Question

I want to sort a Java TreeMap based on some attribute of value. To be specific, I want to sort a TreeMap<Integer, Hashset<Integer>> based on the size of Hashset<Integer>. To achieve this, I have done the following:

A Comparator class:

private static class ValueComparer implements Comparator<Integer> {
    private Map<Integer, HashSet<Integer>>  map = null;
    public ValueComparer (Map<Integer, HashSet<Integer>> map){
        super();
        this.map = map;
    }

@Override
    public int compare(Integer o1, Integer o2) {
        HashSet<Integer> h1 = map.get(o1);
        HashSet<Integer> h2 = map.get(o2);

        int compare = h2.size().compareTo(h1.size());

        if (compare == 0 && o1!=o2){
            return -1;
        }
        else {
            return compare;
        }
    }
}

A usage example:

TreeMap<Integer, HashSet<Integer>> originalMap = new TreeMap<Integer, HashSet<Integer>>();

//load keys and values into map

ValueComparer comp = new ValueComparer(originalMap);
TreeMap<Integer, HashSet<Integer>> sortedMap = new TreeMap<Integer, HashSet<Integer>>(comp);
sortedMap.putAll(originalMap);

The problem:

This doesn’t work when originalMap contains more than 2 values of the same size. For other cases, it works alright. When more than two values in the map are of same size, the third value in the new sorted-map is null and throws NullPointerException when I try to access it.

I can’t figure out what the problem is. Woule be nice if someone could point out.

Update:
Here’s an example that works when two values have the same size: http://ideone.com/iFD9c
In the above example, if you uncomment lines 52-54, this code will fail- that’s what my problem is.

Answer 1

Update: You cannot return -1 from ValueComparator just because you want to avoid duplicate keys to not be removed. Check the contract of Comparator.compare.

---

When you pass a Comparator to TreeMap you compute a (“new”) place to put the entry. No (computed) key can exist more than once in a TreeMap.

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If you want to sort the orginalMap by size of the value you can do as follows:

public static void main(String[] args) throws Exception {

    TreeMap<Integer, HashSet<Integer>> originalMap = 
        new TreeMap<Integer, HashSet<Integer>>();

    originalMap.put(0, new HashSet<Integer>() {{ add(6); add(7); }});
    originalMap.put(1, new HashSet<Integer>() {{ add(6); }});
    originalMap.put(2, new HashSet<Integer>() {{ add(9); add(8); }});


    ArrayList<Map.Entry<Integer, HashSet<Integer>>> list = 
        new ArrayList<Map.Entry<Integer, HashSet<Integer>>>();
    list.addAll(originalMap.entrySet());

    Collections.sort(list, new Comparator<Map.Entry<Integer,HashSet<Integer>>>(){
        public int compare(Map.Entry<Integer, HashSet<Integer>> o1,
                           Map.Entry<Integer, HashSet<Integer>> o2) {

            Integer size1 = (Integer) o1.getValue().size();
            Integer size2 = (Integer) o2.getValue().size();
            return size2.compareTo(size1);
        }
    });

    System.out.println(list);
}