In Java how to join Arrays? 3 ways: Apache Commons ArrayUtils, Java 8 Streams and Simple APIs

In this tutorial we will go over different ways we can join Java Arrays.

If you have any of below questions then you are at right place:

• How can I concatenate two arrays in Java?
• How to Merge Two Arrays in Java?
• 3 Ways to Combine Arrays in Java
• Join Array of Primitives with Separator in Java

Here are the three ways you could you could join arrays in Java.

1. Join Array using Apache Common utility
2. Join Array using Java8 Stream operation
3. Join Array using Standard Java APIs

Let’s get started:

Create java class CrunchifyJoinArrays3Ways.java.

Put below Java code into file.

package crunchify.com.tutorial;

import org.apache.commons.lang3.ArrayUtils;

import java.lang.reflect.Array;
import java.util.Arrays;
import java.util.stream.IntStream;
import java.util.stream.Stream;

/**
 * @author Crunchify.com
 * <p>
 * In Java how to join Arrays? 3 ways: Apache Commons ArrayUtils, Java 8 Streams and Simple APIs.
 * Version: 1.0.3
 */
public class CrunchifyJoinArrays3Ways {
    public static void main(String[] args) {

        crunchifyPrint("Original String Array1: [google, twitter]");
        crunchifyPrint("Original String Array2: [apple, microsoft]");
        crunchifyPrint("Original Integer Array1: [111, 444]");
        crunchifyPrint("Original Integer Array2: [222, 555]\n");
        // Join Array using Apache Common utility
        joinArrayUsingApacheCommon();

        // Join Array using Java8 Stream operation
        joinArrayUsingJava8Stream();

        // Join Array using Standard Java APIs
        joinArrayUsingJavaAPI();
    }

    // Method-1: Join Array using Apache Common utility
    private static void joinArrayUsingApacheCommon() {

        String[] company1 = new String[]{"google", "twitter"};
        String[] company2 = new String[]{"apple", "microsoft"};

        // from org.apache.commons.lang3 maven dependency
        String[] crunchifyResult = ArrayUtils.addAll(company1, company2);

        crunchifyPrint("From Method-1: addAll() - String ==> " + Arrays.toString(crunchifyResult));

        int[] crunchifyArray1 = new int[]{111, 444};
        int[] crunchifyArray2 = new int[]{222, 555};

        int[] crunchifyResult2 = ArrayUtils.addAll(crunchifyArray1, crunchifyArray2);

        crunchifyPrint("From Method-1: addAll() - Integer ==> " + Arrays.toString(crunchifyResult2));
    }

    // Simple Java Print Method
    private static void crunchifyPrint(String result) {
        System.out.println(result);
    }

    // Method-2: Join Array using Java8 Stream operation
    private static void joinArrayUsingJava8Stream() {
        String[] company1 = new String[]{"google", "twitter"};
        String[] company2 = new String[]{"apple", "microsoft"};

        // Stream.of() - returns a sequential ordered stream whose elements are the specified values.
        // A sequence of elements supporting sequential and parallel aggregate operations. The following example illustrates an aggregate operation using Stream and IntStream:
        //
        //     int sum = widgets.stream()
        //                      .filter(w -> w.getColor() == RED)
        //                      .mapToInt(w -> w.getWeight())
        //                      .sum();
        String[] result = Stream.of(company1, company2).flatMap(Stream::of).toArray(String[]::new);
        crunchifyPrint("\nFrom Method-2: Stream.of() ==> " + Arrays.toString(result));

        int[] crunchifyArray1 = new int[]{111, 444};
        int[] crunchifyArray2 = new int[]{222, 555};

        // Arrays.stream() - returns a sequential IntStream with the specified array as its source.
        int[] crunchifyResult2 = IntStream.concat(Arrays.stream(crunchifyArray1), Arrays.stream(crunchifyArray2)).toArray();

        crunchifyPrint("From Method-2: IntStream.concat() ==> " + Arrays.toString(crunchifyResult2));
    }

    // Method-3: Join Array using Standard Java APIs
    private static void joinArrayUsingJavaAPI() {
        String[] company1 = new String[]{"google", "twitter"};
        String[] company2 = new String[]{"apple", "microsoft"};

        String[] crunchifyResult = crunchifyJoinGenericArray(company1, company2);

        crunchifyPrint("\nFrom Method-3: crunchifyJoinArrayusingGeneric() ==> " + Arrays.toString(crunchifyResult));

        int[] crunchifyArray1 = new int[]{111, 444};
        int[] crunchifyArray2 = new int[]{222, 555};

        int[] crunchifyResult2 = crunchifyJoinIntegerArray(crunchifyArray1, crunchifyArray2);

        // Arrays.toString() Returns a string representation of the contents of the specified array.
        // The string representation consists of a list of the array's elements, enclosed in square brackets ("[]").
        // Adjacent elements are separated by the characters ", " (a comma followed by a space).
        // Elements are converted to strings as by String.valueOf(int). Returns "null" if a is null.
        crunchifyPrint("From Method-3: joinArray() ==> " + Arrays.toString(crunchifyResult2));
    }

    @SafeVarargs
    private static <T> T[] crunchifyJoinGenericArray(T[]... crunchifyArrays) {
        int crunchify = 0;
        for (T[] crunchifyArray : crunchifyArrays) {
            crunchify += crunchifyArray.length;
        }

        //T[] result = new T[crunchify];
        final T[] crunchifyResult = (T[]) Array.newInstance(crunchifyArrays[0].getClass().getComponentType(),
                crunchify);

        int crunchifyOffset = 0;
        for (T[] crunchifyArray : crunchifyArrays) {

            // Copies an array from the specified source array, beginning at the specified position, to the specified position of the destination array.
            // A subsequence of array components are copied from the source array referenced by src to the destination array referenced by dest.
            // The number of components copied is equal to the length argument.
            // The components at positions srcPos through srcPos+length-1 in the source array are copied into positions destPos through destPos+length-1, respectively, of the destination array.
            System.arraycopy(crunchifyArray, 0, crunchifyResult, crunchifyOffset, crunchifyArray.length);
            crunchifyOffset += crunchifyArray.length;
        }

        return crunchifyResult;
    }

    private static int[] crunchifyJoinIntegerArray(int[]... crunchifyArrays) {
        int crunchify = 0;
        for (int[] crunchifyArray : crunchifyArrays) {
            crunchify += crunchifyArray.length;
        }

        final int[] crunchifyResult = new int[crunchify];

        int crunchifyOffset = 0;
        for (int[] crunchifyArray : crunchifyArrays) {

            System.arraycopy(crunchifyArray, 0, crunchifyResult, crunchifyOffset, crunchifyArray.length);
            crunchifyOffset += crunchifyArray.length;
        }

        return crunchifyResult;
    }

}

Let’s take a look at some of Java API details.

java.lang.System.arraycopy():

java.util.Arrays.toString():

java.util.stream():

Just run program as a Java Program and you should see a result same as below.

IntelliJ IDEA console output:

/Library/Java/JavaVirtualMachines/jdk-15.jdk/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=50249:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/app/crunchify/github/CrunchifyTutorials/target/classes:/Users/app/crunchify/github/CrunchifyTutorials/WebContent/WEB-INF/lib/zxing-2.1.jar/Users/app/.m2/repository/commons-collections/commons-collections/3.2.1/commons-collections-3.2.1.jar/spring-context-support-5.1.3.RELEASE.jar crunchify.com.tutorial.CrunchifyJoinArrays3Ways

Original String Array1: [google, twitter]
Original String Array2: [apple, microsoft]
Original Integer Array1: [111, 444]
Original Integer Array2: [222, 555]

From Method-1: addAll() - String ==> [google, twitter, apple, microsoft]
From Method-1: addAll() - Integer ==> [111, 444, 222, 555]

From Method-2: Stream.of() ==> [google, twitter, apple, microsoft]
From Method-2: IntStream.concat() ==> [111, 444, 222, 555]

From Method-3: crunchifyJoinArrayusingGeneric() ==> [google, twitter, apple, microsoft]
From Method-3: joinArray() ==> [111, 444, 222, 555]

Process finished with exit code 0

Please let me know if you have any other way to merge Java Arrays or you get any exception.