Java Program to Find Common Elements Between Two Arrays

Given two arrays, the task is to find the elements that are present in both arrays. Each common element is included only once in the result.

• Find elements that are present in both arrays.
• HashSet can be used to store and find common elements efficiently.

Example

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]

Different Approaches to Find Common Elements

1. Using Nested Loops

In this approach, each element of the first array is compared with every element of the second array. When a match is found, the element is added to a HashSet.

import java.util.HashSet;
import java.util.Set;

public class Geeks {

    public static void findCommonElements(String[] arr1, String[] arr2) {

        Set<String> common = new HashSet<>();

        // Compare each element of arr1 with arr2
        for (String element1 : arr1) {
            for (String element2 : arr2) {

                if (element1.equals(element2)) {
                    common.add(element1);
                    break;
                }
            }
        }

        System.out.println(common);
    }

    public static void main(String[] args) {

        String[] arr1 = {"Article", "in", "Geeks", "for", "Geeks"};
        String[] arr2 = {"Geeks", "for", "Geeks"};

        findCommonElements(arr1, arr2);
    }
}

[Geeks, for]

Explanation

• Two arrays are created.
• The outer loop selects each element from arr1.
• The inner loop compares it with every element of arr2.
• If both elements are equal, the element is added to the HashSet.
• HashSet automatically removes duplicate entries.
• break avoids unnecessary comparisons after a match is found.

2. Using HashSet and retainAll()

The retainAll() method keeps only the elements that are common to two collections.

Approach

• Add the elements of the first array to a HashSet.
• Add the elements of the second array to another HashSet.
• Call retainAll() on the first set.
• The first set now contains only the common elements.

import java.util.HashSet;
import java.util.Set;

public class Geeks {

    public static void findCommonElements(int[] arr1, int[] arr2) {

        Set<Integer> set1 = new HashSet<>();
        Set<Integer> set2 = new HashSet<>();

        // Add elements of the first array
        for (int value : arr1) {
            set1.add(value);
        }

        // Add elements of the second array
        for (int value : arr2) {
            set2.add(value);
        }

        // Keep only common elements
        set1.retainAll(set2);

        System.out.println(set1);
    }

    public static void main(String[] args) {

        int[] arr1 = {1, 4, 9, 16, 25, 36, 49, 64, 81, 100};
        int[] arr2 = {100, 9, 64, 7, 36, 5, 16, 3, 4, 1};

        findCommonElements(arr1, arr2);
    }
}

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

Explanation

• set1 stores the unique elements of the first array.
• set2 stores the unique elements of the second array.
• set1.retainAll(set2) removes every element from set1 that is not present in set2.
• Therefore, set1 contains only the common elements.

Note: HashSet does not guarantee insertion order, so the order of elements in the output may vary.

3. Using HashSet and contains()

This approach uses a HashSet to efficiently check whether elements of the second array are present in the first array.

Approach

• Add all elements of the first array to a HashSet.
• Traverse the second array.
• Use contains() to check whether the current element exists in the set.
• Add matching elements to a result set.
• The result set prevents duplicate common elements.

import java.util.HashSet;
import java.util.Set;

public class Geeks {

    public static void findCommonElements(int[] arr1, int[] arr2) {

        Set<Integer> set = new HashSet<>();
        Set<Integer> common = new HashSet<>();

        // Add elements of the first array
        for (int value : arr1) {
            set.add(value);
        }

        // Find elements common to both arrays
        for (int value : arr2) {
            if (set.contains(value)) {
                common.add(value);
            }
        }

        System.out.println(common);
    }

    public static void main(String[] args) {

        int[] arr1 = {1, 2, 3, 4, 5, 6, 7};
        int[] arr2 = {1, 3, 4, 5, 6, 9, 8};

        findCommonElements(arr1, arr2);
    }
}

[1, 3, 4, 5, 6]

Explanation

• A HashSet stores all unique elements from arr1.
• The second array is traversed one element at a time.
• contains() checks whether the current element exists in the first array.
• If it exists, it is added to the common set.
• Since common is a HashSet, duplicate elements are stored only once.