Combine arrays

Two arrays exist. We can combine them into a third array. We can do this in different ways. Some are more efficient, and others have simpler syntax.

By creating another array, we can copy all elements into a single region of storage. For-loops can be used to quickly copy these elements.

For-loop example

Let us begin with this example. We have two arrays—they can have any number of elements, but in this example they both have 6 elements.

import java.util.Arrays;

public class Program {
    public static void main(String[] args) {

        int[] array1 = { 1, 2, 3, 4, 5 };
        int[] array2 = { 5, 4, 3, 2, 1 };

        // Create empty array of required length.
        int[] merged = new int[array1.length + array2.length];

        // Copy first array into new array.
        for (int i = 0; i < array1.length; i++) {
            merged[i] = array1[i];
        }
        // Copy second array into new array.
        // ... Use offset to assign elements.
        for (int i = 0; i < array2.length; i++) {
            merged[array1.length + i] = array2[i];
        }

        // Print results.
        System.out.println(Arrays.toString(array1));
        System.out.println(Arrays.toString(array2));
        System.out.println(Arrays.toString(merged));
    }
}
[1, 2, 3, 4, 5]
[5, 4, 3, 2, 1]
[1, 2, 3, 4, 5, 5, 4, 3, 2, 1]

Collections.addAll

This approach is a little simpler. We create an ArrayList of the required element type. Then we use Collections.addAll to add both arrays to it.

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;

public class Program {
    public static void main(String[] args) {

        // Two string arrays we want to combine.
        String[] array1 = { "cat", "bird", "fish" };
        String[] array2 = { "ant", "bee" };

        System.out.println(Arrays.toString(array1));
        System.out.println(Arrays.toString(array2));

        // Create an ArrayList.
        // ... Add all string arrays with addAll.
        ArrayList<String> list = new ArrayList<>();
        Collections.addAll(list, array1);
        Collections.addAll(list, array2);

        // Display ArrayList contents.
        System.out.println(list.toString());

        // Convert ArrayList to String array.
        // ... This is the final merged array.
        String[] merged = new String[list.size()];
        list.toArray(merged);

        System.out.println(Arrays.toString(merged));
    }
}
[cat, bird, fish]
[ant, bee]
[cat, bird, fish, ant, bee]
[cat, bird, fish, ant, bee]

Notes, Collections.addAll

With Collections.addAll, we cannot add int arrays to an ArrayList of Integers. For int arrays, using for-loops is a better solution.

With for-loops or an ArrayList and Collections.addAll we can combine arrays. For more than two arrays, Collections.addAll may be simpler—fewer offsets will be needed.