How to Merge Two Arrays in Java: Simple Methods Explained

To merge two arrays in Java, you can create a new array with a length equal to the sum of both arrays and copy elements from each array using System.arraycopy. Alternatively, you can use Java 8+ Streams to concatenate arrays easily.

Syntax

Here is the basic syntax to merge two arrays using System.arraycopy:

• Create a new array with length equal to the sum of the two arrays.
• Use System.arraycopy(sourceArray, sourcePos, destArray, destPos, length) to copy elements from the first array.
• Use System.arraycopy again to copy elements from the second array starting at the correct position.

For Java 8 and later, you can use Stream.concat to merge arrays:

Type[] merged = Stream.concat(Arrays.stream(array1), Arrays.stream(array2))
    .toArray(Type[]::new);

java

int[] merged = new int[array1.length + array2.length];
System.arraycopy(array1, 0, merged, 0, array1.length);
System.arraycopy(array2, 0, merged, array1.length, array2.length);

Example

This example shows how to merge two integer arrays using System.arraycopy and print the merged array.

java

import java.util.Arrays;

public class MergeArrays {
    public static void main(String[] args) {
        int[] array1 = {1, 2, 3};
        int[] array2 = {4, 5, 6};

        int[] merged = new int[array1.length + array2.length];
        System.arraycopy(array1, 0, merged, 0, array1.length);
        System.arraycopy(array2, 0, merged, array1.length, array2.length);

        System.out.println(Arrays.toString(merged));
    }
}

Output

[1, 2, 3, 4, 5, 6]

Common Pitfalls

Common mistakes when merging arrays include:

• Not creating a new array with enough length to hold both arrays.
• Copying elements to the wrong position in the new array, causing overwrites or missing elements.
• Trying to merge arrays of different types without proper handling.

Also, using loops manually to copy elements is possible but less efficient and more error-prone than System.arraycopy.

java

/* Wrong way: new array too small */
int[] mergedWrong = new int[array1.length];
System.arraycopy(array1, 0, mergedWrong, 0, array1.length);
System.arraycopy(array2, 0, mergedWrong, array1.length, array2.length); // Throws ArrayIndexOutOfBoundsException

/* Right way: new array with correct size */
int[] mergedRight = new int[array1.length + array2.length];
System.arraycopy(array1, 0, mergedRight, 0, array1.length);
System.arraycopy(array2, 0, mergedRight, array1.length, array2.length);

Quick Reference

Summary tips for merging arrays in Java:

• Use System.arraycopy for fast and safe copying.
• For Java 8+, use Stream.concat for a concise approach.
• Always create a new array with combined length.
• Use Arrays.toString() to print arrays for debugging.

Key Takeaways

Create a new array with length equal to the sum of both arrays before merging.

Use System.arraycopy for efficient copying of array elements.

Java 8+ Streams provide a concise way to merge arrays with Stream.concat.

Avoid array index errors by carefully managing copy positions.

Use Arrays.toString to easily display merged arrays.