Loop Control

Sometimes you need finer control over a loop than just letting its condition decide when it starts and stops. Java provides two keywords, break and continue, that let you jump around inside a loop's execution.

The break Statement

The break statement immediately exits the loop it's inside, skipping any remaining iterations entirely.

for (int i = 0; i < 10; i++) {
    if (i == 5) {
        break;
    }
    System.out.println(i);
}

Output:

0
1
2
3
4

Even though the loop was set up to run 10 times, it stopped as soon as i reached 5, since break exited the loop immediately.

The continue Statement

The continue statement skips the rest of the current iteration and jumps straight to the next one, without exiting the loop entirely.

for (int i = 0; i < 5; i++) {
    if (i == 2) {
        continue;
    }
    System.out.println(i);
}

Output:

0
1
3
4

When i was 2, continue skipped the System.out.println(i); line for that iteration only, and the loop continued on to i = 3.

Labeled Loops

When loops are nested inside one another, a plain break or continue only affects the innermost loop. Labeling a loop lets you target an outer loop specifically.

outer:
for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 3; j++) {
        if (j == 1) {
            break outer;
        }
        System.out.println(i + ", " + j);
    }
}

Output:

0, 0

Here, break outer; exits the entire labeled outer loop as soon as the condition is met, rather than just the inner loop. Labeled loops are relatively uncommon, but useful to recognize when you come across them.

Infinite Loops

An infinite loop is a loop whose condition never becomes false, causing it to repeat forever (or until the program is forcibly stopped).

while (true) {
    System.out.println("This never stops!");
}

Infinite loops are usually created by accident, often because a variable used in the condition is never updated inside the loop body. This is especially dangerous in robot code: WPILib runs your robot's code inside its own periodic loop roughly every 20 milliseconds, and if one of your methods gets stuck inside an accidental infinite loop, it blocks that periodic loop from ever finishing its cycle. This means your entire robot, drivetrain included, can stop responding to commands entirely.

break can be used to intentionally escape a loop that would otherwise run forever, once some condition inside the loop body is met.