Finding the common elements in two arrays.

Here’s a Java program that finds the common elements in two arrays:

import java.util.ArrayList;

import java.util.HashSet;

import java.util.List;

 import java.util.Set;

 public class CommonElementsInArrays {

    public static void main(String[] args) {

        int[] array1 = {1, 2, 3, 4, 5};

        int[] array2 = {4, 5, 6, 7, 8};

        int[] commonElements = findCommonElements(array1, array2);

        System.out.println("Common elements:");

        for (int element : commonElements) {

            System.out.print(element + " ");

        }
    }

    public static int[] findCommonElements(int[] array1, int[] array2) {

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

        for (int element : array1) {

            set1.add(element);
        }

        List<Integer> commonElementsList = new ArrayList<>();

        for (int element : array2) {

            if (set1.contains(element)) {

                commonElementsList.add(element);
          }
        }

        int[] commonElementsArray = new int[commonElementsList.size()];

        for (int i = 0; i < commonElementsList.size(); i++) {

            commonElementsArray[i] = commonElementsList.get(i);

        }

        return commonElementsArray;
    }
}

This program defines a `findCommonElements` method that takes two arrays as input and returns an array containing the common elements between the two input arrays. It uses a `HashSet` to store the elements of the first array for efficient lookup, and then iterates through the second array to check for common elements. The common elements are stored in an `ArrayList` and finally converted to an array before being returned.

In the `main` method, we provide sample input arrays `array1` and `array2`, and then call the `findCommonElements` method to find the common elements. The common elements are then printed to the console. In this example, the output will be `4 5`, as these are the elements common to both arrays.