Non-static nested class in java

Question

I expect the output of the below program to be:

Inside Static Block A
Inside A
Inside Constructor B

But the output is:

Inside Static Block A
Inside A
Inside A
Inside A
.
.
.
Inside A(Infinite times)

The code is:

public class First
{
    public static void main(String args[])
    {
        A op=new A();
    }
}
class A
{
    private int a=100;
    private int b;
    A()
    {
        System.out.print("Inside A");
        B obj=new B();
    }
    static
    {
        System.out.print("Inside Static Block A");
    }
    class B
    {
        B()
        {
            System.out.print("Inside Constructor B");
        }
        A o=new A();
    }
}

Can someone please tell me why the output is so?

B obj=new B(); and A o=new A(); - what do you think this does? — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 28, 2015 at 19:33

Kai Sternad · Accepted Answer · 2015-06-28 19:41:30Z

3

You have a circular reference between B and A. In the constructor of A you create a B and in B you create a field with an A.

In the constructor of A() you do:

B obj = new B();

so, you create a new B().

In B() you create a field with an A(). This creates another instance of A() which again creates a B() in its constructor, and so forth.

edited Jun 28, 2015 at 19:41

answered Jun 28, 2015 at 19:33

Kai Sternad

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6 Comments

Frosted Cupcake Over a year ago

Can you please elaborate? I have called the constructor A after constructor B block,then how it is able to call it ...

Frosted Cupcake Over a year ago

No @Kai Sternad, I have called the constructor A from outside the constructor B block

Andy Turner Over a year ago

@Raj assigning a value to a field when you declare it is effectively like code which is prepended to the constructor. You are still invoking it whenever you create the instance of B.

Kai Sternad Over a year ago

@RajMalhotra the constructor of B doesn't just return because the fields are also initialized. And that's where the problem is.

Andy Turner Over a year ago

@Raj field assignments happen before the constructor is called, in order that you could potentially use the field values in the ctor.

|

Sneh · Accepted Answer · 2015-06-28 19:43:36Z

1

It is due to the fact that you have a circular reference between Class A and Class B. In your constructor of A you, create an object B

 A()
 {
   System.out.print("Inside A");
    B obj=new B(); // You are creating a new object of Class B
 }

In Class B you are creating a new object of class A

 class B
 {
     B()
     {
        System.out.print("Inside Constructor B");
     }
     A o=new A(); 
 }

Here you create an object of class A which again calls the constructor of Class A which creates an object of Class B again and hence the whole process goes into an infinite loop

answered Jun 28, 2015 at 19:43

Sneh

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Comments

Pshemo · Accepted Answer · 2015-06-28 19:54:13Z

Take a look at

A() {
    System.out.print("Inside A");
    B obj = new B();
}

So to create instance of class A you need to create instance of class B.

But code of class B is

class B {
    B() {
        System.out.print("Inside Constructor B");
    }

    A o = new A();
}

which (since initialization of fields happens at start of each constructors) is similar to as

class B {
    B() {
        o = new A();
        System.out.print("Inside Constructor B");
    }

    A o = null;
}

so as you see it also needs to create instance of class A.

So when you call

public static void main(String args[])
{
    A op=new A();
}

new A() first loads A class which executes

static {
    System.out.print("Inside Static Block A");
}

and then tries to execute constructor A(){

System.out.print("Inside A");
B obj = new B();

but before it ends its execution it calls new B() which calls code from constructor B(){ which as already shown is

o = new A();
System.out.print("Inside Constructor B");

and we and up with having to create another instance of A even before we are able to print Inside Constructor B.

So as you see to create instance of class A you need to create instance of class B which needs to create another instance of class A which needs to create another instance of class B ... and so on until stack will overflow.

Roman C · Accepted Answer · 2015-06-28 19:38:36Z

0

The point is not accessible due recursive calls.

B()
{
    System.out.println("Inside Constructor B");
    A o=new A();
}

A o=new A();

This code should return desired output:

Inside Static Block A
Inside A
Inside Constructor B

answered Jun 28, 2015 at 19:38

Roman C

1

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