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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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15.3. Type of an Expression

If an expression denotes a variable or a value, then the expression has a type known at compile time. The rules for determining the type of an expression are explained separately below for each kind of expression.

The value of an expression is assignment compatible (§5.2) with the type of the expression, unless heap pollution (§4.12.2) occurs.

Likewise, the value stored in a variable is always compatible with the type of the variable, unless heap pollution occurs.

In other words, the value of an expression whose type is T is always suitable for assignment to a variable of type T.

Note that an expression whose type is a class type F that is declared final is guaranteed to have a value that is either a null reference ...

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