while and do-while

When you only know 'until what,' use while — with a safety limit so a retry cannot become a hang. Use do-while when you must check at least once, like polling a service.

2 min read

Sometimes you cannot answer "how many times?" — you can only answer "until what?" Wait until the page loads. Retry until the login succeeds. Poll until the service is up. That is a while loop: it repeats as long as a condition stays true, and it is the shape behind every wait in test automation.

public class L4 {
    public static void main(String[] args) {
        int attempts = 0;
        boolean loggedIn = false;
        while (!loggedIn && attempts < 3) {
            attempts++;
            System.out.println("Login attempt " + attempts);
            loggedIn = (attempts == 3);
        }
        System.out.println("Logged in: " + loggedIn + " after " + attempts + " attempts");
    }
}

Login attempt 1
Login attempt 2
Login attempt 3
Logged in: true after 3 attempts

Notice the two-part condition: !loggedIn && attempts < 3. In plain English: keep trying while we are not yet logged in and we still have attempts left. That second part is a safety limit. It is not optional — it is the difference between a retry and a hang. A while that can never make its condition false runs forever.

do-while: when you must run at least once

A plain while checks its condition first, so it can run zero times — if the condition is already false, the body never runs. Occasionally you need the opposite: run the body once, then decide whether to repeat. That is do-while, and polling a service is the natural example — you must check at least once to know anything:

public class L5 {
    public static void main(String[] args) {
        int response;
        int poll = 0;
        do {
            poll++;
            response = (poll < 2) ? 503 : 200;
            System.out.println("Poll " + poll + ": HTTP " + response);
        } while (response != 200);
        System.out.println("Service is up after " + poll + " polls");
    }
}

Poll 1: HTTP 503
Poll 2: HTTP 200
Service is up after 2 polls

The condition sits at the bottom, so the body always runs before it is tested.

"Check, then maybe do" is a while. "Do, then check whether to repeat" is a do-while. In day-to-day automation you will reach for while far more often, but the day you need "run at least once," do-while is exactly right.

Choosing without overthinking

Ask: do I know the number of passes? If yes — a count, a fixed list, every row — use for. If you only know the stopping condition — until it loads, until it succeeds — use while. When both would work, for is usually clearer because its counter and limit sit together on one line.

You can retry with a safety limit and poll until a service is up. The next lesson is the two bugs that cost testers the most time — off-by-one and the infinite loop — plus break and continue for fail-fast and skip-row control.