A DESCRIPTION OF THE PROBLEM :
Since the merge method will not call the resize method when the ++size exceeds the threshold, if the merge method is called in the forEach method at this time, the merge method will call the resize method, which will cause the forEach method to behave abnormally.


---------- BEGIN SOURCE ----------
public class Main {
    public static void main(String[] args) {
        Map<Integer, Integer> map = new HashMap<>(2);

        map.merge(1, 1, Integer::sum);
        map.merge(2, 1, Integer::sum);

        map.forEach((k, v) -> {
            map.merge(k, -1, Integer::sum);
            System.out.println(k);
        });
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Same as the putVal method:
if (++size > threshold) resize();