15.2

Variables as Values

EXPRESSIONS

declared

void

 ( 8.4). Such an expression can be used only as an expression state 

ment ( 14.7), because every other context in which an expression can appear

requires the expression to denote something. An expression statement that is a

method invocation may also invoke a method that produces a result; in this case

the value returned by the method is quietly discarded.

Each expression occurs in the declaration of some (class or interface) type

that is being declared: in a field initializer, in a static initializer, in a constructor

declaration, or in the code for a method.

15.2   Variables as Values

If an expression denotes a variable, and a value is required for use in further eval 

uation, then the value of that variable is used. In this context, if the expression

denotes a variable or a value, we may speak simply of the

value

 of the expression.

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 always assignment compatible ( 5.2) with the

type of the expression, just as the value stored in a variable is always compatible

with the type of the variable. 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 or an object whose class

is

F

 itself, because

final

 types have no subclasses.

15.4   Expressions and Run Time Checks

If the type of an expression is a primitive type, then the value of the expression is

of that same primitive type. But if the type of an expression is a reference type,

then the class of the referenced object, or even whether the value is a reference to

an object rather than

null

, is not necessarily known at compile time. There are a

few places in the Java language where the actual class of a referenced object

affects program execution in a manner that cannot be deduced from the type of the

expression. They are as follows:

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