如何自定义 TreeMap 以按降序对键进行排序？

Question

我有一段代码试图按降序对 TreeMap 中的键进行排序，因为当我只使用 TreeMap 时，它会根据键进行排序，这些键是按升序排列。我得到的错误是：

No Suitable Constructor found for TreeMap.

这是怎么回事？

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

试试这个：

Comparator<Double> comparator = Double::compare;
Comparator<Double> reverseComparator = comparator.reversed();
Map<Double,Integer> reversedMap = new TreeMap(reverseComparator);

Answer 2

您的比较器类型是 Entry<Double, Integer>，它需要是地图键类型 Double 的超类型。由于 Entry<Double, Integer> 不是 Double 的超类型，因此 constructor can't be matched.

相反，尝试：

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

此外，为了有点风格，您可能需要考虑将 coinsMap 声明为 Map<Double, Integer> coinsMap = new TreeMap<>()，就像我在上面的代码片段中所做的那样。查看 Josh Bloch 的精彩著作 Effective Java 2nd Ed.（第 4 章与此处相关）以获得关于 Java 的重要建议。

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