while loops and sentinels

A while loop repeats as long as its condition is true — checked before each pass:

int count = 1;
while (count <= 3) {
    System.out.println(count);
    count++;              // the update MUST move toward the exit
}
// prints 1 2 3

Trace the four contact points: initialize → test → body → update → test...

Two classic failures:

• Infinite loop: the update is missing or moves the wrong way (count-- with a count <= 3 test never exits).
• Off-by-one: < vs <= decides whether the last value is processed.

A sentinel loop reads until a special value appears — the pattern behind "read numbers until the user types -1". On the exam it appears as "process elements of an array until a negative price" style tasks:

int i = 0;
while (i < data.length && data[i] >= 0) {
    total += data[i];
    i++;
}

The && ordering matters: test the index bound before indexing, or the last check reads past the end.