How to Sort HashMap in Java

Java HashMap does not preserve any order by default. If there is a need to sort HashMap we sort it explicitly based on the requirements. Java provides an option to sort HashMap based on keys and values. In this section, we will learn how to sort HashMap according to keys and values.

Sort HashMap by Keys
Sort HashMap by Values

Sort HashMap by Keys

There are following ways to sort HashMap by keys:

By using TreeMap
By using LinkedHashMap

When we use LinkedHashMap, we should follow the process:

When we use LinkedHashMap, then we need to get Key set. Convert the Set into List, sort the list and then add the sorted list into LinkedHashMap in the same order. The same process we have done in the example Sort HashMap by Value.

Example of sorting HashMap by Keys

In the following example, we use TreeMap constructor to sort the elements and pass the object of HashMap class as an argument. This is the simplest way to sort HashMap by Keys.

import java.util.Map;
import java.util.HashMap;
import java.util.TreeMap;
import java.util.Iterator;
public class SortHashMapByKeys
{
public static void main(String args[])
{
//implementation of HashMap
HashMap<Integer, String> hm=new HashMap<Integer, String>();
//addding keys and values to HashMap
hm.put(23, "Yash");
hm.put(17, "Arun");
hm.put(15, "Swarit");
hm.put(9, "Neelesh");
Iterator <Integer> it = hm.keySet().iterator();		
System.out.println("Before Sorting");
while(it.hasNext())
{
int key=(int)it.next();
System.out.println("Roll no:  "+key+"     name:   "+hm.get(key));
}
System.out.println("\n");
Map<Integer, String> map=new HashMap<Integer, String>();
System.out.println("After Sorting");
//using TreeMap constructor to sort the HashMap
TreeMap<Integer,String> tm=new  TreeMap<Integer,String> (hm);
Iterator itr=tm.keySet().iterator();			 
while(itr.hasNext())  
{  
int key=(int)itr.next();
System.out.println("Roll no:  "+key+"     name:   "+hm.get(key));
}  
}
}

Output:

Before Sorting
Roll no:  17     name:   Arun
Roll no:  23     name:   Yash
Roll no:  9     name:   Neelesh
Roll no:  15     name:   Swarit

After Sorting
Roll no:  9     name:   Neelesh
Roll no:  15     name:   Swarit
Roll no:  17     name:   Arun
Roll no:  23     name:   Yash

Sort HashMap by Values using Comparator Interface

In Java, sorting HashMap by values is complicated because there is no direct method is available. To sort the HashMap by values, we need to create a Comparator. It compares two elements based on the values.

After that get the Set of elements from the Map and convert Set into the List. Use the Collections.sort(List) method to sort the list of elements by values by passing customized comparator. Now create a new LinkedHashMap and copy the sorted elements into that. Since LinkedHashMap guarantees the insertion order of mappings. We get a HashMap whose values are in sorted order.

There is a slight difference between sorting HashMap by Keys and Values is that it can have duplicate values but not duplicate Keys. We cannot use TreeMap to sort values because TreeMap sorts elements by Keys.

Example of sort HashMap by values

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;
import java.util.Set;
public class SortHashMapValue 
{
public static void main(String[] args) 
{
//implementing HashMap
HashMap<Integer, String> hm = new HashMap<Integer, String>();
hm.put(6, "Tushar");
hm.put(12, "Ashu");
hm.put(5, "Zoya");
hm.put(78, "Yash");
hm.put(10, "Praveen");
hm.put(67, "Boby");
hm.put(1, "Ritesh");
System.out.println("Before Sorting:");
Set set = hm.entrySet();
Iterator iterator = set.iterator();
while(iterator.hasNext()) 
{
Map.Entry map = (Map.Entry)iterator.next();
System.out.println("Roll no:  "+map.getKey()+"     Name:   "+map.getValue());
}
Map<Integer, String> map = sortValues(hm); 
System.out.println("\n");
System.out.println("After Sorting:");
Set set2 = map.entrySet();
Iterator iterator2 = set2.iterator();
while(iterator2.hasNext()) 
{
Map.Entry me2 = (Map.Entry)iterator2.next();
System.out.println("Roll no:  "+me2.getKey()+"     Name:   "+me2.getValue());
}
}
//method to sort values
private static HashMap sortValues(HashMap map) 
{ 
List list = new LinkedList(map.entrySet());
//Custom Comparator
Collections.sort(list, new Comparator() 
{
public int compare(Object o1, Object o2) 
{
return ((Comparable) ((Map.Entry) (o1)).getValue()).compareTo(((Map.Entry) (o2)).getValue());
}
});
//copying the sorted list in HashMap to preserve the iteration order
HashMap sortedHashMap = new LinkedHashMap();
for (Iterator it = list.iterator(); it.hasNext();) 
{
 Map.Entry entry = (Map.Entry) it.next();
sortedHashMap.put(entry.getKey(), entry.getValue());
} 
return sortedHashMap;
}
}

Output:

Before Sorting:
Roll no:  1     Name:   Ritesh
Roll no:  67   Name:   Boby
Roll no:  5     Name:   Zoya
Roll no:  6     Name:   Tushar
Roll no:  10   Name:   Praveen
Roll no:  12   Name:   Ashu
Roll no:  78   Name:   Yash

After Sorting:
Roll no:  12     Name:   Ashu
Roll no:  67     Name:   Boby
Roll no:  10     Name:   Praveen
Roll no:  1       Name:   Ritesh
Roll no:  6       Name:   Tushar
Roll no:  78     Name:   Yash
Roll no:  5       Name:   Zoya

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