how to jump from one for loop to another for loop using label in java?

Question

when i am trying to access for loop from another loop i get following errors. how can i do that can somebody explain.

public class Test {
   public static void main(String...rDX) {
      runing:
      for (int i = 1; i < 10; i++)
         System.out.print(i);
      for(int i = 1; i < 10; i++) {
         if (i == 5) {
            continue runing;
         }
      }
   }
}

error: java:28: error: undefined label: runing continue runing; ^ 1 error

Second loop starts iterating after first one **already finished**. There is nothing to continue. — Pshemo, Oct 19 '18 at 06:35
First, don't put random text inside your code. If you are going to enumerate things like "running", then use comments to delineate that. Secondly, can you be more clear as to what exactly you want? — Matthew, Oct 19 '18 at 06:35
@Matthew That is not random text, `runing:` is *label* (part of Java syntax). — Pshemo, Oct 19 '18 at 06:36
Might be worth mentioning that using labels is generally considered a bad practice. Of course there are some exceptions for some very specific situations, like when you want to shortcut multiple nested loops. — Bas de Groot, Oct 19 '18 at 06:37
[This contains some examples of how to use labels in Java](https://www.geeksforgeeks.org/g-fact-64/), but please think about different solutions... — deHaar, Oct 19 '18 at 06:43

LuCio · Answer 1 · 2018-10-19T08:26:01.447

The JLS says:

The Identifier is declared to be the label of the immediately contained Statement. ... Unlike C and C++, the Java programming language has no goto statement; identifier statement labels are used with break or continue statements ... appearing anywhere within the labeled statement.

The scope of a label of a labeled statement is the immediately contained Statement.

In your case the for-loop immediatly follwoing the label runing doesn't mention the label. The second for loop tries to continue to the label. But it is not part of the first for-loop, thus the immendiatly following statement.

This results in a compile-time error.

Thus in order to syntactically correct jump from one for loop to another for loop using label you need an outer loop containing the label. But I would argue that's not the right approach.

score 2 · Accepted Answer · answered Oct 19 '18 at 06:53

You can't do that because you can only abort a chain of nested fors and continue on some outer / parent for. You can't continue with other fors that happen to be simply nearby. But you could use that to do e.g.

public class Test {
    public static void main(String...rDX) {
        runing: for (;;) { // infinite loop
            for (int i = 1; i < 10; i++)
                System.out.print(i);
            for(int i = 1; i < 10; i++) {
                if (i == 5) {
                    continue runing;
                }
            }
            break; // = break runing; effectively never reached
                   // because above "continue" will always happen
        }
    }
}

This has the desired(?) effect since it continues with the newly added outer loop which goes back the the first loop.

(?) = at least what it would do when it would compile - I doubt you actually want that because you'll still print 1..10, then in the next step invisibly count to 5 and do nothing with that number.

score 0 · Answer 3 · answered Oct 19 '18 at 06:46

As LuCio has already stated, the problem here is that the second loop is not part of the first loop.

If you nest the for loops from your example it will work:

public static void main(String[] args) {
    runing:
    for (int i = 1; i < 10; i++) {
        System.out.print(i);
        for (int j = 1; j < 10; i++) {
            if (j == 5) {
                continue runing;
            }
        }
    }
}

Of course this is just an example of how it could work, i'm not sure if this is the actual logic your application requires.

how to jump from one for loop to another for loop using label in java?

3 Answers3