A map is an data structure class that stores associations between keys and values which are widely used in java programming language. As map stores key-value pair, we can can sort map by two ways.

1. Sort by keys
2. Sort by values

Let us see example to sort map in both ways.

1) Sort a Map by keys

There are several ways to sort a map in java. I would like to use TreeMap class to sort a unsorted map.

package com.code2succeed.sorting.map;

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

public class OrderByKey {
	public static void main(String[] args) {
		Map<Integer, String> unsortedMap = new HashMap<Integer, String>();
		unsortedMap.put(1, "a");
		unsortedMap.put(20, "c");
		unsortedMap.put(11, "e");
		unsortedMap.put(15, "b");
		unsortedMap.put(7, "d");
		
		System.out.println("Unsorted Map : "+unsortedMap);
		Map<Integer, String> sortedMap = new TreeMap<Integer, String>(unsortedMap);
		System.out.println("Sorted Map : "+sortedMap);
	}
}

Output

Unsorted Map : {1=a, 20=c, 7=d, 11=e, 15=b}
Sorted Map : {1=a, 7=d, 11=e, 15=b, 20=c}

In above example we have sorted a map which contains Integer as key and String as value. What if I have vice versa, String as key and Integer as value? The answer of this question is same use TreeMap class to sort the unsorted map.

Here is the example to sort unsorted map with String as key and Integer as value.

package com.code2succeed.sorting.map;

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

public class OrderByKey {
	public static void main(String[] args) {
		Map<String, Integer> unsortedMap = new HashMap<String, Integer>();
		unsortedMap.put("a", 1);
		unsortedMap.put("c", 20);
		unsortedMap.put("e", 11);
		unsortedMap.put("b", 15);
		unsortedMap.put("d", 7);
		
		System.out.println("Unsorted Map : "+unsortedMap);
		Map<String, Integer> sortedMap = new TreeMap<String, Integer>(unsortedMap);
		System.out.println("Sorted Map : "+sortedMap);
	}
}

Output

Unsorted Map : {d=7, e=11, b=15, c=20, a=1}
Sorted Map : {a=1, b=15, c=20, d=7, e=11}

2) Sort a Map by values

package com.code2succeed.sorting.map;

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

public class OderByValue {
	public static void main(String[] args) {
		Map<Integer, String> unsortedMap = new HashMap<Integer, String>();
		unsortedMap.put(1, "a");
		unsortedMap.put(20, "c");
		unsortedMap.put(11, "e");
		unsortedMap.put(15, "b");
		unsortedMap.put(7, "d");
		
		System.out.println("Unsorted Map: "+unsortedMap);
		
		// Convert Map to List
		List<Map.Entry<Integer, String>> list = new ArrayList<Map.Entry<Integer, String>>(unsortedMap.entrySet());
		
		// Sort list with comparator, to compare the Map values
		Collections.sort(list, new Comparator<Map.Entry<Integer, String>>() {
			public int compare(Map.Entry<Integer, String> o1,
                                           Map.Entry<Integer, String> o2) {
				return (o1.getValue()).compareTo(o2.getValue());
			}
		});
		
		// Convert sorted map back to a Map
		Map<Integer, String> sortedMap = new LinkedHashMap<Integer, String>();
		for (Iterator<Map.Entry<Integer, String>> it = list.iterator(); it.hasNext();) {
			Map.Entry<Integer, String> entry = it.next();
			sortedMap.put(entry.getKey(), entry.getValue());
		}
		
		System.out.println("Sorted Map by Value: "+sortedMap);
	}
}

Output

Unsorted Map: {1=a, 20=c, 7=d, 11=e, 15=b}
Sorted Map by Value: {1=a, 15=b, 20=c, 7=d, 11=e}