What exactly is null in Java memory [duplicate]

Question

If I declare some string variable like

String str = null; 

• Is there memory allocated for variable str?
• If allocated, how many bytes will be allocated?
• If not allocated, how does the JVM know that there is some variable called str been declared?
• If allocated and if there is null value inside the memory, then what exactly is the representation of null in binary?

Answer 1

When you do something like:

String str = null;

The only thing allocated is a reference to a string (this is analogous to a string-pointer in languages which contain pointer-types). This reference is a 32 or 64 bit variable (depending upon your hardware architecture and Java version) which resides on the stack (probably, depending upon the exact context in which your declaration is placed).

No memory should be allocated for the null itself, because null is not actually a valid object instance. It is simply a placeholder that indicates that the object reference is not currently referring to an object. Probably it is simply the literal value 0, such that assigning an object reference to null is equivalent to setting a pointer type to NULL in C/C++. But that last bit is conjecture on my part.