Java<TreeMap按key排序与按照value排序>

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1. TreeMap默认按照key递增排序

public class Main {

    public static void main(String[] args) {
        TreeMap<Integer, String> treeMap = new TreeMap<>();
        TreeMap<String, Integer> treeMap1 = new TreeMap<>();
        treeMap.put(7, "h");
        treeMap.put(8, "g");
        treeMap.put(9, "f");
        treeMap.put(10, "e");
        treeMap.put(14, "a");
        treeMap.put(1, "w");
        treeMap.put(2, "v");
        treeMap.put(3, "u");
        treeMap.put(11, "d");
        treeMap.put(12, "c");
        treeMap.put(13, "b");
        treeMap.put(4, "k");
        treeMap.put(5, "j");
        treeMap.put(6, "i");
        System.out.println("----------------------*------------------------------");
        while (treeMap.size() != 0) {
            //treemap自动按照key进行递增排序
            System.out.println(treeMap.firstEntry().getKey() + " - " + treeMap.firstEntry().getValue());
            treeMap1.put(treeMap.firstEntry().getValue(), treeMap.firstEntry().getKey());
            treeMap.remove(treeMap.firstKey());
        }
        System.out.println("----------------------*------------------------------");
        while (treeMap1.size() != 0) {
            //treemap自动按照key进行递增排序
            System.out.println(treeMap1.firstEntry().getKey() + " - " + treeMap1.firstEntry().getValue());
            treeMap1.remove(treeMap1.firstKey());
        }
        System.out.println("----------------------*------------------------------");
    }
}

得到结果：

----------------------*------------------------------
1 - w
2 - v
3 - u
4 - k
5 - j
6 - i
7 - h
8 - g
9 - f
10 - e
11 - d
12 - c
13 - b
14 - a
----------------------*------------------------------
a - 14
b - 13
c - 12
d - 11
e - 10
f - 9
g - 8
h - 7
i - 6
j - 5
k - 4
u - 3
v - 2
w - 1
----------------------*------------------------------

2.按照value排序

public class Main {

    public static void main(String[] args) {
        TreeMap<String, Integer> treeMap = new TreeMap<>();
        treeMap.put("s", 2);
        treeMap.put("w", 5);
        treeMap.put("d", 1);
        treeMap.put("f", 0);
        treeMap.put("h", 9);
        treeMap.put("q", 22);
        treeMap.put("a", 25);
        //按照value排序
        List<Map.Entry<String, Integer>> entryArrayList = new ArrayList<>(treeMap.entrySet());
        Collections.sort(entryArrayList, Comparator.comparing(Map.Entry::getValue));
        System.out.println("----------------------*------------------------------");
        for (Map.Entry<String, Integer> entry : entryArrayList) {
            System.out.println(entry.getKey() + " - " + entry.getValue());
        }
        System.out.println("----------------------*------------------------------");

    }
}

结果：

----------------------*------------------------------
f - 0
d - 1
s - 2
w - 5
h - 9
q - 22
a - 25
----------------------*------------------------------

注意：

List<Map.Entry<String, Integer>> entryArrayList = new ArrayList<>(treeMap.entrySet());
        Collections.sort(entryArrayList, Comparator.comparing(Map.Entry::getValue));

等价于下面的lamda：

  List<Map.Entry<String, Integer>> entryArrayList = new ArrayList<>(treeMap.entrySet());
        Collections.sort(entryArrayList, (o1, o2) -> o1.getValue().compareTo(o2.getValue()));

也等价于：

List<Map.Entry<String, Integer>> entryArrayList = new ArrayList<>(treeMap.entrySet());
        Collections.sort(entryArrayList, new Comparator<Map.Entry<String, Integer>>() {
            @Override
            public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
                return o1.getValue().compareTo(o2.getValue());
            }
        });