We all know that HashMap will not save key-value pairs in any sort of order neither preserve the insertion order. In this tutorial we will learn how to sort a HashMap based on Keys. We will be using two approaches.
- TreeMap Collection class (Which has in-built support for sorting the elements using Comparable and Comparator Interface)
- Implementing the Comparator Interface
- Using Collections.sort() method
1. HashMap Sorting by Keys Example – Using TreeMap
In this example we will sort the keys of the HashMap using TreeMap. Sorting a HashMap keys using a TreeMap is very simple, simply add the unsorted hashMap(unsortedMap) to the TreeMap to get it sorted.
import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;
public class SortByKeysHashMapExample
{
public static void main(String[] args) {
Map<Integer, String> unsortedMap = new HashMap<Integer, String>();
unsortedMap.put(5, "asd");
unsortedMap.put(1, "cfd");
unsortedMap.put(7, "gdf");
unsortedMap.put(55, "qwe");
unsortedMap.put(66, "weq");
unsortedMap.put(3, "wer");
unsortedMap.put(8, "yes");
unsortedMap.put(93, "nsa");
unsortedMap.put(50, "tes");
unsortedMap.put(12, "mds");
unsortedMap.put(43, "fsa");
//Print the Elements of the Map before Sorting
System.out.println("Elements of the HashMap before Sorting");
printMap(unsortedMap);
//Create a Treemap of unsortedMap to get it sorted
Map<Integer,String> sortedMap = new TreeMap<Integer,String>(unsortedMap);
//Print the Elements of the Map after Sorting
System.out.println("Elements of the HashMap after Sorting");
printMap(sortedMap);
}
public static void printMap(Map<Integer, String> map) {
System.out.println("**************************************");
for (Map.Entry<Integer, String> entry : map.entrySet()) {
System.out.println("Key : " + entry.getKey()
+ " Value : " + entry.getValue());
}
System.out.println();
}
}
Output :
Elements of the HashMap before Sorting ************************************** Key : 50 Value : tes Key : 1 Value : cfd Key : 3 Value : wer Key : 55 Value : qwe Key : 5 Value : asd Key : 66 Value : weq Key : 7 Value : gdf Key : 93 Value : nsa Key : 8 Value : yes Key : 43 Value : fsa Key : 12 Value : mds Elements of the HashMap after Sorting ************************************** Key : 1 Value : cfd Key : 3 Value : wer Key : 5 Value : asd Key : 7 Value : gdf Key : 8 Value : yes Key : 12 Value : mds Key : 43 Value : fsa Key : 50 Value : tes Key : 55 Value : qwe Key : 66 Value : weq Key : 93 Value : nsa
2. HashMap Sorting by Keys Example – Using TreeMap and Comparator
We will be overriding the compare() method of the Comparator
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;
public class SortByKeysHashMapExample
{
public static void main(String[] args) {
Map<Integer, String> unsortedMap = new HashMap<Integer, String>();
unsortedMap.put(5, "asd");
unsortedMap.put(1, "cfd");
unsortedMap.put(7, "gdf");
unsortedMap.put(55, "qwe");
unsortedMap.put(66, "weq");
unsortedMap.put(3, "wer");
unsortedMap.put(8, "yes");
unsortedMap.put(93, "nsa");
unsortedMap.put(50, "tes");
unsortedMap.put(12, "mds");
unsortedMap.put(43, "fsa");
//Print the Elements of the Map before Sorting
System.out.println("Elements of the HashMap before Sorting");
printMap(unsortedMap);
Map<Integer,String> sortedMap =
new TreeMap<Integer,String>(new Comparator<Integer>()
{
@Override
public int compare(Integer i1, Integer i2)
{
return i1.compareTo(i2);
}
}
);
sortedMap.putAll(unsortedMap);
//Print the Elements of the Map after Sorting
System.out.println("Elements of the HashMap after Sorting");
printMap(sortedMap);
}
public static void printMap(Map<Integer, String> map) {
System.out.println("**************************************");
for (Map.Entry<Integer, String> entry : map.entrySet()) {
System.out.println("Key : " + entry.getKey()
+ " Value : " + entry.getValue());
}
System.out.println();
}
}
Output :
Elements of the HashMap before Sorting ************************************** Key : 50 Value : tes Key : 1 Value : cfd Key : 3 Value : wer Key : 55 Value : qwe Key : 5 Value : asd Key : 66 Value : weq Key : 7 Value : gdf Key : 93 Value : nsa Key : 8 Value : yes Key : 43 Value : fsa Key : 12 Value : mds Elements of the HashMap after Sorting ************************************** Key : 1 Value : cfd Key : 3 Value : wer Key : 5 Value : asd Key : 7 Value : gdf Key : 8 Value : yes Key : 12 Value : mds Key : 43 Value : fsa Key : 50 Value : tes Key : 55 Value : qwe Key : 66 Value : weq Key : 93 Value : nsa
3. HashMap Sorting by Keys Example – Collections.sort() method
In this approach we will be getting the EntrySet and store it to a List(sortedList)and pass the list along with the comparator to Collections.sort() method. Finally add the sortedList to the LinkedHashMap(sortedMap) as it will maintain the insertion order.
package com.javainterviewpoint.HashMap;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
public class SortByKeysHashMapExample
{
public static void main(String[] args) {
Map<Integer, String> unsortedMap = new HashMap<Integer, String>();
unsortedMap.put(5, "asd");
unsortedMap.put(1, "cfd");
unsortedMap.put(7, "gdf");
unsortedMap.put(55, "qwe");
unsortedMap.put(66, "weq");
unsortedMap.put(3, "wer");
unsortedMap.put(8, "yes");
unsortedMap.put(93, "nsa");
unsortedMap.put(50, "tes");
unsortedMap.put(12, "mds");
unsortedMap.put(43, "fsa");
//Print the Elements of the Map before Sorting
System.out.println("Elements of the HashMap before Sorting");
printMap(unsortedMap);
List<Entry<Integer,String>> unsortedList = new ArrayList<Entry<Integer,String>>(unsortedMap.entrySet());
Collections.sort(unsortedList,new Comparator<Entry<Integer,String>>()
{
@Override
public int compare(Entry<Integer,String> e1,Entry<Integer,String> e2)
{
return e1.getKey().compareTo(e2.getKey());
}
}
);
Map<Integer,String> sortedMap = new LinkedHashMap<Integer,String>();
for(Entry<Integer,String> entry:unsortedList){
sortedMap.put(entry.getKey(),entry.getValue());
}
//Print the Elements of the Map after Sorting
System.out.println("Elements of the HashMap after Sorting");
printMap(sortedMap);
}
public static void printMap(Map<Integer, String> map) {
System.out.println("**************************************");
for (Map.Entry<Integer, String> entry : map.entrySet()) {
System.out.println("Key : " + entry.getKey()
+ " Value : " + entry.getValue());
}
System.out.println();
}
}
Output :
Elements of the HashMap before Sorting ************************************** Key : 50 Value : tes Key : 1 Value : cfd Key : 3 Value : wer Key : 55 Value : qwe Key : 5 Value : asd Key : 66 Value : weq Key : 7 Value : gdf Key : 93 Value : nsa Key : 8 Value : yes Key : 43 Value : fsa Key : 12 Value : mds Elements of the HashMap after Sorting ************************************** Key : 1 Value : cfd Key : 3 Value : wer Key : 5 Value : asd Key : 7 Value : gdf Key : 8 Value : yes Key : 12 Value : mds Key : 43 Value : fsa Key : 50 Value : tes Key : 55 Value : qwe Key : 66 Value : weq Key : 93 Value : nsa
How to Sort HashMap having Object Keys ?
We have learnt how to sort Wrapper Objects, but in the real world situations you will be in a situation to sort object based on the particular attribute lets now see how we can achieve this.
Lets take a class Car which has two attributes color and wheels, we implement sorting of Car object based on wheels attribute
Car.java
public class Car
{
private String color;
private Integer wheels;
public Car(String color, int wheels) {
this.color = color;
this.wheels = wheels;
}
public String getColor() {
return color;
}
public Integer getWheels() {
return wheels;
}
@Override
public String toString()
{
return ""+color+""+wheels;
}
}
SortObjectKeyHashMapExample.java
import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.TreeMap;
public class SortObjectKeyHashMapExample
{
public static void main(String args[])
{
Map<Car,String> unsortedMap = new HashMap<Car,String>();
Car c1 = new Car("Red",3);
Car c2 = new Car("Blue",1);
Car c3 = new Car("Green",4);
Car c4 = new Car("Yellow",2);
unsortedMap.put(c1, "Red Car");
unsortedMap.put(c2, "Blue Car");
unsortedMap.put(c3, "Green Car");
unsortedMap.put(c4, "Yellow Car");
//Print the Elements of the Map before Sorting
System.out.println("Elements of the HashMap before Sorting");
printMap(unsortedMap);
Map<Car,String> sortedMap = new TreeMap<Car,String>(
new Comparator()
{
@Override
public int compare(Car c1,Car c2)
{
return c1.getWheels().compareTo(c2.getWheels());
}
});
sortedMap.putAll(unsortedMap);
//Print the Elements of the Map after Sorting
System.out.println("Elements of the HashMap after Sorting");
printMap(sortedMap);
}
public static void printMap(Map<Car, String> map) {
System.out.println("**************************************");
for (Entry<Car, String> entry : map.entrySet()) {
System.out.println("Key : " + entry.getKey()
+ " Value : " + entry.getValue());
}
System.out.println();
}
}
Output :
Elements of the HashMap before Sorting ************************************** Key : Blue1 Value : Blue Car Key : Yellow2 Value : Yellow Car Key : Green4 Value : Green Car Key : Red3 Value : Red Car Elements of the HashMap after Sorting ************************************** Key : Blue1 Value : Blue Car Key : Yellow2 Value : Yellow Car Key : Red3 Value : Red Car Key : Green4 Value : Green Car
Note :
If you want the HashMap keys to be sorted in Descending (Reverse) order just reverse the condition like below
return c2.getWheels().compareTo(c1.getWheels());
What if key is String?
The code should work fine, even if the key is a String