Memory allocation : How much space does a reference occupy in Java?

Question

In Java we have written a code:

A a1;
a1 = new A();

How many bytes of memory is reserved when compiler compiles the code:

A a1;

Answer 1

That's not specified by the Java standard and thus you should not worry about it.

Technically, references are usually as big as the machine's word size, i.e. 32 bit on a 32 bit machine and 64 bit on a 64 bit machine, though some 64 bit JVMs use special magic to allow 32 bit references.

Answer 2

One pointer's worth of memory is used on the stack. That should be 32 bits (4 bytes) unless your machine's in 64-bits mode.

edit:

I see that some people are confused and think that the A object itself is allocated on the stack. That is not the case in Java: all objects are allocated on the heap (modulo JIT optimizations of course). The line A a1; simply allocates pointer a1, initially set to NULL. The pointer itself is in the stack, though of course what it points to will be on the heap. The later call to new A() will allocate an A object on the heap, and the size of that allocation does depend on what's in A.