How to compare two hashmaps in Java

Learn to compare two hashmaps in Java by keys, values and key-value pairs. Also learn to compare while allowing or restricting duplicate values.

1. Compare hashmap for same key-values – HashMap.equals()

By default, HashMap.equals() method compares two hashmaps by key-value pairs. It means both hashmap instances must have exactly same key-value pairs and both must be of same size.

The order of key-value pairs can be different and does not play in role in comparison.

import java.util.HashMap;

public class HashMapExample 
{
    public static void main(String[] args) throws CloneNotSupportedException 
    {
        HashMap<Integer, String> map1 = new HashMap<>();

        map1.put(1, "A");
        map1.put(2, "B");
        map1.put(3, "C");
        
        //Same as map1
        HashMap<Integer, String> map2 = new HashMap<>();

 		map2.put(3, "C");
        map2.put(1, "A");
        map2.put(2, "B");
        
        //Different from map1
        HashMap<Integer, String> map3 = new HashMap<>();

        map3.put(1, "A");
        map3.put(2, "B");
        map3.put(3, "C");
        map3.put(3, "D");
        
        System.out.println(map1.equals(map2));  //true
        System.out.println(map1.equals(map3));  //false
    }
}

Program output.

true
false

2. Compare two hashmaps for same keys – HashMap.keySet()

2.1. Are both hashmaps equal?

If we want to compare hashmaps by keys i.e. two hashmaps will be equals if they have exactly same set of keys, we can use HashMap.keySet() function. It returns all the map keys in HashSet.

We can compare the hashset of keys for both maps using Set.equals() method. It returns true if the two sets have the same size, and every element of the specified set is contained in another set.

import java.util.HashMap;

public class HashMapExample 
{
    public static void main(String[] args) throws CloneNotSupportedException 
    {
        HashMap<Integer, String> map1 = new HashMap<>();

        map1.put(1, "A");
        map1.put(2, "B");
        map1.put(3, "C");
        
        //Same keys as map1	
        HashMap<Integer, String> map2 = new HashMap<>();

 		map2.put(3, "C");
        map2.put(1, "A");
        map2.put(2, "B");
        
        //Different keys than map1
        HashMap<Integer, String> map3 = new HashMap<>();

        map3.put(1, "A");
        map3.put(2, "B");
        map3.put(3, "C");
        map3.put(3, "D");
        
        System.out.println( map1.keySet().equals( map2.keySet() ));  //true
        System.out.println( map1.keySet().equals( map3.keySet() ));  //false
    }
}

Program output.

true
false

2.2. Find out extra keys

We may be interested in finding out what extra keys first hashmap has than second hashmap. So get this difference, first do a union of keys from both hashmaps, and then remove all keys present in first hashmap.

Java program to find out the difference between two hashmaps.

//map 1 has 3 keys
HashMap<Integer, String> map1 = new HashMap<>();

map1.put(1, "A");
map1.put(2, "B");
map1.put(3, "C");

//map 2 has 4 keys
HashMap<Integer, String> map2 = new HashMap<>();

map2.put(1, "A");
map2.put(2, "B");
map2.put(3, "C");
map2.put(4, "C");

//Union of keys from both maps
HashSet<Integer> unionKeys = new HashSet<>(map1.keySet());
unionKeys.addAll(map2.keySet());

unionKeys.removeAll(map1.keySet());

System.out.println(unionKeys);

Program output.

[4]

3. Compare hashmaps for values – HashMap.values()

If we want to compare hashmaps by values i.e. two hashmaps will be equals if they have exactly same set of values. Please note that HashMap allows duplicate values, so decide if you want to compare hashmaps with duplicate or without duplicate values.

3.1. Duplicates are NOT allowed

Add all values from HashMap.values() to an arraylist for both maps. Now compare both arraylists for equality.

HashMap<Integer, String> map1 = new HashMap<>();

map1.put(1, "A");
map1.put(2, "B");
map1.put(3, "C");

//Same values as map1
HashMap<Integer, String> map2 = new HashMap<>();

map2.put(4, "A");
map2.put(5, "B");
map2.put(6, "C");

//Different values than map1 - C is added twice
HashMap<Integer, String> map3 = new HashMap<>();

map3.put(1, "A");
map3.put(2, "B");
map3.put(3, "C");
map3.put(4, "C");

System.out.println( new ArrayList<>( map1.values() ).equals(new ArrayList<>( map2.values() )) );       //true
System.out.println( new ArrayList<>( map1.values() ).equals(new ArrayList<>( map3.values() )) );       //false

Program output.

true
false

3.2. Duplicates are allowed

If you want to remove duplicate values before comparing the hashmaps, the add all values into a HashSet which automatically ignores duplicate values.

HashMap<Integer, String> map1 = new HashMap<>();

map1.put(1, "A");
map1.put(2, "B");
map1.put(3, "C");

//Same values as map1
HashMap<Integer, String> map2 = new HashMap<>();

map2.put(4, "A");
map2.put(5, "B");
map2.put(6, "C");

//Duplicate values  - C is added twice
HashMap<Integer, String> map3 = new HashMap<>();

map3.put(1, "A");
map3.put(2, "B");
map3.put(3, "C");
map3.put(4, "C");

System.out.println( new ArrayList<>( map1.values() ).equals(new ArrayList<>( map2.values() )) );       //true
System.out.println( new ArrayList<>( map1.values() ).equals(new ArrayList<>( map3.values() )) );       //true

Program output.

true
true

Drop me your questions related to comparing hashmaps in Java.

Happy Learning !!

Read More :

A Guide to Java HashMap
HashMap Java Docs