Java TreeMap Comparator

I need a comparator for a TreeMap. Should I write this anonymously in the constructor for my TreeMap? How else could I write my comparator. Currently, Java does not like my code (can I do this anonymously?):

SortedMap<String, Double> myMap = new TreeMap<String, Double>(new Comparator<Entry<String, Double>>() { public int compare(Entry<String, Double> o1, Entry<String, Double> o2) { return o1.getValue().compareTo(o2.getValue()); } });

1. Can I do the above anonymously?
2. How else could I do this?
3. I want to sort myMap by the Value not the Key

3 Answers

You can not sort TreeMap on values.

A Red-Black tree based NavigableMap implementation. The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used You will need to provide comparator for Comparator<? super K> so your comparator should compare on keys.

To provide sort on values you will need SortedSet. Use

SortedSet<Map.Entry<String, Double>> sortedset = new TreeSet<Map.Entry<String, Double>>( new Comparator<Map.Entry<String, Double>>() { @Override public int compare(Map.Entry<String, Double> e1, Map.Entry<String, Double> e2) { return e1.getValue().compareTo(e2.getValue()); } }); sortedset.addAll(myMap.entrySet());

To give you an example

 SortedMap<String, Double> myMap = new TreeMap<String, Double>(); myMap.put("a", 10.0); myMap.put("b", 9.0); myMap.put("c", 11.0); myMap.put("d", 2.0); sortedset.addAll(myMap.entrySet()); System.out.println(sortedset);

Output:

 [d=2.0, b=9.0, a=10.0, c=11.0]

The comparator should be only for the key, not for the whole entry. It sorts the entries based on the keys.

You should change it to something as follows

SortedMap<String, Double> myMap = new TreeMap<String, Double>(new Comparator<String>() { public int compare(String o1, String o2) { return o1.compareTo(o2); }
});

Update

You can do something as follows (create a list of entries in the map and sort the list base on value, but note this not going to sort the map itself) -

List<Map.Entry<String, Double>> entryList = new ArrayList<Map.Entry<String, Double>>(myMap.entrySet()); Collections.sort(entryList, new Comparator<Map.Entry<String, Double>>() { @Override public int compare(Entry<String, Double> o1, Entry<String, Double> o2) { return o1.getValue().compareTo(o2.getValue()); } });

you can swipe the key and the value. For example

 String[] k = {"Elena", "Thomas", "Hamilton", "Suzie", "Phil"}; int[] v = {341, 273, 278, 329, 445}; TreeMap<Integer,String>a=new TreeMap(); for (int i = 0; i < k.length; i++) a.put(v[i],k[i]); System.out.println(a.firstEntry().getValue()+"\t"+a.firstEntry().getKey()); a.remove(a.firstEntry().getKey()); System.out.println(a.firstEntry().getValue()+"\t"+a.firstEntry().getKey());