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());
}
});
- Can I do the above anonymously?
- How else could I do this?
- I want to sort myMap by the Value not the Key
Solution 1:
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
forComparator<? 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]
Solution 2:
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());
}
});