While Loops

Integer stock = 20;

while (stock > 0) {
    stock -= 3;
    System.debug('Stock remaining: ' + stock);
}

1. Integer stock = 20; — the starting quantity.
2. (blank line)
3. while (stock > 0) { — checked fresh before every pass; the loop keeps going as long as this is true.
4. stock -= 3; — unlike a for loop, there's no built-in counter — you're fully responsible for making sure something inside the loop eventually makes the condition false. This is that something: it shrinks stock toward (and past) zero.
5. System.debug('Stock remaining: ' + stock); — logs the new value each pass.
6. } — closes the loop.

Integer callsInQueue = 7;

while (callsInQueue > 0) {
    System.debug('Connecting next caller. Calls remaining: ' + callsInQueue);
    callsInQueue--;
}

System.debug('Queue cleared.');

1. Integer callsInQueue = 7; — calls waiting.
2. (blank line)
3. while (callsInQueue > 0) { — keep going as long as calls remain.
4. System.debug('Connecting next caller. Calls remaining: ' + callsInQueue); — logs the current count.
5. callsInQueue--; — decreases the count by one each pass — the thing that eventually makes line 3 false.
6. } — closes the loop.
7. (blank line)
8. System.debug('Queue cleared.'); — runs once, after the loop finishes.

A call center doesn't know how many calls will be in the queue at any moment — that number changes constantly. while fits this naturally: keep connecting callers as long as there are calls waiting.

• Forgetting to change the condition variable inside the loop. If stock was never decremented above, stock > 0 would stay true forever, and Apex would eventually hit a governor limit and throw an error rather than actually loop infinitely.
• Using while when for fits better. If you already know the exact number of iterations up front, a for loop usually communicates that more clearly than a while loop with a manually managed counter.