What are the advantages when we do like this?

[bilal haider]

package mypack;

public class A {
private B bObj;
public A() {
// TODO Auto-generated constructor stub
bObj=new B();
}
}

package mypack;

import java.util.HashSet;
import java.util.Set;

public class B {
private Set<A> aLists;

public B() {
aLists = new HashSet<A>();
}
}

[Mohamed Sanaulla]

Bilal, Welcome to JavaRanch

Please TellTheDetails - And what is that you want to do? Also any exceptions you have with the code?

[bilal haider]

A is contating an object of b which is containing a hashset of A.
There is no error or exception ... I just want to know that what are the advantages of this kind of containment?

public static void main(String[] args) {
	// TODO Auto-generated method stub

	try {

		A a=new A();
		B b=new B();
		a.setBObj(b);


	} catch (Exception e) {

		System.out.println(e.getMessage());
	}
}

[Wouter Oet]

Let's say you have this:

public class Group {
   private Set<Child> children = new HashSet<Child>();

   // ...
}

public class Child {
   private Group isInGroup;

   // ...
}

Can you see now what it is used for?

[bilal haider]

Thank you very much for the quick response, and moving me one step ahead.
What I understand is that Group class is containing distinct child objects.

But why we are containing group object in child object?

[Paul Sturrock]

Its is so you can see an association from both sides. From the Group instance you know what Child object the Group contains. From the Child object, you know what Group this particular Child object belongs to.