14.19

Unreachable Statements

BLOCKS AND STATEMENTS

One might expect the

if

 statement to be handled in the following manner, but

these are not the rules that Java actually uses:

HYPOTHETICAL: An

if then

 statement can complete normally iff at least

one of the following is

true

:

N

The

if

then

 statement is reachable and the condition expression is not a

constant expression whose value is

true

.

N

The

then

 statement can complete normally.

The

then

 statement is reachable iff the

if

then

 statement is reachable and

the condition expression is not a constant expression whose value is

false

.

HYPOTHETICAL: An

if

then

else

 statement can complete normally iff

the

then

 statement can complete normally or the

else

 statement can com 

plete normally. The

then

 statement is reachable iff the

if

then

else

 state 

ment is reachable and the condition expression is not a constant expression

whose value is

false

. The

else

 statement is reachable iff the

if

then

else

statement is reachable and the condition expression is not a constant expres 

sion whose value is

true

.

This approach would be consistent with the treatment of other control structures

in Java. However, in order to allow the if statement to be used conveniently for

 conditional compilation  purposes, the actual rules are as follows:

ACTUAL: An

if

then

 statement can complete normally iff it is reachable.

The

then

 statement is reachable iff the

if

then

 statement is reachable.

ACTUAL: An

if

then

else

 statement can complete normally iff the

then

statement can complete normally or the

else

 statement can complete nor 

mally. The

then

 statement is reachable iff the

if

then

else

 statement is

reachable. The

else

 statement is reachable iff the

if

then

else

 statement

is reachable.

As an example, the following statement results in a compile time error:

while (false) { x=3; }

because the statement

x=3;

 is not reachable; but the superficially similar case:

if (false) { x=3; }

does not result in a compile time error. An optimizing compiler may realize that

the statement

x=3;

 will never be executed and may choose to omit the code for

that statement from the generated

class

 file, but the statement

x=3;

 is not

regarded as  unreachable  in the technical sense specified here.

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