Concatenating two lists means merging two lists into a single list.
Consider the given lists:
LIST 1
LIST 2
LIST AFTER CONCATENATION
There are several methods to perform concatenation operation:
- Using addAll() method
- Using stream
- Using union()
Method 1: Using addAll() method
Syntax:
addAll ( list name )
This method takes name of list as argument and add all the elements of the specified list in the same order as the original list.Â
- Create a new empty list ( concatenated_list)
- Use addAll () method to concatenate the given list1 and list2 into the newly created list.
concatenated_list.addAll (list1) // concatenates first listÂ
concatenated_list.addAll (list2) // concatenates second listÂ
After performing the above steps our empty list now contains both the list.
Java
// Java Program to Concatenate Two List// using addAll() method  import java.io.*;import java.util.ArrayList;import java.util.List;  public class GFG {      public static void main(String[] args)    {        // given list 1        List<Integer> list1 = new ArrayList<Integer>();        list1.add(1);        list1.add(2);        list1.add(3);        list1.add(4);                // given list 2        List<Integer> list2 = new ArrayList<Integer>();        list2.add(5);        list2.add(6);        list2.add(7);        list2.add(8);          // creating new empty list        List<Integer> concatenated_list            = new ArrayList<Integer>();          // using addAll( ) method to concatenate the lists        concatenated_list.addAll(list1);        concatenated_list.addAll(list2);          System.out.println("list1: " + list1);        System.out.println("list2: " + list2);        System.out.println("Concatenated list: "                           + concatenated_list);    }} |
Output
list1: [1, 2, 3, 4] list2: [5, 6, 7, 8] Concatenated list: [1, 2, 3, 4, 5, 6, 7, 8]
Method 2: Using streams
Stream.concat(list1.stream(),list2.stream()).collect(Collectors.toList())
It takes two streams as argument and creates a concatenated stream out of them.Second list is appended to the first list.Â
Java
// Java Program to Concatenate Two List // using streams  import java.io.*;import java.util.ArrayList;import java.util.List;import java.util.stream.Collectors;import java.util.stream.Stream;  public class GFG {      public static void main(String[] args)    {        // given list 1        List<Integer> list1 = new ArrayList<Integer>();        list1.add(1);        list1.add(2);        list1.add(3);        list1.add(4);                // given list 2        List<Integer> list2 = new ArrayList<Integer>();        list2.add(5);        list2.add(6);        list2.add(7);        list2.add(8);          // creating new empty list        List<Integer> concatenated_list = new ArrayList<Integer>();          // using Stream.concat() method to concatenate the lists        concatenated_list = Stream.concat(list1.stream(), list2.stream())                  .collect(Collectors.toList());          System.out.println("list1: " + list1);        System.out.println("list2: " + list2);        System.out.println("Concatenated list: "+ concatenated_list);            }} |
Output
list1: [1, 2, 3, 4] list2: [5, 6, 7, 8] Concatenated list: [1, 2, 3, 4, 5, 6, 7, 8]
Method 3: Using union()Â
ListUtils.union( list1, list2)
 It takes two list as argument and return new concatenated list. Second list is appended to the first list.Â
Java
// Java program to concatenate two lists// using union()  import java.io.*;import java.util.*;import java.lang.*;import org.apache.commons.collections.ListUtils;  public class GFG {      public static void main(String[] args)    {        // given list 1        List<Integer> list1 = new ArrayList<Integer>();        list1.add(1);        list1.add(2);        list1.add(3);        list1.add(4);          // given list 2        List<Integer> list2 = new ArrayList<Integer>();        list2.add(5);        list2.add(6);        list2.add(7);        list2.add(8);          // creating new empty list        List<Integer> concatenated_list = new ArrayList<Integer>();          // using ListUtils.union() method to concatenate        // the lists        concatenated_list = ListUtils.union(list1, list2);          System.out.println("list1: " + list1);        System.out.println("list2: " + list2);        System.out.println("Concatenated list: "                           + concatenated_list);    }} |
Output:
Prerequisites for running the above code:
- Download and Install the following Apache Commons Collections library http://commons.apache.org/proper/commons-collections/download_collections.cgi
- Now go to the properties of your project.
- Select Java Build Path.
- Under Java Build Path go to libraries.
- Under libraries select Classpath.
- Now go to add external JARs and add the JAR executable file, downloaded from the given link.
