While(false) causes unreachable statement compilation error

Question

I was recently removing a block of code from our code base before a release and used an if(false) statement to prevent execution:

if (false) {
    ArrayList<String> list = new ArrayList<String>();
    ...
}

This compiles fine and will prevent execution of the offending block of code (right or wrong, that's not the current argument).

However, kind of by accident, I changed the block above to:

while (false) {
    ArrayList<String> list = new ArrayList<String>();
    ...
}

and received an unreachable statement compilation error.

I appreciate the compilation error and understand the reasons, however, I'm struggling to comprehend the difference between the two blocks and why the former compiles fine but the latter does not when they both have unreachable statements.

Answer 1

In both case the compiler should raise an error, because the code between braces is essentially pointless, but if (false) was kept in Java to simulate C/C++ preprocessor #if 0, quite a common way of disabling parts of code for testing or debugging.

EDIT: for reference, "conditional compiling" is detailed at the end of chapter 14.21 of the Java Language Specification.

Answer 2

"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 @ http://docs.oracle.com/javase/specs/#14.21."

Quoted from http://www.coderanch.com/t/266678/java-programmer-SCJP/certification/false-false