Java Conversions and Promotions

Every expression written in the Java programming language 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. In other cases, the context may be able to accept a type that is related to the type of the expression; as a convenience, rather than requiring the programmer to indicate a type conversion explicitly, the Java programming 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.

Conversions at Compile Time and Run Time

1. A conversion from type Object to type Thread 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.

2. A conversion from type Thread to type Object requires no run-time action; Thread is a subclass of Object, so any reference produced by and expression of type Thread is a valid reference value of type Object.

3. 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.

4. A conversion from type double to type long requires a nontrivial translation from a 64-bit floating-point value to the 64-bit integer representation. Depending on the actual run-time value, information may be lost.

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