16

C H A P T E R

Definite Assignment

All the evolution we know of proceeds from the vague to the definite.

 Charles Peirce

E

ACH local variable must have a

definitely assigned

 value when any access of

its value occurs. An access to its value consists of the simple name of the variable

occurring anywhere in an expression except as the left hand operand of the simple

assignment operator

=

.

A Java compiler must carry out a specific conservative flow analysis to make

sure that, for every access of a local variable, the local variable is definitely

assigned before the access; otherwise a compile time error must occur.

The remainder of this chapter is devoted to a precise explanation of the words

 definitely assigned before . The idea is that an assignment to the local variable

must occur on every possible execution path to the access from the beginning of

the constructor, method, or static initializer that contains the access. The analysis

takes into account the structure of statements and expressions; it also provides a

special treatment of the expression operators

!

,

&&

,

||

, and

? :

, the operators

&

,

|

,

^

,

==

, and

!=

 with

boolean

 operands, and boolean valued constant expressions.

For example, a Java compiler recognizes that

k

 is definitely assigned before its

access (as an argument of a method invocation) in the code:

{

int k;

if (v > 0 && (k = System.in.read()) >= 0)

System.out.println(k);

}

because the access occurs only if the value of the expression:

v > 0 && (k = System.in.read()) >= 0

is true, and the value can be

true

 only if the assignment to

k

 is executed (more

properly, evaluated). Similarly, a Java compiler will recognize that in the code:

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