5

C H A P T E R

Conversions and Promotions

Thou art not for the fashion of these times,

Where none will sweat but for promotion.

 William Shakespeare,

As You Like It

, Act II, scene iii

E

VERY Java expression has a type that can be deduced from the structure of the

expression and the types of the literals, variables, and methods mentioned in the

expression. It is possible, however, to write an expression in a context where the

type of the expression is not appropriate. In some cases, this leads to an error at

compile time; for example, if the expression in an

if

 statement ( 14.8) has any

type other than

boolean

, a compile time error occurs. In other cases, the context

may be able to accept a type that is related to the type of the expression; as a con 

venience, rather than requiring the programmer to indicate a type conversion

explicitly, the Java language performs an implicit

conversion

 from the type of the

expression to a type acceptable for its surrounding context.

A specific conversion from type

S

 to type

T

 allows an expression of type

S

 to

be treated at compile time as if it had type

T

 instead. In some cases this will

require a corresponding action at run time to check the validity of the conversion

or to translate the run time value of the expression into a form appropriate for the

new type

T

. For example:

A conversion from type

Object

 ( 20.1) to type

Thread

 ( 20.20) requires a

run time check to make sure that the run time value is actually an instance of

class

Thread

 or one of its subclasses; if it is not, an exception is thrown.

A conversion from type

Thread

 to type

Object

 requires no run time action;

Thread

 is a subclass of

Object

, so any reference produced by an expression

of type

Thread

 is a valid reference value of type

Object

.

A conversion from type

int

 to type

long

 requires run time sign extension of

a 32 bit integer value to the 64 bit

long

 representation. No information is

lost.

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