Hi Sonali,
When argument is of Object type, a copy of reference(on stack) get created which points to same object(on heap) and which can modify that object as it points to same mem. allocation.
e.g.
Obj o=new Obj();
o.somevar=23;
S.o.p(o.somevar); //will print the 23
method(o);
S.o.p(o.somevar); //will print the 12
//somewhere in class
void method(Object o)
{
o.somevar=12;
}
I Hope this will suffice.....
-Sachin!!!