Why Java identifies unreachable code only in case of while loop?

Question

If I have code like

public static void main(String args[]){
    int x = 0;
    while (false) { x=3; }  //will not compile  
}

compiler will complaint that x=3 is unreachable code but if I have code like

public static void main(String args[]){
    int x = 0;
    if (false) { x=3; }
    for( int i = 0; i< 0; i++) x = 3;   
}

then it compiles correctly though the code inside if statement and for loop is unreachable. Why is this redundancy not detected by java workflow logic ? Any usecase?

Weird enough, the `if (false) { x=3; }` part looks as if it was copied right out of [*JSL*](http://docs.oracle.com/javase/specs/jls/se7/html/jls-14.html#jls-14.21) (scroll all the way to the bottom). — Nir Alfasi, Jun 01 '14 at 07:19
Naah. I encountered this question in OCJP certification demo exam. Thanks for the JSL link though. — Aniket Thakur, Jun 01 '14 at 14:07
Well, I guess that one of the guys that were writing the demo was lazy... :) — Nir Alfasi, Jun 01 '14 at 17:00

score 7 · Accepted Answer · answered Jun 01 '14 at 07:09

As described in Java Language Specification, this feature is reserved for "conditional compilation".

An example, described in the JLS, is that you may have a constant

static final boolean DEBUG = false;

and the code that uses this constant

if (DEBUG) { x=3; }

The idea is to provide a possibility to change DEBUG from true to false easily without making any other changes to the code, which would not be possible if the above code gave a compilation error.

score 2 · Answer 2 · edited May 23 '17 at 10:32

The use case with the if condition is debugging. AFAIK it is explicitly allowed by the spec for if-statements (not for loops) to allow code like this:

class A {
    final boolean debug = false;

    void foo() {
        if (debug) {
            System.out.println("bar!");
        }
        ...
    }
}

You can later (or via debugger at runtime) change the value of debug to get output.

EDIT As Christian pointed out in his comment, an answer linking to the spec can be found here.

score 2 · Answer 3 · answered Jun 01 '14 at 07:09

Regarding the for loop, I thinks it's just that it's not as easy to detect as the use of a false constant inside a while loop.

Regarding the if, that was a deliberate choice to authorize it in order to be able to remove debugging code from the byte code at compilation time:

private static final boolean DEBUG = false; // or true

...

if (DEBUG) {
    ...
}

Why Java identifies unreachable code only in case of while loop?

3 Answers3

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