// make it thread safe
public class ThreadSafetyPuzzler {
    private List<Integer> numbers;

    public ThreadSafetyPuzzler() {
        numbers = new ArrayList<>();
    }

    public void addNumber(int number) {
        numbers.add(number);
    }

    public int getSum() {
        int sum = 0;
        for (int number : numbers) {
            sum += number;
        }
        return sum;
    }

    public void clearNumbers() {
        numbers.clear();
    }
}