How does Java know where to jump to when breaking out of a loop?

Question

while (condition) {

    if (condition) {
        statement1;
        statement2;

        break;
    } else {
        statement3;
        statement4;
    }

}

By using break in the if clause, we ensure the loop is halted and exited.

I don't understand how the break statement "knows" that it is within a loop for it to exit out of in the first place, or how it "knows" where to jump to. How does this happen?

Answer 1

I don't understand how the break statement "knows" that it is within a loop for it to exit out of in the first place.

The break statement does not know that it's within a switch or loop statement. The compiler verifies that the break statement is within a switch or loop statement. If it encounters a break statement that is not within a loop statement, it will emit a compile-time error.

If no switch, while, do, or for statement in the immediately enclosing method, constructor, or initializer contains the break statement, a compile-time error occurs.

If the compiler is able to verify that the break statement is within a switch or loop statement, it will then emit JVM instructions to jump abruptly to the first statement immediately after the nearest enclosing loop.

Thus:

for(int i = 0; i < 10; i++) {
    if(i % 2 == 0) {
         break;
    }
}

would be translated by the compiler into:

0:  iconst_0        # push integer 0 onto stack
1:  istore_1        # store top of stack in local 1 as integer                  
                    # i = 0
2:  iload_1         # push integer in local 1 onto stack
3:  bipush 10       # push integer 10 onto stack
5:  if_icmpge 23    # pop and compare top two (as integers), jump if first >= second
                    # if i >= 10, end for
8:  iload_1         # push integer in local 1 onto stack
9:  iconst_2        # push integer 2 onto stack
10: irem            # pop top two and computes first % second and pushes result
                    # i % 2
11: ifne 17         # pop top (as integer) and jump if not zero to 17
                    # if(i % 2 == 0) 
14: goto 23         # this is the break statement
17: iinc 1, 1       # increment local 1 by 1
                    # i++
20: goto 2          # go to top of loop
                    # loop
23: return          # end of loop body