Labeled break just after loop not working in Java

Question

for(int x = 0; x < 5; x++)
        stop2: {
                System.out.println("Count");
                if (x == 2)
                    break stop2;
        }

        stop3: for(int x = 0; x < 5; x++) {
                System.out.println("Second");
                if (x == 2)
                    break stop3;
        }

While I was learning the labeled break in Java, I noticed that putting the label just before and after the for statement would have different results. In the code above, the for loop with stop2 does not stop after printing "Count" 3 times while the other one does. Can somebody explain the difference between these two? I thought the labeled break breaks the body right after the label...

@user207421 - Try it and see .... and check the JLS link in my answer. — Stephen C, Jan 25 '20 at 06:24

Elliott Frisch · Answer 1 · 2020-01-25T15:23:20.067

4

You have the syntax incorrect (for your desired behavior) in the first example. This

for(int x = 0; x < 5; x++)
stop2: {
    System.out.println("Count");
    if (x == 2)
        break stop2;
}

should be

stop2: for (int x = 0; x < 5; x++) {
    System.out.println("Count");
    if (x == 2) {
        break stop2;
    }
}

or just

for (int x = 0; x < 5; x++) {
    System.out.println("Count");
    if (x == 2) {
        break;
    }
}

Since an unlabeled break applies to the first (that is the innermost) loop when no label is applied to the break.

edited Jan 25 '20 at 15:23

answered Jan 25 '20 at 04:14

Elliott Frisch

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'Without a label' should be removed from this. – user207421 Jan 25 '20 at 04:18
@user207421 How so? A `break` without a label, means the loop also does not require a label. – Elliott Frisch Jan 25 '20 at 04:23
You're implying that if a loop has a label it isn't considered when finding the innermost. I'm sure you don't mean that. – user207421 Jan 25 '20 at 04:24

score 3 · Accepted Answer · edited Jun 20 '20 at 09:12

I thought the labeled break breaks the body right after the label.

No. It breaks the statement (or block) to which the label is attached.

Thus:

  for (int x = 0; x < 5; x++)
      stop2: {
            // stuff
            break stop2;
      }

breaks the body statement of the for loop, but

 stop3: for (int x = 0; x < 5; x++)
      {
          //stuff
          break stop3;
      }

breaks the for statement.

Contrast the above this with

for (int x = 0; x < 5; x++)
      {
          //stuff
          break;
      }

An ordinary (non-labelled) break can only be used in a loop or a switch statement, and it breaks the immediately enclosing loop or switch.

This is what JLS 14.15 says about this:

"A break statement with no label attempts to transfer control to the innermost enclosing switch, while, do, or for statement of the immediately enclosing method or initializer; this statement, which is called the break target, then immediately completes normally."

"A break statement with label Identifier attempts to transfer control to the enclosing labeled statement (§14.7) that has the same Identifier as its label; this statement, which is called the break target, then immediately completes normally. In this case, the break target need not be a switch, while, do, or for statement."

Thus, the key thing in determining what a break statement (ordinary or labelled) does is what the break target is.

This is all a bit obscure ... and another good reason to use labelled statements only when they are absolutely necessary. I cannot see the justification for using them in your examples (except as illustrations). One of them should be written as an ordinary break and the other should be written as a continue.

But (for the doubters) both of those examples are valid, compilable Java code and both have well defined meaning according to the JLS.

Thanks, the example was just an illustration btw. – mikeeei Jan 26 '20 at 14:48 — mikeeei, Jan 26 '20 at 14:48

score 1 · Answer 3 · answered Jan 25 '20 at 04:19

In your case you dont need a labeled break. in Java the break statement breaks the loop that contains it. So your stop2 code block can really be.

for (int x = 0; x < 5; x++) {
    System.out.println("Count");
    if (x == 2) {
        break stop2;
    }
}

An example of a labelled break would be this.

Consider you have a loop within a loop and on some condition you want to break out of the outer loop. Example:

stop2:
for (int x=0; x < 5; x++) {
  for (int y=0; y<5;y++) {
    if (x == 3 && y == 2)  { //just making up some condition for the example
        break stop2;
    }
  }
}

score 1 · Answer 4 · answered Jan 25 '20 at 04:37

      for(int x = 0; x < 5; x++)
        stop2: {
                System.out.println("Count");
                if (x == 2)
                    break stop2;
        }

In this case, when you break label stop2 the control goes back again to for loop and starts with the next iteration (x = 3).

        stop3: for(int x = 0; x < 5; x++) {
                System.out.println("Second");
                if (x == 2)
                    break stop3;
        }

In case 2, when you break the label stop3 the control doesn't go back again to for loop.

Labeled break just after loop not working in Java

4 Answers4