Removal discipline

ArrayList is where the exam harvests points. Two API facts separate calm students from panicked ones:

remove(int) vs remove(Integer). list.remove(2) removes the element AT INDEX 2. To remove the value 2 from an ArrayList<Integer>, write list.remove(Integer.valueOf(2)). When a list holds {10, 20, 30}, remove(1) leaves {10, 30} — it did not remove the value 1 (which was never there).

Index invalidation. Every removal shifts all later elements left by one. size() changes the moment remove fires. The classic broken loop:

for (int i = 0; i < list.size(); i++) {
    if (list.get(i).equals(target)) {
        list.remove(i);     // shifts left; next element is now at i
    }
}

With {"a", "a", "b"} and target "a": i=0 removes, list is "a", "b"; i=1 checks "b" — the SECOND "a" at index 1 was skipped entirely. Consecutive matches are the case that exposes this.

Forward-with-fix (re-decrement i): correct but subtle. Backward removal — the exam's preferred clean solution:

for (int i = list.size() - 1; i >= 0; i--) {
    if (list.get(i).equals(target)) {
        list.remove(i);
    }
}

Removing at i only shifts elements after i — and you have already visited all of them. Safe, no index gymnastics.