"Rohit Gupta" wrote...

Only partially...

You ask "how" to do a specific thing technically, without telling us *why*
you want to do it. Bi-directional coupling is mostly a bad idea...

As an aside, newbies are usually better off in the group
comp.lang.java.help...

E.g.:

1. Let the instance of B get a reference
  to the instance of A

This can be accomplished by e.g. providing the reference at instantiation of
the B object, or by providing it at a later stage, depending on how the
sequences of messages will go further on in the application.

2. Let there be a mutator (e.g. a public set-method)
  for 'x' in the A class.

/// Bjorn A

I think you already know that to change the value of x you simply assign it,
as in

   x = ...

And because x is private to class A, the assignment must be done within one
of A's methods.  Either A performs this operation itself or some other code
calls a method of A on an instance of A to do it.  For some other code to
invoke the method, it must have a reference to an instance of A.

So the real questions are, what are you trying to assign to x? from where is
the assignment being made? does the source have a reference to an instance
of A?

Cheers,
Matt Humphrey matth@ivizNOSPAM.com  http://www.iviz.com/

As Bjorn notes, bi-directional coupling is not a good idea. You can
still get much the same result, but do it in a much better way, by using
the idea of a 'listener'.
In other words, class A creates class B. Class B offers a way for
interested parties to listen out for a certain something happening - A
registers itself as an interested party... then, when the interesting
thing happens (like some sort of change in B), class A gets notified,
and can inquire of B what state it is in.
If you want more details of this, google for "listener pattern" or
"subscriber/observer".

Telling us your requirement in more detail wouldn't hurt! Even if you
provided an example scenario, that would allow us to communicate more
clearly...