Usage of an unlabeled break statement causes compilation to fail

Question

There are tons of answers on StackOverflow related to the of usage break statement in Java, yet unfortunately, I was not able to discover a clear explanation to my Java beginner type of a question.

// Main.java
public class Main {

    public static void main(String[] args) {
        foo: {
            break foo; // compiles and executes OK
        }

        bar: {
            // Java compiler produces an error at this point
            break;
        }


        // perhaps Java compiler complains on unused label, but...

        baz: {
            System.out.println("Hello World"); // compiles and executes OK
        }
    }
}

javac Main.java 

Main.java:10: error: break outside switch or loop
            break;
            ^
1 error

What am I missing?

Answer 1

There are 2 forms of the break statement, one without a label and one with a label.

Your first case compiles because there is a corresponding label of the same name labeling the statement to break from.

But in the second case, without a label, needs a specific kind of statement to break out of. According to the JLS, Section 14.15:

A break statement with no label attempts to transfer control to the innermost enclosing switch, while, do, or for statement; this enclosing 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 enclosing 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.

It is a compile-time error if a break statement has no break target.

There is no enclosing switch, while, do, or for; a block doesn't meet this requirement, so there is no break target. Because there is no break target, there is a compiler error.

The break statement with an explicit label only requires a labeled statement, which can include a block with braces.