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How to sort a map using value (<key, value>) in Java?

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How to sort a map using value (<key, value>) in Java?
posted Mar 25, 2016 by anonymous

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1 Answer

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1. Sort a Map by Keys

To sort a Map by its keys, uses TreeMap, it sort Map keys automatically.

1.1 Review a “Map keys” in “String” example.

SortMapOnKeyStringExample.java

package com.mkyong;

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

public class SortMapOnKeyStringExample {

    public static void main(String[] args) {

        Map<String, String> unsortMap = new HashMap<String, String>();
        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......");
        Map<String, String> treeMap = new TreeMap<String, String>(unsortMap);
        printMap(treeMap);

    }

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

}

OUTPUT:

Unsort Map......
Key : D Value : d
Key : E Value : e
Key : F Value : f
Key : A Value : a
Key : B Value : b
Key : C Value : c
Key : M Value : m
Key : N Value : n
Key : Y Value : y
Key : J Value : j
Key : Z Value : z

OUT PUT:

Sorted Map......
Key : A Value : a
Key : B Value : b
Key : C Value : c
Key : D Value : d
Key : E Value : e
Key : F Value : f
Key : J Value : j
Key : M Value : m
Key : N Value : n
Key : Y Value : y
Key : Z Value : z

1.2 Review a “Map keys” in “Integer” example.

SortMapOnKeyIntegerExample.java

package com.mkyong;

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

public class SortMapOnKeyIntegerExample {

    public static void main(String[] args) {

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

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

        System.out.println("\nSorted Map......");
        Map<Integer, String> treeMap = new TreeMap<Integer, String>(
            new Comparator<Integer>() {

            @Override
            public int compare(Integer o1, Integer o2) {
                return o2.compareTo(o1);
            }

        });
        treeMap.putAll(unsortMap);

        printMap(treeMap);

    }

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

}

OUTPUT:

Unsort Map......
Key : 50 Value : j
Key : 1 Value : d
Key : 2 Value : m
Key : 99 Value : n
Key : 20 Value : c
Key : 5 Value : b
Key : 6 Value : a
Key : 7 Value : e
Key : 8 Value : y
Key : 9 Value : f
Key : 10 Value : z

Sorted Map......
Key : 99 Value : n
Key : 50 Value : j
Key : 20 Value : c
Key : 10 Value : z
Key : 9 Value : f
Key : 8 Value : y
Key : 7 Value : e
Key : 6 Value : a
Key : 5 Value : b
Key : 2 Value : m
Key : 1 Value : d
By default, it will sorts the “Integer” keys by ascending order, 0-10. In this example, we add a Comparator to TreeMap and make it sorts keys by descending order.

2. Sort a Map by Values

TreeMap is unable to sort the Map values, instead, we should use Comparator.

The overall idea is, converts the Map into a List, sorts the List by Comparator and put the sorted list back to a Map.

Map ---> List ---> Sort --> SortedList ---> Map

2.1 Review a Map values in “Integer” example.

SortMapOnValueIntegerExample.java

package com.hostingcompass.jobs;

import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

public class SortMapOnValueIntegerExample {

    public static void main(String[] args) {

        Map<String, Integer> unsortMap = new HashMap<String, Integer>();
        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......");
        Map<String, Integer> sortedMap = sortByComparator(unsortMap);
        printMap(sortedMap);

    }

    private static Map<String, Integer> sortByComparator(Map<String, Integer> unsortMap) {

        // Convert Map to List
        List<Map.Entry<String, Integer>> list = 
            new LinkedList<Map.Entry<String, Integer>>(unsortMap.entrySet());

        // Sort list with comparator, to compare the Map values
        Collections.sort(list, new Comparator<Map.Entry<String, Integer>>() {
            public int compare(Map.Entry<String, Integer> o1,
                                           Map.Entry<String, Integer> o2) {
                return (o1.getValue()).compareTo(o2.getValue());
            }
        });

        // Convert sorted map back to a Map
        Map<String, Integer> sortedMap = new LinkedHashMap<String, Integer>();
        for (Iterator<Map.Entry<String, Integer>> it = list.iterator(); it.hasNext();) {
            Map.Entry<String, Integer> entry = it.next();
            sortedMap.put(entry.getKey(), entry.getValue());
        }
        return sortedMap;
    }

    public static void printMap(Map<String, Integer> map) {
        for (Map.Entry<String, Integer> entry : map.entrySet()) {
            System.out.println("[Key] : " + entry.getKey() 
                                      + " [Value] : " + entry.getValue());
        }
    }

}

Output

Unsort Map......
Sorted Map......

answer Apr 6, 2016 by Karthick.c
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