Question

What happen at background(in case of memory) when I declare variable and then create object for that variable . Is reference variable store anywhere and in which format and how this variable points to the memory on heap. Please clarify below doubts in the comments.

For example

ClassA instance;       // Where this variable store and how much memory occupies
instance=new ClassA(); //How instance variable points to memory

EDIT

What will effect on my program memory if my program contains so many unused variable.

Answer 1

The instance variable is just a pointer at runtime, it points to the object allocated in the GC heap. The variable can live anywhere, stack, CPU register, inside another object that’s on the heap or in the loader heap if it is static.

The big deal about the garbage collector is that it is capable of finding this pointer during a garbage collection. And can thus see that the object is still referenced and can adjust the pointer value when it compacts the heap. That’s fairly straight-forward when the reference is static on inside another object. Harder when it is on the stack or a register, the jitter provides sufficient info to let the GC find it.