out.println("A");    //I 
}

A(int x){
//super will be called implicitly at the first line of this constructor 
System.out.println("AA");
}}


class HelloWorld {
    public static void main(String[] args) {
        new A(5);
    }
}
OUTPUT:
A
AA