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The Java® Language Specification, Java SE 7 Edition, Fourth Edition by Alex Buckley, Gilad Bracha, Guy Steele, Bill Joy, James Gosling

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4.1. The Kinds of Types and Values

There are two kinds of types in the Java programming language: primitive types (§4.2) and reference types (§4.3). There are, correspondingly, two kinds of data values that can be stored in variables, passed as arguments, returned by methods, and operated on: primitive values (§4.2) and reference values (§4.3).

Type:    PrimitiveType    ReferenceType

There is also a special null type, the type of the expression null3.10.7, §15.8.1), which has no name.

Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type.

The null reference is the only possible value of an expression of null type.

The null reference can always undergo a widening reference conversion ...

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