What is null in Java?

Question

What is null?

Is null an instance of anything?

What set does null belong to?

How is it represented in the me

Answer 1

Bytecode representation

Java's null has direct JVM support: three instructions are used to implement it:

• aconst_null: e.g. to set a variable to null as in Object o = null;
• ifnull and ifnonnull: e.g. to compare an object to null as in if (o == null)

Chapter 6 "The Java Virtual Machine Instruction Set " then mentions the effects of null on other instructions: it throws a NullPointerException for many of them.

2.4. "Reference Types and Values" also mentions null in generic terms:

A reference value may also be the special null reference, a reference to no object, which will be denoted here by null. The null reference initially has no run-time type, but may be cast to any type. The default value of a reference type is null.