DEFINITE ASSIGNMENT

Definite Assignment

16

void flow(boolean flag) {

int k;

if (flag)

k = 3;

if (!flag)

k = 4;

System.out.println(k); // k

 is not  definitely assigned  before here

}

and so compiling this program must cause a compile time error to occur.

In order to precisely specify all the cases of definite assignment, the rules in

this section define two technical terms:

whether a local variable is

definitely assigned before

 a statement or expres 

sion, and

whether a local variable is

definitely assigned after

a statement or expression.

In order to specify boolean valued expressions, the latter notion is refined into

two cases:

whether a local variable is

definitely assigned after

 the expression

when true

,

and

whether a local variable is

definitely assigned after

 the expression

when false

.

Here

when true

 and

when false

 refer to the value of the expression. For example,

the local variable k is definitely assigned a value after evaluation of the expression

a && ((k=m) > 5)

when the expression is

true

 but not when the expression is

false

 (because if

a

 is

false

, then the assignment to

k

 is not executed (more properly, evaluated)).

The statement  

V

 is definitely assigned after

X

  (where

V

 is a local variable

and

X

 is a statement or expression) means  

V

 is definitely assigned after

X

 if

X

completes normally . If

X

 completes abruptly, the assignment may not have

occurred, and the rules stated here take this into account. A peculiar consequence

of this definition is that  

V

 is definitely assigned after

break;

  is always true!

Because a

break

 statement never completes normally, it is vacuously true that

V

has been assigned a value if the

break

 statement completes normally.

To shorten the rules, the customary abbreviation  iff  is used to mean  if and

only if .

Let

V

 be a local variable. Let

a

,

b

,

c

, and

e

be expressions. Let

S

 and

T

 be

statements.

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