TreeMap 上的 ContainsKey 抛出 NPE

Question

So i have a Comparator that i defined for my TreeMap to sort it by values, but when i use containsKey(anyKey) on a key that it does not contain it doesn't return false, but it throws a NullPointerException . Now what i don't understand is why it goes in my Comparator for the containsKey() method and how i could find a solution... I've been stuck on this for a couple of hours and any help would be appriciated.

Here is the code :

public class test {

    public static void main(String[] args) {

        HashMap<Integer, Integer> map = new HashMap<>();
        map.put(1,2);
        map.put(2, 3);
        Comparator<Integer> comp = new ValueComparator<Integer, Integer>(map);
        TreeMap<Integer, Integer> tm = new TreeMap<Integer, Integer>(comp);
        tm.putAll(map);
        System.out.println(tm.containsKey(1)); // returns true
        System.out.println(tm.containsKey(3)); //Throws NPE
    }


}

class ValueComparator<K, V extends Comparable<V>> implements Comparator<K>{

    HashMap<K, V> map = new HashMap<K, V>();

    public ValueComparator(HashMap<K, V> map){
        this.map.putAll(map);
    }

    @Override
    public int compare(K s1, K s2) {

        return map.get(s1).compareTo(map.get(s2));//descending order    
    }
}

Answer 1

@Override
public int compare(K s1, K s2) {
    // Here is the problem you need to check if map.get(s1) returns null.
    return map.get(s1).compareTo(map.get(s2));//descending order    
}