While Loops

for loops are great when we know how many times we want to repeat. But sometimes we want to keep going as long as something is true, without knowing in advance how many iterations that means. This is the job of while

public class Main {
    public static void main(String[] args) {
        int i = 0;
        while (i < 10) {
            System.out.println(i);
            i++;
        }
    }
}

Output

Why no 10? Because when i = 10, the condition 10 < 10 is false, so we exit. If we wanted 10 included, we'd use the condition i <= 10

while runs as long as the condition is true

Be very careful. If we forget to update i inside the loop, we get an infinite loop

public class Main {
    public static void main(String[] args) {
        int i = 0;
        while (i <= 100) {
            System.out.println(i);
            // we forgot i++
        }
    }
}

i stays 0 forever, so the condition is always true, and the program prints 0 non-stop. Whenever you write a while, ask yourself: "what makes this condition eventually become false?". If the answer is "nothing", you have a problem

When to choose for vs while?

for when you know the count ("do this 10 times", "go through every element of an array")
while when the stopping condition depends on something computed inside the loop ("keep asking the user until they type quit", "keep dividing by 2 until the number drops below 1")

Both are equally powerful. Style and readability decide which one you pick

Sometimes, in the middle of a loop, we want to say "that's enough, stop" or "skip this one, move to the next". For that we have break and continue

break stops the loop completely. The remaining iterations never happen

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            if (i == 5) {
                break;
            }
            System.out.println(i);
        }
    }
}

Output

We told the loop to go up to 99, but as soon as i became 5, break kicked in and the loop ended. The numbers 5, 6, 7, ..., 99 never printed

A real example: searching a name in an array. As soon as we find the target, there's no point checking the rest

public class Main {
    public static void main(String[] args) {
        String[] names = { "Tommy", "Lance", "Cortez", "Phil", "Sonny" };
        String target = "Cortez";

        for (String name : names) {
            if (name.equals(target)) {
                System.out.println("Found " + target);
                break;
            }
            System.out.println("Checking " + name);
        }
    }
}

Output

Checking Tommy
Checking Lance
Found Cortez

The loop didn't check Phil and Sonny: we already found what we wanted

continue is different. It doesn't stop the loop, it just skips the rest of the current iteration and jumps to the next one

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            if (i == 5) {
                continue;
            }
            System.out.println(i);
        }
    }
}

Output

5 is missing. When i was 5, continue fired, jumped over System.out.println, and the loop continued from i = 6

A real example: print only even numbers

public class Main {
    public static void main(String[] args) {
        for (int i = 0; i <= 10; i++) {
            if (i % 2 != 0) {
                continue;
            }
            System.out.println(i);
        }
    }
}

Output: 0 2 4 6 8 10

break and continue work the same way in while. A handy combination is while (true), a loop that would normally be infinite, which we escape with break

public class Main {
    public static void main(String[] args) {
        int i = 0;
        while (true) {
            if (i >= 5) {
                break;
            }
            System.out.println(i);
            i++;
        }
    }
}

One trap: inside a while, continue jumps straight back to the condition. If you haven't already updated the counter, the condition never changes and you get an infinite loop. So inside a while, update the counter before continue

Mission: Cracking the Safe

Tommy found Diaz's safe and tries the combinations one by one: 1, then 2, then 3, and so on upward. You don't know in advance how many tries it takes: exactly the kind of problem for while (true) plus break.

Store the safe's secret combination in an int named secretCombination. Then use a while (true) loop that counts the attempts starting from 1. On each attempt:

if the current attempt equals the secret combination → print Safe open and leave the loop with break
otherwise → print Trying N, where N is the attempt number, then move to the next

Example for secret combination 3:

Trying 1
Trying 2
Safe open

Example for secret combination 1 (opens on the first try):

Safe open

Example for secret combination 5:

Trying 1
Trying 2
Trying 3
Trying 4
Safe open