if(false) vs. while(false): unreachable code vs. dead code

Question

I tried the following in Eclipse:

if (false) {}: warning 'dead code'
while (false) {}: compilation error 'unreachable code'

I was wondering whether there is a real 'reason' for this difference. I already found this...

...but why not allow while (false) for the same debugging purpose?

score 33 · Accepted Answer · answered Nov 30 '13 at 11:57

33

The JLS section on unreachable code explains the rationale. Essentially, Java normally shouldn't use conditional compilation like C routinely does with #ifdef, but there are some situations (such as debugging, and in particular backward binary compatibility) where allowing the compiler to entirely strip out code is needed, and so the specific construct if(false) is permitted for that purpose.

answered Nov 30 '13 at 11:57

chrylis -cautiouslyoptimistic-

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+1 `but why not allow while (false) for the same debugging purpose?` I didn't get that part? – Narendra Pathai Nov 30 '13 at 12:02
@MarkRotteveel I'm not always in the mood for JLS archaeology... *most interesting man meme* – chrylis -cautiouslyoptimistic- Nov 30 '13 at 12:03
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Does that mean that `if(false) {}` is the only exception where the compiler 'allows' unreachable code? – luukburger Nov 30 '13 at 12:03
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@NarendraPathai `while(false)` doesn't really make sense for debugging; in a debugging (or binary-compatibility) situation, you'll be either turning off sections or alternating between version (such as the Android compatibility libraries do). In both of these cases, any loops would remain in place, and it's best to keep the exception to the reachability rule as narrow as possible. – chrylis -cautiouslyoptimistic- Nov 30 '13 at 12:04
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@luukburger Not `if(false)` literally, but `if(compile-time-false-expression)`. – chrylis -cautiouslyoptimistic- Nov 30 '13 at 12:04
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@NarendraPathai The why is explained at the end of the linked paragraph in the JLS: conditional compilation – Mark Rotteveel Nov 30 '13 at 12:04
Thanks! On a side note, `while(true)` is another unreachable code scenario, for the code that follows the loop. :-) – asgs Mar 09 '16 at 20:33

Rahul Tripathi · Answer 2 · 2013-11-30T12:04:57.327

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You must read the Unreachable Statements. Although with while(false) the compiler will throw an error but with if(false) it wil show a warning to the user.

Although if (false) was kept in Java to simulate C/C++ preprocessor #if 0

The specification says that:

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.

The rationale for this differing treatment is to allow programmers to define "flag variables" such as:

static final boolean DEBUG = false; and then write code such as:

if (DEBUG) { x=3; } The idea is that it should be possible to change the value of DEBUG from false to true or from true to false and then compile the code correctly with no other changes to the program text.

edited Nov 30 '13 at 12:04

answered Nov 30 '13 at 11:58

Rahul Tripathi

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The compiler won't throw an error for `if (false {}`. That's the point. – luukburger Nov 30 '13 at 12:00
I still have the downvote because the compiler will *not* raise an error in the case of `if(false)`. – chrylis -cautiouslyoptimistic- Nov 30 '13 at 12:00
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@RahulTripathi `while(false)` is a compile-time error. `if(false)` *may* produce a warning but is explicitly *not* a compile-time error. – chrylis -cautiouslyoptimistic- Nov 30 '13 at 12:05
@luukburger:- Yes you are correct. I got it wrong. I have updated my answer and the rationale why the compiler will not throw an error! – Rahul Tripathi Nov 30 '13 at 12:17
It's worth noting that `if (compileTimeConstant) {...}` makes sense as a construct, and `while(true) {...}` and `do {...} while(compileTimeConstant)` make sense, but `while(compileTimeConstant) {...}` doesn't make sense in any case [other than the literal-`true` one] that couldn't be better written as `if (compileTimeConstant) { while(true) {} }`. – supercat Jan 24 '14 at 17:34

score 0 · Answer 3 · answered Dec 04 '20 at 11:12

0

However, the 'do-while loop' will compile and execute.

do {
    System.out.println("its work!");
} while (false);                       //compile and execute

answered Dec 04 '20 at 11:12

Java bee

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if(false) vs. while(false): unreachable code vs. dead code

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