How to Merge Two Arrays in Java

In Java, there are several ways to merge or add two arrays: with Java home resources prior to Java 8, with Java 8 streams, or with the help of the Guava or Apache Commons libraries.
 

Merge two arrays in Java

To merge two arrays into one, we use two methods of the Java Standard Edition: Arrays.copyOf() and System.arraycopy(). Arrays.copyOf() creates a new array result with the contents of the first array one, but with the length of both arrays. System.arraycopy() then does the real work of copying: it copies the second array into the result array just created with the length of both arrays.
 

 

Example :

import java.util.Arrays;

public class MergeArrays 
{
    public static void main(String[] args) 
    {
        String[] birds = {"Angel", "Buddy", "Sunny", "Sunshine"};
        String[] cats = {"Lions", "Tigers", "Leopards", "Cheetahs"};
        String[] animals = concatArrays(birds, cats);
        System.out.println(Arrays.toString(animals));
    }

    public static <T> T[] concatArrays(T[] arr1, T[] arr2) 
    {
        T[] result = Arrays.copyOf(arr1, arr1.length + arr2.length);
        System.arraycopy(arr2, 0, result, arr1.length, arr2.length);
        return result;
    }
}

Output:

[Angel, Buddy, Sunny, Sunshine, Lions, Tigers, Leopards, Cheetahs]

Why the method copyOf() is in the Util class Arrays, but the method arraycopy() in the class System, is illogical. The reason should be quite simply historical: the System class has been around since Java 1.0, the Arrays class only since Java 1.2.
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Merge two arrays in Java with Java 8 streams

In Java 8, the API was extended to include streams, which represent an elegant declarative fluent interface for processing collections.

Arrays can be converted into a stream quite easily using Arrays.stream(). This stream contains all elements of the array.
 

Example :

import java.util.Arrays;
import java.util.stream.Stream;

public class MergeArrays 
{
    public static void main(String[] args) 
    {
        String[] birds = {"Angel", "Buddy", "Sunny", "Sunshine"};
        String[] cats = {"Lions", "Tigers", "Leopards", "Cheetahs"};
        
        String[] animals = Stream.concat(
                Arrays.stream(birds), 
                Arrays.stream(cats)
            ).toArray(String[]::new);
            
        System.out.println(Arrays.toString(animals));
    }
}

Output:

[Angel, Buddy, Sunny, Sunshine, Lions, Tigers, Leopards, Cheetahs]

Stream.concat() creates a new stream containing the elements of the first stream before the elements of the second stream. The method toArray() converts the stream back into an array.
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Merge two arrays in Java with Google Guava

Google Guava offers an easy way to merge two arrays with the ObjectArrays.concat() method.

import java.util.Arrays;
import com.google.common.collect.ObjectArrays;

public class MergeArrays 
{
    public static void main(String[] args) 
    {
        String[] birds = {"Angel", "Buddy", "Sunny", "Sunshine"};
        String[] cats = {"Lions", "Tigers", "Leopards", "Cheetahs"};
        String[] animals = ObjectArrays.concat(birds, cats, String.class);
        System.out.println(Arrays.toString(animals));
    }
}

Output:

[Angel, Buddy, Sunny, Sunshine, Lions, Tigers, Leopards, Cheetahs]

 

Merge two arrays in Java with Apache Commons Lang ArrayUtils

Apache Commons also provides a method for merging arrays in the ArrayUtils class:

import java.util.Arrays;
import org.apache.commons.lang3.ArrayUtils;

public class MergeArrays 
{
    public static void main(String[] args) 
    {
        String[] birds = {"Angel", "Buddy", "Sunny", "Sunshine"};
        String[] cats = {"Lions", "Tigers", "Leopards", "Cheetahs"};
        String[] animals;

        animals = ArrayUtils.addAll(birds, cats);
        
        System.out.println(Arrays.toString(animals));
    }
}

Output:

[Angel, Buddy, Sunny, Sunshine, Lions, Tigers, Leopards, Cheetahs]

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