15

C H A P T E R

Expressions

When you can measure what you are speaking about,

and express it in numbers, you know something about it;

but when you cannot measure it, when you cannot express it in numbers,

your knowledge of it is of a meager and unsatisfactory kind:

it may be the beginning of knowledge, but you have scarcely,

in your thoughts, advanced to the stage of science.

 William Thompson, Lord Kelvin

M

UCH of the work in a Java program is done by evaluating

expressions

, either

for their side effects, such as assignments to variables, or for their values, which

can be used as arguments or operands in larger expressions, or to affect the execu 

tion sequence in statements, or both.

This chapter specifies the meanings of Java expressions and the rules for their

evaluation.

15.1   Evaluation, Denotation, and Result

When an expression in a Java program is

evaluated

 (

executed

), the

result

 denotes

one of three things:

A variable ( 4.5) (in C, this would be called an

lvalue

)

A value ( 4.2,  4.3)

Nothing (the expression is said to be

void

)

Evaluation of an expression can also produce side effects, because expres 

sions may contain embedded assignments, increment operators, decrement opera 

tors, and method invocations.

An expression denotes nothing if and only if it is a method invocation

( 15.11) that invokes a method that does not return a value, that is, a method

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