How to sort a treemap based on its values?

Question

How can I sort a treemap using its values rather than the key?

Answer 1

Swap values and keys.

More seriously, please provide some context what you want to achieve. Maybe it is enough to sort after other processing is finished.

Answer 2

You could try giving a Comparator that compare values instead of keys when you create the TreeMap.

 final TreeMap<Integer,String> tree = new TreeMap<Integer,String>();
 tree.put(1, "1");
 tree.put(2, "2");
 tree.put(3, "3");
 tree.put(4, "4");

 final TreeMap<Integer,String> treeSortedByValues = new TreeMap<Integer,String>(new Comparator<Integer>()
 {
  public int compare(Integer o1, Integer o2)
  {
   return tree.get(o1).compareTo(tree.get(o2));
  }
 });
 treeSortedByValues.putAll(tree);

 for ( Entry<Integer, String> e : treeSortedByValues.entrySet() )
 {
  System.out.println(e.getKey() + ": " + e.getValue());
 }

Answer 3

Apache Commons Collections has a TreeBidiMap:

This class guarantees that the map will be in both ascending key order and ascending value order, sorted according to the natural order for the key's and value's classes.

There's a Java5-generics port of it here.

Answer 4

You cannot as the TreeMap's comparator is run against the keys only, e.g. see this constructor.

Anyway, you can use multiple Collections, use the TreeMap (or rather HashMap) for looking up elements by keys, and have a SortedSet to iterate on the values.

Answer 5

Few weeks ago I also had this question and this link help me a lot:

http://paaloliver.wordpress.com/2006/01/24/sorting-maps-in-java/

Hope it helps you too.:)

Answer 6

Google Collections provides a TreeMultiMap.

You could also use two collections. What are you trying to accomplish? Can you explain your use cases?

Answer 7

Here is a solution:

public static <K, V extends Comparable<V>> Map<K, V> sortByValues(final Map<K, V> map) {
Comparator<K> valueComparator =  new Comparator<K>() {
    public int compare(K k1, K k2) {
        int compare = map.get(k2).compareTo(map.get(k1));
        if (compare == 0) return 1;
        else return compare;
    }
};
Map<K, V> sortedByValues = new TreeMap<K, V>(valueComparator);
sortedByValues.putAll(map);
return sortedByValues;

}

Note that the map is sorted from the highest value to the lowest.