How can I concatenate two arrays in Java?

In Java, you can concatenate two arrays using several methods. Here are the most common approaches:

1. Using `System.arraycopy` (All Java Versions)

This method efficiently copies elements from both arrays into a new array.

public static int[] concatenate(int[] array1, int[] array2) {
    int[] result = new int[array1.length + array2.length];
    System.arraycopy(array1, 0, result, 0, array1.length);
    System.arraycopy(array2, 0, result, array1.length, array2.length);
    return result;
}

// Example usage:
int[] array1 = {1, 2, 3};
int[] array2 = {4, 5};
int[] concatenated = concatenate(array1, array2); // [1, 2, 3, 4, 5]

Works for any array type (e.g., String[], Object[]):

public static <T> T[] concatenate(T[] array1, T[] array2) {
    T[] result = Arrays.copyOf(array1, array1.length + array2.length);
    System.arraycopy(array2, 0, result, array1.length, array2.length);
    return result;
}

2. Using Java 8+ Streams

Concatenate arrays using the Stream API for a functional approach.

For primitive arrays (e.g., int[]):

int[] concatenated = IntStream.concat(
    Arrays.stream(array1), 
    Arrays.stream(array2)
).toArray();

For object arrays (e.g., String[]):

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

3. Using `Arrays.copyOf` (Combine with `arraycopy`)

Create a new array and copy elements sequentially:

public static int[] concatenate(int[] array1, int[] array2) {
    int[] result = Arrays.copyOf(array1, array1.length + array2.length);
    System.arraycopy(array2, 0, result, array1.length, array2.length);
    return result;
}

4. Using Apache Commons Lang (Third-Party Library)

For simplicity, use ArrayUtils.addAll from Apache Commons Lang:

import org.apache.commons.lang3.ArrayUtils;

int[] concatenated = ArrayUtils.addAll(array1, array2);

Key Considerations

Null Handling: Check for null arrays before concatenation to avoid errors.
Performance: System.arraycopy is the fastest method for large arrays.
Java Version: Use streams for Java 8+; otherwise, stick to arraycopy.

Example with Generic Method

public static <T> T[] concatenate(T[] array1, T[] array2) {
    T[] result = Arrays.copyOf(array1, array1.length + array2.length);
    System.arraycopy(array2, 0, result, array1.length, array2.length);
    return result;
}

// Usage:
String[] arr1 = {"a", "b"};
String[] arr2 = {"c", "d"};
String[] combined = concatenate(arr1, arr2); // ["a", "b", "c", "d"]

Choose the method that best fits your use case and Java version!

1. Using System.arraycopy (All Java Versions)