Blocks and Statements - The Java Language Specification - page 416

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 .
• An if-then-else statement can complete normally iff the then -statement
can complete normally or the else -statement can complete normally.
The then -statement is reachable iff the if-then-else statement 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 expression whose value
is true .
This approach would be consistent with the treatment of other control struc-
tures. However, in order to allow the if statement to be used conveniently for
“conditional compilation” purposes, the actual rules differ.
The rules for the if statement are as follows:
• An if-then statement can complete normally iff it is reachable.
The then -statement is reachable iff the if-then statement is reachable.
• An if-then-else statement can complete normally iff the then -statement
can complete normally or the else -statement can complete normally.
The then -statement is reachable iff the if-then-else statement is reach-
able.
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 re-
garded as “unreachable” in the technical sense specified here.
The rationale for this differing treatment is to allow programmers to define
“flag variables” such as: