Please explain the usage of Labeled Statements

JLS 14.7 Labeled statements

(edited for clarity)

Statements may have label prefixes (Identifier : Statement). 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 (§14.15) or continue (§14.16) statements appearing anywhere within the labeled statement.

So the JLS is clear that labels are used with break or continue, and no other grammatical element of the Java programming language uses it.

Strictly speaking, break and continue, labeled or not, are NEVER necessary. They can always be written out of the code. Used idiomatically, however, they can lead to more readable code.

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Here’s an illustrative example: given an int[], we want to :

• print "One (1)" on 1
• print "Two (2)" on 2
• print "Zero " on 0

• immediately stop processing on any other number

  int[] arr = { 1, 2, 0, 1, -1, 0, 2 };
  loop:
  for (int num : arr) {
      switch (num) {
      case 1:
          System.out.print("One ");
          break;
      case 2:
          System.out.print("Two ");
          break;
      case 0:
          System.out.print("Zero ");
          continue loop;
      default:
          break loop;
      }
      System.out.print("(" + num + ") ");
  }
  // prints "One (1) Two (2) Zero One (1) "
  

Here we see that:

• The different numbers are processed in a switch
• Unlabeled break in the switch is used to avoid “fall-through” between cases
• Labeled continue loop; is used to skip post-processing on case 0: (the label is not necessary here)
• Labeled break loop; is used to terminate the loop on default: (the label is necessary here; otherwise it’s a switch break)

So labeled break/continue can also be used outside of nested loops; it can be used when a switch is nested inside a loop. More generally, it’s used when there are potentially multiple break/continue target, and you want to choose one that is not immediately enclosing the break/continue statement.

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Here’s another example:

    morningRoutine: {
        phase1: eatBreakfast();
        if (grumpy) break morningRoutine;
        phase2: kissWife();
        phase3: hugChildren();
    }
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Here’s another case of a labeled break being used not within an iterative statement, but rather within a simple block statement. One may argue that the labels lead to better readability; this point is subjective.

And no, the last line DOES NOT give compile time error. It’s actually inspired by Java Puzzlers Puzzle 22: Dupe of URL. Unfortunately, the puzzle does not go into “proper” use of labeled statements in more depth.