Take a look at this Java IAQ (infrequently asked question) "Is null an object?".
To answer your questions:
- Only memory to store the reference is allocated
- The java standard doesn't specify how much memory a reference takes up (although, in practice it is usually 32 or 64 bits depending on your JVM - although some JVMs try to beat this)
- The memory representation of null doesn't matter, since you can't do bit level operations on it (or assign it to anything that isn't a reference)
