How to make an object mutable?

Question

Can any one tell me how to make an object mutable?
Following is Class X & Y?
class Y
public static void main(String arg[]) {
X a1=new X();
Object a=a1.get();
System.out.println(a.toString());
a1.set(a);
System.out.println(a.toString());
class X implements Serializable
public Object get(){
return new Object();
public synchronized void set(Object o)
o=null;
In my class Y when i say
a1.set(a);
I want local Object a of main method should be nullified.
Can it be possible if yes what is the way or code to be applied so that
my next a.toString() statement will give me NullpointerException.

Answer 1

Isn't it more accurate to say that object references are passed by value?
OP -- Basically you can't to what you want to do. When you "pass an object" as a method parameter, what you're really passing is a copy of the reference that points to the object. You now have two refs pointing to the same object--one in the caller and one in the method being executed. Setting either of those refs to null does NOT affect the object itself, and does NOT affect the other ref. It just means that the ref that's been set to null no longer points to any object.
If you want the called method to make a change that the caller can see, you need to either 1) return a value from the method, 2) encapsulate the object to be changed as a member of new class, or 3) pass the object to be changed as the single element of an array. I would STRONGLY recommend against (3) as a way to simulate pass by reference. Better to examine your design and determine whether (1) or (2) more closely matches what you're really trying to accomplish.