Java Program to Find Common Elements Between Two Arrays

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]

Using Iterative Methods         

Approach: 

1. Get the two java Arrays.
2. Iterate through each and every element of the arrays one by one and check whether they are common in both.
3. Add each common element in the set for unique entries.

Java

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

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

class GFG {
    private static void FindCommonElemet(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] == arr2[j]) {

                    // add common elements
                    set.add(arr1[i]);
                    break;
                }
            }
        }
        for (String i : set) {
            System.out.print(i + " ");
        }
    }

    // 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" };

        // print Array 1
        System.out.println("Array 1: "
                           + Arrays.toString(arr1));

        // print Array 2
        System.out.println("Array 2: "
                           + Arrays.toString(arr2));

        System.out.print("Common Elements: ");

        // Find the common elements
        FindCommonElemet(arr1, arr2);
    }
}

Output

Array 1: [Article, in, Geeks, for, Geeks]
Array 2: [Geeks, for, Geeks]
Common Elements: Geeks for 

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.

Using Hashsets:

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 :

1. Get the two Arrays.
2. Create two hashsets and add elements from arrays tp those sets.
3. Find the common elements in both the sets using Collection.retainAll() method. This method keeps only the common elements of both Collection in Collection1.
4. 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("Common elements- " + 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 };

        // print Array 1
        System.out.println("Array 1: "
                           + Arrays.toString(arr1));
        // print Array 2
        System.out.println("Array 2: "
                           + Arrays.toString(arr2));
        FindCommonElements(arr1, arr2);
    }
}

Output

Array 1: [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
Array 2: [100, 9, 64, 7, 36, 5, 16, 3, 4, 1]
Common elements- [16, 64, 1, 4, 36, 100, 9]

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

Using HashSet:

Approach:

1. Add all elements of first array into a hashset.
2. 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);
            }
        }

        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] + " ");
            }
        }
    }
}

Output

1
3
4
5
6

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