15.15

Cast Expressions

EXPRESSIONS

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 Dims

opt

 )

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.

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