Java How to Concatenate two Arrays

Tags: ArrayUtils Array

In this Java tutorial, we learn how to write a Java utility class to concatenate two arrays of objects in Java programming language.

How to concatenate two arrays in Java

At this first step, we create a new Java class named ArrayUtils, and implement a new static method named concat(T[] firstArray, T[] secondArray), this method to concatenate two arrays of generic object type T with the following steps:

  • Create a new ArrayList object.
  • Use the Collections.addAll(Collection<? super T> c, T… elements) method to add first array and seconds array to the ArrayList object.
  • Use the ArrayList.toArray(T[] a) method to convert the ArrayList object to expected array value.

ArrayUtils.java 

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class ArrayUtils {

    /**
     * This method to concatenate two arrays
     * @param firstArray the first array of T
     * @param secondArray the second array of T
     * @param <T>
     * @return the concatenate result array
     */
    public static <T> T[] concat(T[] firstArray, T[] secondArray) {
        List<T> result = new ArrayList<>();
        Collections.addAll(result, firstArray);
        Collections.addAll(result, secondArray);

        T[] emptyArray = (T[])Array.newInstance(firstArray.getClass().getComponentType(), 0);
        return result.toArray(emptyArray);
    }
}

In the following example Java program, we learn how to use the above ArrayUtils.concat(T[] firstArray, T[] secondArray) method in Java program.

ArrayUtilsConcatExample.java 

import java.util.Arrays;

public class ArrayUtilsConcatExample {
    public static void main(String... args) {
        String[] array1 = new String[]{"Java", "Kotlin"};
        String[] array2 = new String[]{"Spring", "Spring Boot"};

        // Concatenate two Arrays
        String[] result = ArrayUtils.concat(array1, array2);

        System.out.println("array1: " + Arrays.toString(array1));
        System.out.println("array2: " + Arrays.toString(array2));
        System.out.println("result: " + Arrays.toString(result));
    }
}
The output as below. 
array1: [Java, Kotlin]
array2: [Spring, Spring Boot]
result: [Java, Kotlin, Spring, Spring Boot]

Happy Coding 😊

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