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Showing content from https://www.geeksforgeeks.org/java/java-program-to-find-common-elements-between-two-arrays/ below:

Java Program to Find Common Elements Between Two Arrays

Last Updated : 23 Jul, 2025

Try it on GfG Practice

Given two arrays and our task is to find their common elements.

Examples:

Input: Array1 = ["Article", "for", "Geeks", "for", "Geeks"],
Array2 = ["Article", "Geeks", "Geeks"]
Output: [Article, Geeks]

Input: Array1 = ["a", "b", "c", "d", "e", "f"],
Array2 = ["b", "d", "e", "h", "g", "c"]
Output: [b, c, d, e]

Example to Find Common Elements Between Two Arrays Java


 // Java Program to find common elements
// in two Arrays
// Using iterative method

import java.io.*;
import java.util.*;

// Driver Class
class GFG {
  	
  	// Function to Find Common Elements 
    private static void FindCommonElement(String[] arr1,String[] arr2)
    {
        Set<String> set = new HashSet<>();
        for (int i = 0; i < arr1.length; i++) {
            for (int j = 0; j < arr2.length; j++) {
                if (arr1[i].equals(arr2[j])) {

                    // add common elements
                    set.add(arr1[i]);
                    break;
                }
            }
        }
      
      	// Common Elements in Arrays
      	System.out.println(set.toString());
    }

    // Main Method
    public static void main(String[] args)
    {

        // Create Array 1
        String[] arr1 = { "Article", "in", "Geeks", "for", "Geeks" };

        // Create Array 2
        String[] arr2 = { "Geeks", "for", "Geeks" };

        // Find the Common Elements
        FindCommonElement(arr1, arr2);
    }
}

Below is the explanation of the above Method:

Get the Two Java Arrays.
Iterate through each and every element of the arrays one by one and check whether they are common in both.
Add each common element in the set for unique entries.

Complexity of the above Method:

Time Complexity: O(m * n) where m and n are lengths of the input arrays.
Auxiliary Space: O(d) where d is the number of common elements.

Other Methods to Find the Common Elements Between Arrays 1. Using retainAll() method of HashSet

By using the retainAll() method of the HashSet we can find the common elements between two arrays.

Syntax:

// This method keeps only the common elements
// of both Collection in Collection1.
Collections1.retainAll(Collections2)

Approach :

Get the two Arrays.
Create two hashsets and add elements from arrays tp those sets.
Find the common elements in both the sets using Collection.retainAll() method. This method keeps only the common elements of both Collection in Collection1.
Set 1 now contains the common elements only.

Below is the implementation of the above approach:

Java


 // Java Program to find common elements
// in two Arrays using hashsets
// and retainAll() method
import java.io.*;
import java.util.*;

class GFG {

    // function to create hashsets
    // from arrays and find
    // their common element
    public static void FindCommonElements(int[] arr1,
                                          int[] arr2)
    {
        // create hashsets
        Set<Integer> set1 = new HashSet<>();
        Set<Integer> set2 = new HashSet<>();

        // Adding elements from array1
        for (int i : arr1) {
            set1.add(i);
        }

        // Adding elements from array2
        for (int i : arr2) {
            set2.add(i);
        }

        // use retainAll() method to
        // find common elements
        set1.retainAll(set2);
        System.out.println(set1);
    }

    // main method
    public static void main(String[] args)
    {
        // create Array 1
        int[] arr1 = { 1, 4, 9, 16, 25, 36, 49, 64, 81, 100 };

        // create Array 2
        int[] arr2 = { 100, 9, 64, 7, 36, 5, 16, 3, 4, 1 };

        FindCommonElements(arr1, arr2);
    }
}

Output

[16, 64, 1, 4, 36, 100, 9]

Complexity of the above method:

Time Complexity: O(m + n)
Auxiliary Space: O(m + n)

2. Using HashSet

Approach:

Add all elements of first array into a HashSet.
Iterate the second array and check whether element present in HashSet using contains method. If contains == true, add the element to result in array.

Below is the implementation of the above approach:

Java


 import java.util.HashMap;

public class CommonElementsOfArrays {
    public static void main(String[] args)
    {
        int[] arr1 = new int[] { 1, 2, 3, 4, 5, 6, 7 };
        int[] arr2 = new int[] { 1, 3, 4, 5, 6, 9, 8 };
        findCommonElements(arr1, arr2);
        /* expected output {1,3,4,5,6}
           coming from the above written code 
           that is right if not please correct me
        */
    }

    public static void findCommonElements(int arr1[],
                                          int arr2[])
    {
        HashMap<Integer, Integer> hashMap = new HashMap<>();
        for (int i = 0; i < arr1.length; i++) {
            if (hashMap.containsKey(arr1[i])) {
                hashMap.put(arr1[i],
                            hashMap.get(arr1[i]) + 1);
            }
            else {
                hashMap.put(arr1[i], 1);
            }
        }

      	System.out.print("[ ");
        
      	for (int i = 0; i < arr2.length; i++) {
            
          	if (hashMap.containsKey(arr2[i])) {
               	hashMap.remove(arr2[i]);
                
              	// remove common elements from hashmap
               	// to avoid duplicates*/
                System.out.print(arr2[i] + " ");
              
            }
          	
        }
            	
      	System.out.print("]");
      
    }
}

Complexity of the above Method:

Time Complexity: O(m + n)
Auxiliary Space: O(m)

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