16

Definite Assignment

DEFINITE ASSIGNMENT

{

int k;

while (true) {

k = n;

if (k >= 5) break;

n = 6;

}

System.out.println(k);

}

the variable

k

 is definitely assigned by the

while

 statement because the condition

expression

true

 never has the value

false

, so only the

break

 statement can

cause the

while

 statement to complete normally, and

k

 is definitely assigned

before the

break

 statement.

Except for the special treatment of certain boolean operators and of boolean 

valued constant expressions, the values of expressions are not taken into account

in the flow analysis. For example, a Java compiler must produce a compile time

error for the code:

{

int k;

int n = 5;

if (n > 2)

k = 3;

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

 is not  definitely assigned  before this

}

even though the value of

n

 is known at compile time, and in principle it can be

known at compile time that the assignment to

k

 will always be executed (more

properly, evaluated). A Java compiler must operate according to the rules laid out

in this section. The rules recognize only constant expressions; in this example, the

expression

n > 2

 is not a constant expression as defined in  15.27.

As another example, a Java compiler will accept the code:

void flow(boolean flag) {

int k;

if (flag)

k = 3;

else

k = 4;

System.out.println(k);

}

as far as definite assignment of

k

 is concerned, because the rules outlined in this

section allow it to tell that

k

 is assigned no matter whether the flag is

true

 or

false

. But the rules do not accept the variation:

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