Question

我有一段代码，我试图按降序对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的订购。我收到的错误是：

找不到适合TreeMap的构造函数。

这里有什么问题？

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

比较器的类型是Entry<Double, Integer>，它需要是地图密钥类型的超类型，即Double。由于Entry<Double, Integer>不是Double的超类型，constructor can't be matched。

相反，请尝试：

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

另外，对于一些样式，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段中所做的那样。查看Josh Bloch的精彩书籍Effective Java 2nd Ed.（第4章与此相关），以获得有关Java的精彩建议。

如何自定义TreeMap以降序排序键？

2 个答案: