Why does this Java code using HashMap cause NullPointerException?

Question

This is what I encountered solving leetcode problem 954. Array of Doubled Pairs.

Here's a global map which stores the count on each value. I use this function below to check if the values in the list (which is also in the map) are all paired (if the map has value and value * 2).

    Map<Integer, Integer> map;
    
    private boolean isPaired( List<Integer> list) {
         for(int key : list) {
            if (map.containsKey(key)) {
                if (map.containsKey(key * 2)) {
                    updateMap(key);
                    updateMap(key * 2);
                } else {
                    return false;
                }
            }
        }
        return true;
    }
    
    private void updateMap(int key) {
        int value = map.get(key);
        if (value - 1 == 0) {
            map.remove(key); 
        } else {
            map.put(key , value - 1); 
        }
    }

with the case (9945 0s, 10052 1s and 10003 2s), it throws NullPointerException. Here's the exception message:

java.lang.NullPointerException
  at line 54, Solution.updateMap
  at line 44, Solution.isPaired
  at line 26, Solution.canReorderDoubled
  at line 54, __DriverSolution__.__helper__
  at line 84, __Driver__.main

And as I changed the function isPaired into the code below:

 private boolean isPaired(List<Integer> list) {
         for(int key : list) {
            if (map.containsKey(key)) {
                updateMap(key);
                if (map.containsKey(key * 2)) {
                    updateMap(key * 2);
                } else {
                    return false;
                }
            }
        }
        return true;
    }

NullPointerException doesn't occur anymore. Why do these happen?

Answer 1

The NullPointerException (NPE) occurs in the first case because key and key*2 are the same value when key=0. The second version of the code tests for the presence of key*2 after the first mapping has already been removed so it doesn't encounter the NPE.