使用自定义比较器时，TreeSet 抛出空指针异常

Question

I got an error msg when I trying to remove a specific element in treeset (not null). I'm not sure why this happened, and I tried to use contains to check if there has the same element in the set already which would work fine. The exception throws when calling reset method.

java.lang.NullPointerException
  at line 55, Leaderboard$1.compare
  at line 52, Leaderboard$1.compare
  at line 374, java.base/java.util.TreeMap.getEntryUsingComparator
  at line 343, java.base/java.util.TreeMap.getEntry
  at line 601, java.base/java.util.TreeMap.remove
  at line 276, java.base/java.util.TreeSet.remove
  at line 86, Leaderboard.reset
  at line 71, __Driver__.__helperSelectMethod__
  at line 91, __Driver__.__helper__
  at line 112, __Driver__.main

Here is my code:

class Leaderboard {
    TreeSet<Integer> TS;
    HashMap<Integer, Integer> HS = new HashMap<>();
    public Leaderboard() {
        TS = new TreeSet<>(new Comparator<Integer>(){
           @Override
            public int compare(Integer a, Integer b){
                if(HS.get(a).equals(HS.get(b))){
                    return Integer.compare(a, b);
                }
                return HS.get(a).compareTo(HS.get(b));
            }
        });
    }
    
    public void addScore(int playerId, int score) {
        HS.put(playerId, HS.getOrDefault(playerId, 0) + score);
        TS.add(playerId);
        TreeSet<Integer> test = TS;
        HashMap<Integer, Integer> test2 = HS;
    }
    
    public int top(int K) {
        Iterator<Integer> iterator = TS.iterator();

        int res = 0;
        while(K > 0 && iterator.hasNext()){
            res += HS.get(iterator.next());
            K--;
        }
        return res;
    }
    
    public void reset(int playerId) {
        Integer id = new Integer(playerId); //tried, not work
        System.out.println(HS.containsKey(id)); // true        
        System.out.println(TS.contains(id)); // true
        HS.remove(id);
        TS.remove(id);
    }
}

/**
 * Your Leaderboard object will be instantiated and called as such:
 * Leaderboard obj = new Leaderboard();
 * obj.addScore(playerId,score);
 * int param_2 = obj.top(K);
 * obj.reset(playerId);
 */

Answer 1

As @VGR mentioned: Are you certain that HS.get(a) never returns null in your compare method?

This is because that the TreeMap is based on HashMap to compare. If delete the HS value using HS.remove(a), whenever the treemap call HS.get(a) there will be a null as return value. which when called in compare for HS.get(a).compareTo will be null.compareTo, and this is the reason that throws an NullPointerException.