How to Customize a TreeMap to sort keys in descending order?

Question

I have a piece of code with which I am trying to sort the keys in the TreeMap in descending order because when I just use the TreeMap, it sorts based on keys, which are in ascending order. The I am getting error is:

No Suitable Constructor found for TreeMap.

What's wrong here?

import java.util.TreeMap;
import java.util.Map;
import java.util.Comparator;
import java.util.Map.Entry;
import java.util.SortedMap;

public class ChangeMachine {


    public TreeMap<Double, Integer> dispenseChange(Double changeAmount, TreeMap<Double, Integer> coinsMap) {
        TreeMap<Double, Integer> coinDispenserMap = new TreeMap<>();
        for (Map.Entry<Double, Integer> coin : coinsMap.entrySet()) {
            if (!(changeAmount > coin.getKey())) {
                coinDispenserMap.put(coin.getKey(), 0);
                continue;
            }
            int noOfCoins = (int) (changeAmount / coin.getKey());
            coinDispenserMap.put(coin.getKey(), noOfCoins);
            Double remainder = changeAmount % coin.getKey();
            changeAmount = remainder;
            if (changeAmount == 0.0) {
                break;
            }
        }
        return coinDispenserMap;

    }


    public static void main(String[] args) {
        ChangeMachine ref = new ChangeMachine();
        TreeMap<Double, Integer> coinsMap = new TreeMap<Double, Integer>(new Comparator<Entry<Double, Integer>>() {
            public int compare(Entry<Double, Integer> coin1, Entry<Double, Integer> coin2) {
                return (coin2.getKey().compareTo(coin1.getKey()));
            }
        });
        coinsMap.put(0.25, 10);
        coinsMap.put(0.01, 10);
        coinsMap.put(0.05, 10);
        coinsMap.put(0.10, 10);

        TreeMap<Double, Integer> coinDispenserMap = ref.dispenseChange(0.86, coinsMap);

        for (Map.Entry<Double, Integer> coin : coinDispenserMap.entrySet()) {
            System.out.println(coin.getKey() + " : " + coin.getValue());
        }
    }
}

Answer 1

Your comparator's type is Entry<Double, Integer> which needs to be a super-type of the map's key type, which is Double. Since Entry<Double, Integer> isn't a super-type of Double, the constructor can't be matched.

Instead, try:

Map<Double, Integer> coinsMap =
    new TreeMap<Double, Integer>(new Comparator<Double>() {
        public int compare(Double coin1, Double coin2) {
            return (coin2.compareTo(coin1));
        }
    });

Also, for a bit of style, you might want to consider declaring coinsMap as Map<Double, Integer> coinsMap = new TreeMap<>() as I did in the code snippet above. Check out Josh Bloch's awesome book Effective Java 2nd Ed. (chapter 4 is relevant here) for great advice on Java.