while(false) vs. if(false)

[Adam Tkaczyk]

Hi, I don't know why
statement
if(false) {...} compiles fine, but
while(false){...} makes compilation error.
Is this a bug?

[Jari Timonen]

    
while(false){
}

Code is never reached aka. executed. (Compiler error, Unreachable statement)

[Kelvin Chenhao Lim]

Hi Adam,

Java uses a simple flow analysis algorithm to find most common cases of unreachable code, and all such unreachable code blocks will be flagged as compile-time errors. That's why your "while (false) { ... }" statement produces an error.

However, Java makes a special exception for "if (false) { ... }", because programmers often use this construct during development to temporarily disable part of the program. That's why the compiler accepts this statement.

If you're interested in the nitty-gritty details, refer to the Java Language Specification's description of unreachable statements.

[Rahul Shilpakar]

If has ELSE statement in optional but while doesn't have.

while (false) doesn't make any sense because the code below it not gonna execute anyway. this is not the case with IF it has ELSE in option.

[Kelvin Chenhao Lim]

Hi Rahul,

Your argument is reasonable as well. But Sun's explicitly stated intention for allowing "if (false) { ... }" was to allow "conditional compilation" of code, i.e. having code that can be enabled or disabled based on a debug flag value that's a compile-time constant. For example:

class Foo {
  private static final boolean DEBUG = false;  // enable for debug output
 
  public static void doFoo() { 
    if (DEBUG) {
      System.out.println("Debug: doFoo() called with this stack trace:");
      Thread.dumpStack();
    }
    System.out.println("Fooed"); 
  }
 
  public static void main(String... args) {
    if (DEBUG) {
      System.out.println("Debug: args = " + Arrays.toString(args));
    }
    doFoo();
  }
}

Note that because DEBUG is a compile-time constant, you'll get a compiler error if you replace any of the "if (DEBUG) { ... }" statements with "while (DEBUG) { ... }".
[ December 11, 2007: Message edited by: Kelvin Lim ]