15.15 Cast Expressions

My days among the dead are passed;
Around me I behold,
Where'er these casual eyes are cast,
The mighty minds of old . . .
—Robert Southey (1774-1843), Occasional Pieces, xviii

A cast expression converts, at run time, a value of one numeric type to a similar value of another numeric type; or confirms, at compile time, that the type of an expression is boolean; or checks, at run time, that a reference value refers to an object whose class is compatible with a specified reference type.

CastExpression:

	( PrimitiveType Dimsopt ) UnaryExpression

	( ReferenceType ) UnaryExpressionNotPlusMinus

See §15.14 for a discussion of the distinction between UnaryExpression and UnaryExpressionNotPlusMinus.

The type of a cast expression is the type whose name appears within the parentheses. (The parentheses and the type they contain are sometimes called the cast operator.) The result of a cast expression is not a variable, but a value, even if the result of the operand expression is a variable.

At run time, the operand value is converted by casting conversion (§5.4) to the type specified by the cast operator.

Not all casts are permitted by the Java language. Some casts result in an error at compile time. For example, a primitive value may not be cast to a reference type. Some casts can be proven, at compile time, always to be correct at run time. For example, it is always correct to convert a value of a class type to the type of its superclass; such a cast should require no special action at run time. Finally, some casts cannot be proven to be either always correct or always incorrect at compile time. Such casts require a test at run time. A ClassCastException is thrown if a cast is found at run time to be impermissible.