java 中对 map根据key或者value进行排序的测试

By：Roy.LiuLast updated：2016-08-16

以前在java中对map进行排序通常会自己单独定义一个方法出来处理，特别是根据 value 进行排序更是麻烦一些，但在java 8 总似乎可以用简单一点的方法处理了，直接用测试代码。

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;

public class SortByKeyExample {

    public static void main(String[] argv) {

        Map unsortMap = new HashMap<>();
        unsortMap.put("z", 10);
        unsortMap.put("b", 5);
        unsortMap.put("a", 6);
        unsortMap.put("c", 20);
        unsortMap.put("d", 1);
        unsortMap.put("e", 7);
        unsortMap.put("y", 8);
        unsortMap.put("n", 99);
        unsortMap.put("j", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

        System.out.println("Original...");
        System.out.println(unsortMap);

        Map result = new LinkedHashMap<>();

        //sort by key, a,b,c..., and put it into the "result" map
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByKey())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

        System.out.println("Sorted...");
        System.out.println(result);

    }

}

结果如下:

original...
{a=6, b=5, c=20, d=1, e=7, f=9, y=8, z=10, j=50, m=2, n=99}

Sorted...
{a=6, b=5, c=20, d=1, e=7, f=9, j=50, m=2, n=99, y=8, z=10}

当然上面是根据key进行排序的。但如果根据value进行排序呢，如何处理.

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;

public class SortByValueExample {

    public static void main(String[] argv) {

        Map unsortMap = new HashMap<>();
        unsortMap.put("z", 10);
        unsortMap.put("b", 5);
        unsortMap.put("a", 6);
        unsortMap.put("c", 20);
        unsortMap.put("d", 1);
        unsortMap.put("e", 7);
        unsortMap.put("y", 8);
        unsortMap.put("n", 99);
        unsortMap.put("j", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

        System.out.println("Original...");
        System.out.println(unsortMap);

        Map result = new LinkedHashMap<>();

        //sort by value, and reserve, 10,9,8,7,6...
        unsortMap.entrySet().stream()
                .sorted(Map.Entry.comparingByValue().reversed())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

        System.out.println("Sorted...");
        System.out.println(result);

    }

}

结果如下：

original...
{a=6, b=5, c=20, d=1, e=7, f=9, y=8, z=10, j=50, m=2, n=99}

Sorted...
{n=99, j=50, c=20, z=10, f=9, y=8, e=7, a=6, b=5, m=2, d=1}

再继续，如果要写一个通用的方法呢，按下面的处理吧:

import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Stream;

public class SortByGenericExample {

    public static void main(String[] argv) {

        Map unsortMap = new HashMap<>();
        unsortMap.put("z", 10);
        unsortMap.put("b", 5);
        unsortMap.put("a", 6);
        unsortMap.put("c", 20);
        unsortMap.put("d", 1);
        unsortMap.put("e", 7);
        unsortMap.put("y", 8);
        unsortMap.put("n", 99);
        unsortMap.put("j", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

        System.out.println("Original...");
        System.out.println(unsortMap);

        System.out.println("Sort By Key...");
        Map resultKey = compareByKey(unsortMap);
        System.out.println(resultKey);

        System.out.println("Sort By Value...");
        Map resultValue = compareByValue(unsortMap);
        System.out.println(resultValue);
    }

    //Reference from java.util.Map source code, try dig inside, many generic examples.
    public static > Map compareByValue(Map map) {

        Map result = new LinkedHashMap<>();

        Stream> mapInStream = map.entrySet().stream();

        mapInStream.sorted(Map.Entry.comparingByValue())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

        return result;

    }

    public static , V> Map compareByKey(Map map) {

        Map result = new LinkedHashMap<>();
        Stream> mapInStream = map.entrySet().stream();

        mapInStream.sorted(Map.Entry.comparingByKey())
                .forEachOrdered(x -> result.put(x.getKey(), x.getValue()));

        return result;

    }

}

结果如下：

original...
{a=6, b=5, c=20, d=1, e=7, f=9, y=8, z=10, j=50, m=2, n=99}

Sort By Key...
{a=6, b=5, c=20, d=1, e=7, f=9, j=50, m=2, n=99, y=8, z=10}

Sort By Value...
{d=1, m=2, b=5, a=6, e=7, y=8, f=9, z=10, c=20, j=50, n=99}

以上是java8 中才使用的，如果不是用的java8 怎么办，下面附录中有方法，也就是以前的传统方法:
根据key排序，利用 TreeMap:

import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class SortByKeyExample1 {

    public static void main(String[] args) {

        Map unsortMap = new HashMap();
        unsortMap.put("Z", "z");
        unsortMap.put("B", "b");
        unsortMap.put("A", "a");
        unsortMap.put("C", "c");
        unsortMap.put("D", "d");
        unsortMap.put("E", "e");
        unsortMap.put("Y", "y");
        unsortMap.put("N", "n");
        unsortMap.put("J", "j");
        unsortMap.put("M", "m");
        unsortMap.put("F", "f");

        System.out.println("Unsort Map......");
        printMap(unsortMap);

        System.out.println("\nSorted Map......By Key");
        Map treeMap = new TreeMap(unsortMap);
        printMap(treeMap);

    }

    //pretty print a map
    public static  void printMap(Map map) {
        for (Map.Entry entry : map.entrySet()) {
            System.out.println("Key : " + entry.getKey()
				+ " Value : " + entry.getValue());
        }
    }

}

如果是根据value进行排序呢, 将map转为 List, 然后对List 利用 Comparator 排序，把结果放到一个LinkedHashMap 中去, 也就是这个过程:
Map ---> List ---> Collections.sort() --> List (Sorted) ---> LinkedHashMap

import java.util.*;

public class SortByValueExample1 {

    public static void main(String[] args) {

        Map unsortMap = new HashMap();
        unsortMap.put("z", 10);
        unsortMap.put("b", 5);
        unsortMap.put("a", 6);
        unsortMap.put("c", 20);
        unsortMap.put("d", 1);
        unsortMap.put("e", 7);
        unsortMap.put("y", 8);
        unsortMap.put("n", 99);
        unsortMap.put("j", 50);
        unsortMap.put("m", 2);
        unsortMap.put("f", 9);

        System.out.println("Unsort Map......");
        printMap(unsortMap);

        System.out.println("\nSorted Map......By Value");
        Map sortedMap = sortByValue(unsortMap);
        printMap(sortedMap);

    }

    private static Map sortByValue(Map unsortMap) {

        // 1. Convert Map to List of Map
        List> list =
                new LinkedList>(unsortMap.entrySet());

        // 2. Sort list with Collections.sort(), provide a custom Comparator
        //    Try switch the o1 o2 position for a different order
        Collections.sort(list, new Comparator>() {
            public int compare(Map.Entry o1,
                               Map.Entry o2) {
                return (o1.getValue()).compareTo(o2.getValue());
            }
        });

        // 3. Loop the sorted list and put it into a new insertion order Map LinkedHashMap
        Map sortedMap = new LinkedHashMap();
        for (Map.Entry entry : list) {
            sortedMap.put(entry.getKey(), entry.getValue());
        }

        /*
        //classic iterator example
        for (Iterator> it = list.iterator(); it.hasNext(); ) {
            Map.Entry entry = it.next();
            sortedMap.put(entry.getKey(), entry.getValue());
        }*/


        return sortedMap;
    }

    public static  void printMap(Map map) {
        for (Map.Entry entry : map.entrySet()) {
            System.out.println("Key : " + entry.getKey()
                    + " Value : " + entry.getValue());
        }
    }

}

当然 sortByValue 也可以写成一个通用的方法:

public static > Map sortByValue(Map unsortMap) {

    List> list =
            new LinkedList>(unsortMap.entrySet());

    Collections.sort(list, new Comparator>() {
        public int compare(Map.Entry o1, Map.Entry o2) {
            return (o1.getValue()).compareTo(o2.getValue());
        }
    });

    Map result = new LinkedHashMap();
    for (Map.Entry entry : list) {
        result.put(entry.getKey(), entry.getValue());
    }

    return result;

}

From：一号门

Tags: map sort

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