Java Program to Join Two Lists

May 27, 2023

What is an List in Java ?

In Java, a list is a collection of elements of the same type, which are ordered and can be accessed using an index. A list is a part of the Java Collection Framework and is implemented by the List interface.
There are several classes in Java that implement the List interface, such as ArrayList, LinkedList, and Vector. These classes provide different ways of storing and manipulating lists of elements, but they all have common methods defined in the List interface, such as adding and removing elements, getting the size of the list, accessing elements by index, and iterating over the elements in the list.

Ways to Add two Lists in Java :

1. Using the addAll() method: This method is available in the List interface and can be used to add all the elements from one list to another

Syntax :

List list1 = new ArrayList<>();
list1.add(1);
list1.add(2);
list1.add(3);

List list2 = new ArrayList<>();
list2.add(4);
list2.add(5);
list2.add(6);

list1.addAll(list2); // add all elements of list2 to list1
System.out.println(list1); // Output: [1, 2, 3, 4, 5, 6]

2. Using the Collections.addAll() method: This method is available in the java.util.Collections class and can be used to add all the elements from an array to a list.

Syntax :

List list = new ArrayList<>();
String[] array = {"one", "two", "three"};
Collections.addAll(list, array); // add all elements of array to list
System.out.println(list); // Output: [one, two, three]

3. Using the Arrays.asList() method: This method is available in the java.util.Arrays class and can be used to convert an array to a list.

Syntax :

String[] array = {"one", "two", "three"};
List list = Arrays.asList(array); // convert array to list
System.out.println(list); // Output: [one, two, three]

Example 1 : Using addAll() and Collections.addAll() Method :

Run

import java.util.*;

public class Main {

    public static void main(String[] args) {
        // Creating the first list
        List< Integer > list1 = new ArrayList<>();
        list1.add(1);
        list1.add(2);
        list1.add(3);
        System.out.println("List 1: " + list1);

        // Creating the second list
        List< Integer > list2 = new ArrayList<>();
        list2.add(4);
        list2.add(5);
        list2.add(6);
        System.out.println("List 2: " + list2);

        // Adding all elements of the second list to the first list using addAll() method
        list1.addAll(list2);
        System.out.println("After adding list2 to list1 using addAll() method: " + list1);

        // Creating an array of strings
        String[] array = {"prepinsta", "prime", "prepinstaprime"};

        // Creating a new list and adding all elements of the array to it using Collections.addAll() method
        List< String > list3 = new ArrayList<>();
        Collections.addAll(list3, array);
        System.out.println("List 3: " + list3);
    }
}

Output :

List 1: [1, 2, 3]
List 2: [4, 5, 6]
After adding list2 to list1 using addAll() method: [1, 2, 3, 4, 5, 6]
List 3: [prepinsta, prime, prepinstaprime]

Explanation : In this example, we create two lists (list1 and list2) of integers, add some elements to them, and then use the addAll() method to add all elements of list2 to list1. We then print the resulting list1 to the console. We also create an array of strings, create a new list (list3), and use the Collections.addAll() method to add all elements of the array to it.

Example 2 : Using the Arrays.asList() method:

Run

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        // Creating the first list
        List< Integer > list1 = new ArrayList<>();
        list1.add(1);
        list1.add(2);
        list1.add(3);
        System.out.println("List 1: " + list1);

        // Creating the second list
        List< Integer > list2 = new ArrayList<>();
        list2.add(4);
        list2.add(5);
        list2.add(6);
        System.out.println("List 2: " + list2);

        // Using the Arrays.asList() method to add both lists
        List < List < Integer >> listOfLists = new ArrayList<>();
        listOfLists.add(list1);
        listOfLists.add(list2);
        List < Integer > flattenedList = new ArrayList<>();
        for (List < Integer > list : listOfLists) {
            flattenedList.addAll(list);
        }
        System.out.println("Flattened list: " + flattenedList);
    }
}

Output :

List 1: [1, 2, 3]
List 2: [4, 5, 6]
Flattened list: [1, 2, 3, 4, 5, 6]

Explanation : In this example, we create two lists (list1 and list2) of integers and then use the Arrays.asList() method to add both lists to a list of lists (listOfLists). We then use a for loop to iterate over the listOfLists and add all the elements to a new list (flattenedList) using the addAll() method. Finally, we print the flattened list to the console.

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Java Program to Join Two Lists

What is an List in Java ?

Ways to Add two Lists in Java :

Syntax :

Syntax :

Syntax :

Example 1 : Using addAll() and Collections.addAll() Method :

Output :

Example 2 : Using the Arrays.asList() method:

Output :

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