Why is declaration of the variable required inside a for-each loop in java

Question

The usual form the of for each loop is this:

for(Foo bar: bars){
    bar.doThings();
}

But if I want to retain bar until after the loop, I can not use the for each loop:

Foo bar = null;
// - Syntax error on token "bar", Identifier expected after this token
for(bar: bars){ 
    if(bar.condition())
        break;
}
bar.doThings();

The for loop gets the syntax error mentioned above.

Why is this? I'm not interested in workarounds, but just curious to the considerations behind this limitation.

In contrast, with an ordinary for loop, the variable can be declared outside or not at all...

int i = 1;
for(;i<max;i++){
    for(;;){
        // Do things
    }
}

Answer 1

This is a good question and I would be happy to see some in-depth answer. However, the official documentation says:

These shortcomings were known by the designers, who made a conscious decision to go with a clean, simple construct that would cover the great majority of cases.

The great majority of cases is the answer for me.

On a side note, personally, I think that foreach loop in Java is just a nice syntax for a standard iterator loop. So, the compiler creates the iterator for the structure and uses the variable to get the value for current iteration. To ensure that the variable has been initialised you need to declare it for the scope of loop (and I think this prevents the variable from being used somewhere else, eg in another thread). Because of that, you cannot use the variable after the loop. But, this is just my opinion and I would be very happy to hear from someone who knows it better. :)

Edit Here is an interesting article about foreach loops in Java.

Another edit I analysed (with jclasslib the bytecode of these methods:

 private static void testForEach(ArrayList<String> als) {
  for(String s: als)
    System.out.println(s);
 }

 private static void testIterator(ArrayList<String> als) {
  for(Iterator<String> is = als.iterator(); is.hasNext();) {
   String s = is.next();
   System.out.println(s);
  } 
 }

Both methods are represented by the same bytecode:

 0 aload_0
 1 invokevirtual #2 <java/util/ArrayList.iterator>
 4 astore_1
 5 aload_1
 6 invokeinterface #3 <java/util/Iterator.hasNext> count 1
11 ifeq 34 (+23)
14 aload_1
15 invokeinterface #4 <java/util/Iterator.next> count 1
20 checkcast #5 <java/lang/String>
23 astore_2
24 getstatic #6 <java/lang/System.out>
27 aload_2
28 invokevirtual #7 <java/io/PrintStream.println>
31 goto 5 (-26)
34 return

The difference is in line 1, the latter method uses invokevirtual #8 . However, both invocations result in calling the same method of Iterator . So, it seems that foreach is nothing more than a syntactic sugar that the compiler just translates into a predefined construct, just as the documentation says. This does not answer the question why it is the way it is . I though this is just interesting and might be worth to know, in the context of the discussion.

Answer 2

I guess they just wanted to promote "good practices": from the Java language guide :

These shortcomings were known by the designers, who made a conscious decision to go with a clean, simple construct that would cover the great majority of cases.

As we know, the behaviour you are asking for can be simulated by just expanding the for-each loop to the old iterator - based style, and probably they just thought its Good Enough.

Answer 3

The way to iterate over a list in this case is:

Foo bar = null;
for(Iterator<Foo> barIterator = bars.iterator(); barIterator.hasNext();){ 
   bar = barIterator.next(); // or whatever else you want to do
   ...
}

Don't iterate over a list using the index unless the list implements RandomAccess!

The enhanced for loop is just syntactic sugar for this iterator approach, which keeps the variable (here bar) local to the loop. (Keeping variables as local as possible is a good thing, generally.)

Answer 4

The purpose of this new syntax is for beautification since it doesn't really add anything new to Java. My guess is they didn't allow the syntax you did for the sole reason: It's not beautiful! :)