Programming Tutorials

Using break as a Form of Goto

By: Baski in Java Tutorials on 2007-09-07  

In addition to its uses with the switch statement and loops, the break statement can also be employed by itself to provide a "civilized" form of the goto statement. Java does not have a goto statement, because it provides a way to branch in an arbitrary and unstructured manner. This usually makes goto-ridden code hard to understand and hard to maintain. It also prohibits certain compiler optimizations. There are, however, a few places where the goto is a valuable and legitimate construct for flow control. For example, the goto can be useful when you are exiting from a deeply nested set of loops.

To handle such situations, Java defines an expanded form of the break statement. By using this form of break, you can break out of one or more blocks of code. These blocks need not be part of a loop or a switch. They can be any block. Further, you can specify precisely where execution will resume, because this form of break works with a label. As you will see, break gives you the benefits of a goto without its problems. The general form of the labeled break statement is shown here:

break label;

Here, label is the name of a label that identifies a block of code. When this form of break executes, control is transferred out of the named block of code. The labeled block of code must enclose the break statement, but it does not need to be the immediately enclosing block. This means that you can use a labeled break statement to exit from a set of nested blocks. But you cannot use break to transfer control to a block of code that does not enclose the break statement.

To name a block, put a label at the start of it. A label is any valid Java identifier followed by a colon. Once you have labeled a block, you can then use this label as the target of a break statement. Doing so causes execution to resume at the end of the labeled block. For example, the following program shows three nested blocks, each with its own label. The break statement causes execution to jump forward, past the end of the block labeled second, skipping the two println() statements.

// Using break as a civilized form of goto.
class Break {
    public static void main(String args[]) {
        boolean t = true;
        first: {
            second: {
                third: {
                    System.out.println("Before the break.");
                    if (t)
                        break second; // break out of second block
                    System.out.println("This won't execute");
                }
                System.out.println("This won't execute");
            }
            System.out.println("This is after second block.");
        }
    }
}

Running this program generates the following output:

Before the break.
This is after second block.

One of the most common uses for a labeled break statement is to exit from nested loops. For example, in the following program, the outer loop executes only once:

// Using break to exit from nested loops
class BreakLoop4 {
    public static void main(String args[]) {
        outer: for (int i = 0; i < 3; i++) {
            System.out.print("Pass " + i + ": ");
            for (int j = 0; j < 100; j++) {
                if (j == 10)
                    break outer; // exit both loops
                System.out.print(j + " ");
            }
            System.out.println("This will not print");
        }
        System.out.println("Loops complete.");
    }
}

This program generates the following output:

Pass 0: 0 1 2 3 4 5 6 7 8 9 Loops complete.

As you can see, when the inner loop breaks to the outer loop, both loops have been terminated.
Keep in mind that you cannot break to any label which is not defined for an enclosing block. For example, the following program is invalid and will not compile:

// This program contains an error.
class BreakErr {
    public static void main(String args[]) {
        one: for (int i = 0; i < 3; i++) {
            System.out.print("Pass " + i + ": ");
        }
        for (int j = 0; j < 100; j++) {
            if (j == 10)
                break one; // WRONG
            System.out.print(j + " ");
        }
    }
}

Since the loop labeled one does not enclose the break statement, it is not possible to transfer control to that block.