TreeMap <String,Integer> object的get方法返回null值

Question

import java.util.*;

public class Sort {

    static class ValueComparator implements Comparator<String> {

        Map<String, Integer> base;

        ValueComparator(Map<String, Integer> base) {
            this.base = base;
        }

        @Override
        public int compare(String a, String b) {
            if (base.get(a) >= base.get(b)) {
                return 1;
            } else {
                return -1;
            }
        }
    }

    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<String, Integer>();
        ValueComparator vc = new ValueComparator(map);
        TreeMap<String, Integer> sorted = new TreeMap<String, Integer>(vc);
        map.put("A", 1);
        map.put("B", 2);
        sorted.putAll(map);
        for (String key : sorted.keySet()) {
            System.out.println(key + " : " + sorted.get(key)); // why null values here?
        }
        System.out.println(sorted.values()); // But we do have non-null values here!
    }
}

输出:

A : null
B : null
[1, 2]
BUILD SUCCESSFUL (total time: 0 seconds)

我想知道为什么我们在第一个注释行获取空值,而我们确实有非空值,如第二个注释行所示.

编辑:@ null的版本似乎无法正常工作.我已将代码更改为:

        public int compare(String a, String b) {
            if (a.equals(b)) return 0;
            if (base.get(a) >= base.get(b)) {
                return 1;
            } else return -1;
        }

它似乎工作,但我不确定.

Answer 1

我的猜测是你的ValueComparator.compare()方法永远不会返回0,表示相等,导致Map.get()方法找不到匹配项.