Description Sebastian Scheid 2006-07-08 04:10:52 EDT

The compiler seems to optimize an empty for-each loop body by removing the complete for statement, so the initialization, which may have side effects, is skipped. 
The compiler behaves different with basic for loops where the init part is executed even if the body is empty. More important, the Sun compiler (1.5_05) does not remove the for-each init.

Since the JLS states that "The meaning of the enhanced for statement is given by translation into a basic for statement" here http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.14.2 that must be a bug due to the different behavior of a for-each loop compared to the same loop translated to a basic for.


Here is a demo class which behaves different when compiled with the Eclipse or the Sun compiler:

==============================
public class Demo {

	public static void main(String[] args) {
		for (Object o : initForEach()) {
//			System.out.println("for-each loop");
		}
		for (Iterator<Object> iter = initBasicFor().iterator(); iter.hasNext(); ) {
//			System.out.println("for loop");
			iter.next(); // exact translation of for-each. Can be omitted when iter.isEmpty() and even then initBasicFor() is called.
		}
	}
	
	static Set<Object> initForEach()	{
		System.out.println("initForEach");
		return new HashSet<Object>();
	}

	static Set<Object> initBasicFor()	{
		System.out.println("initBasicEach");
		return new HashSet<Object>();
	}
	
}
==============================

The init() method is only called if the for-each loop is not empty when compiled by Eclipse whereas it is always called when compiled by Sun.

Sun output:
initForEach
initBasicEach

Eclipse output:
initBasicEach