Hi everyone,

In this post, we are going to try to solve an easy programming interview question that is likely to be asked in Java interviews. But before, I suggest you to take a look at 40+ Essential Core Java Interview Questions to refresh your memory on core concepts.

The question is:

Given two arrays, write a function to compute their intersection. The resulting array needs to contain unique integers.

There are a lot of different ways to approach this question, but I prefer to use a Set to store the intersection results. In the following code, first I check the base cases that the input arrays are null or empty. Then I create two sets: one to store the unique elements of the first array and the intersection of the input arrays. As you know, the elements stored in a Set have to be unique, and this is the reason we are using Sets in this example.

package intersection;

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

public class Solution {
    public int[] intersection(int[] nums1, int[] nums2) {
        
        if(nums1==null || nums2==null || nums1.length==0 || nums2.length==0) return new int[0];
        
        Set set = new HashSet<>();
        Set intersect = new HashSet<>();
        
        // Iterate through the first array and add the elements to the first set.
        for (int i = 0; i < nums1.length; i++) {
            set.add(nums1[i]);
        }
        
        // Iterate through the second array and whenever there is a match add those numbers to the intersect set.
        for (int i = 0; i < nums2.length; i++) {
            if (set.contains(nums2[i])) {
                intersect.add(nums2[i]);
            }
        }
        // Create and populate the result array
        int[] result = new int[intersect.size()];
        int i = 0;
        for (Integer num : intersect) {
            result[i++] = num;
        }
        return result;
    }
}

After writing the base cases and creating our empty Sets, we iterate through the first array and add the elements to the first set using a for loop. Then we iterate through the second array and whenever there is a match, add those numbers to the intersect set. Finally, we create a new result array of type int, populate this array using the intersect set and return this array.

package intersection;

import static org.junit.Assert.*;

import org.junit.BeforeClass;
import org.junit.Test;

public class TestSolution {
 private static Solution s; 
 
 @BeforeClass
 public static void setUpBeforeClass() throws Exception{
  s = new Solution();
  
 }
 @Test
 public void testIntersection1() {
  int[] nums1 = {4,8,1,2,5,12,15,3,8,121};
  int[] nums2 = {123,456,678,78,15,99,12};
  int[] result =  {12,15};
  assertArrayEquals(result,s.intersection(nums1, nums2));
 }
 // second array is empty
 @Test
 public void testIntersection2() {
  int[] nums1 = {4,8,1,2,5,12,15,3,99,121};
  int[] nums2 = {};
  int[] result =  {};
  assertArrayEquals(result,s.intersection(nums1, nums2));
 }
 // Empty arrays
 @Test
 public void testIntersection3() {
  int[] nums1 = {};
  int[] nums2 = {};
  int[] result =  {};
  assertArrayEquals(result,s.intersection(nums1, nums2));
 }
 
 //first array is empty
 @Test
 public void testIntersection4() {
  int[] nums1 = {};
  int[] nums2 = {123,456,678,78,15,99,12};
  int[] result =  {};
  assertArrayEquals(result,s.intersection(nums1, nums2));
 }
 //first array is null
 @Test
 public void testIntersection5() {
  int[] nums1 = null;
  int[] nums2 = {123,456,678,78,15,99,12};
  int[] result =  {};
  assertArrayEquals(result,s.intersection(nums1, nums2));
 }
 //arrays are null
 @Test
 public void testIntersection6() {
  int[] nums1 = null;
  int[] nums2 = null;
  int[] result =  {};
  assertArrayEquals(result,s.intersection(nums1, nums2));
 }
}

Here is the test result:



Hope this post is helpful. Please leave a comment if you have any comments or suggestions. Happy coding!