Code unreachable in while and if statements

Question

Code below fails to compile since x=3 is unreachable

while (false) { x=3; }

But why for( int i = 0; i< 0; i++) {x = 3;} compiles fine? In this code x=3 is also unreachable.

Of course it is unreachable, try while (true) – SPlatten Nov 11 '19 at 08:13 — SPlatten, Nov 11 '19 at 08:13

score 2 · Accepted Answer · answered Nov 11 '19 at 08:12

See JLS 14.21, Unreachable Statements.

The contained statement [in a while loop] is reachable iff the while statement is reachable and the condition expression is not a constant expression whose value is false.

false is a constant expression whose value is false, so x=3; is unreachable.

The contained statement [in a basic for loop] is reachable iff the for statement is reachable and the condition expression is not a constant expression whose value is false.

i<0 isn't a constant expression, so the contained statement is considered reachable, even if it is not actually reachable.

Ssr1369 · Answer 2 · 2019-11-11T08:21:14.243

0

The while statement can be proven unreachable in compilation time, whereas the for statement needs to be checked in running time to find it unreachable.

If you try this

int i = 0;
while(i>1) { i = 4; }

The code will compile just fine.

edited Nov 11 '19 at 08:21

answered Nov 11 '19 at 08:13

Ssr1369

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Code unreachable in while and if statements

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