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Java Program to Merge Two Arrays

Last Updated : 14 Nov, 2024
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Given two arrays, the task is to merge or concatenate them and store the result into another array.

Example:

Java 
// Java Program to demonstrate merging
// two array using System.arraycopy() method
import java.util.Arrays;

public class MergeTwoArrays1 {
    public static void main(String[] args) {
        
        int[] a = { 10, 20, 30, 40 };
        int[] b = { 50, 60, 70, 80 };

        // determines length of both arrays
        int a1 = a.length;
        int b1 = b.length;
      
        // resultant array size
        int c1 = a1 + b1;

        // create the resultant array
        int[] c = new int[c1];

        // using the pre-defined function arraycopy
        System.arraycopy(a, 0, c, 0, a1);
        System.arraycopy(b, 0, c, a1, b1);

        System.out.println("" + Arrays.toString(c));
    }
}

Output
[10, 20, 30, 40, 50, 60, 70, 80]

Explanation: The simplest and most efficient way to merge two arrays is by using the in-built System.arraycopy() function.

  • First, we initialize two arrays, a and b, and store values in them.
  • Then, we calculate their lengths and create a third array to hold the merged result.
  • Then we use System.arraycopy() to merge both arrays into the third array.

Other Methods to Merge Two Arrays

1. Without Using Pre-Defined Function

This method is useful when we need full control over the merging process without relying on built-in functions. It is better than a user-defined function because it avoids extra function overhead. It directly merging the arrays.

Java 
// Java Program to demonstrate merging
// two array without using pre-defined method
import java.util.Arrays;

public class MergeTwoArrays2 {
    public static void main(String[] args) {

        int a[] = { 10, 20, 30 };
        int b[] = { 40, 50, 60, 70, 80 };

        // determining length of both arrays
        int a1 = a.length;
        int b1 = b.length;

        // resultant array size
        int c1 = a1 + b1;

        // Creating a new array
        int[] c = new int[c1];

        // Loop to store the elements of first
        // array into resultant array
        for (int i = 0; i < a1; i = i + 1) {
          
            // Storing the elements in 
            // the resultant array
            c[i] = a[i];
        }

        // Loop to concat the elements of second 
        // array into resultant array
        for (int i = 0; i < b1; i = i + 1) {

            // Storing the elements in the 
            // resultant array
            c[a1 + i] = b[i];
        }

        System.out.println("" + Arrays.toString(c));
    }
}

Output
[10, 20, 30, 40, 50, 60, 70, 80]

Explanation:

  • First, we initialize two arrays, a and b. Then we calculate their lengths.
  • Then, we create a new array c to store the merged result.
  • The first loop copies elements from a to c and the second loop copies elements from b to c.
  • At last we print the merged array.

2. Using Java Streams

This method utilizes Java Streams to concatenate or merge two arrays. Java Streams were introduced in Java 8 and provide a declarative way to process collections of elements.

Java 
// Java Program to demonstrate merging
// two array using Java Stream 
import java.util.Arrays;
import java.util.stream.IntStream;

public class MergeTwoArrays3 {

    public static void main(String[] args) {

        int a[] = {10, 20, 30};
        int b[] = {40, 50, 60, 70, 80};

        // Merging arrays using Java Streams
        int[] c = mergeArraysUsingStreams(a, b);
		
      	System.out.println("" + Arrays.toString(c));
    }

    public static int[] mergeArraysUsingStreams(int[] arr1, int[] arr2) {
        return IntStream.concat(Arrays.stream(arr1), Arrays.stream(arr2)).toArray();
    }
}

Output
[10, 20, 30, 40, 50, 60, 70, 80]

Explanation:

  • In this example, we merge two integer arrays a and b using Java Streams by concatenating them with IntStream.concat() method.
  • Then we convert the result back to an array.
  • The mergeArraysUsingStreams method combines the two arrays and returns the merged array as an int[].

3. Using ArrayList

In this method, we have used an ArrayList to facilitate the merging of two arrays. An ArrayList is a dynamic data structure in Java that can grow or shrink in size as needed.

Java 
// Java Program to demonstrate merging
// two array using ArrayList
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class MergeTwoArrays4 {
 
    public static int[] mergeArraysUsingArrayList(int[] a,
                                                  int[] b)
    {
        
        // Create an ArrayList to store the merged
        // elements
        List<Integer> r = new ArrayList<>();

        // Iterate through a and add each element to
        // resultList
        for (int n : a) {
            r.add(n);
        }

        // Iterate through b and add each element to
        // resultList
        for (int n : b) {
            r.add(n);
        }

        // Convert the ArrayList to an array using
        // streams
        return r.stream()
          .mapToInt(Integer::intValue).toArray();
    }

    public static void main(String[] args)
    {

        int a[] = { 10, 20, 30 };
        int b[] = { 40, 50, 60, 70, 80 };
      
        int[] r1 = mergeArraysUsingArrayList(a, b);

      	System.out.println("" + Arrays.toString(r1));
    }
}

Output
[10, 20, 30, 40, 50, 60, 70, 80]

Explanation:

  • In this example, we use an ArrayList to store the merged elements from two arrays.
  • The for-each loop iterates through both arrays by adding their elements to the ArrayList.
  • Then the stream().mapToInt(Integer::intValue).toArray() method converts the ArrayList back into an integer array and prints the merged result.


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