Join two ArrayLists in Java

An Array is a collection of certain elements that can be anything which takes up the adjacent memory locations. Here we store multiple elements of the same type together. An ArrayList is a class that is resizable unlike the in-built Array. An essential concept to comprehend involves recognizing how an ArrayList separates itself from a typical Array since only the former allows modifications through adding or deleting its elements. Importantly, one can easily access numerous variations of highly-functionalized ArrayLists through the java.util package capable of performing diverse operations. In this particular article we will demonstrate different approaches to join two ArrayLists using Java language.

Syntax

import java.util.ArrayList;

ArrayList<String>(object name) = new ArrayList<String>();

Algorithm for addAll() method

• Step 1 − Create an ArrayList join1.

• Step 2 − Add elements to the list.

• Step 3 − Print the elements of the ArrayList.

• Step 4 − Create another ArrayList join2.

• Step 5 − Add elements to the list.

• Step 6 − Print the elements of the ArrayList.

• Step 7 − Append elements of join2 with join1, using addAll.

• Step 8 − Print the combined ArrayList.

Algorithm for removeAll()/addAll method

• Step 1 − Create an ArrayList join1.

• Step 2 − Add elements to the list.

• Step 3 − Print the elements of the ArrayList.

• Step 4 − Create another ArrayList join2.

• Step 5 − Add elements to the list.

• Step 6 − Print the elements of the ArrayList.

• Step 7 − Create another ArrayList join3.

• Step 8 − Add the elements of join2 to join3.

• Step 9 − Remove elements of join1.

• Step 10 − Append elements of join1 with join3, using addAll.

• Step 11 − Print the combined ArrayList.

Algorithm for Stream flatMap() method

• Step 1 − Create an ArrayList join1.

• Step 2 − Add elements to the list.

• Step 3 − Print the elements of the ArrayList.

• Step 4 − Create another ArrayList join2.

• Step 5 − Add elements to the list.

• Step 6 − Print the elements of the ArrayList.

• Step 7 − Get elements of join1 and join2 in a single stream.

• Step 8 − Collect stream elements to an ArrayList.

• Step 9 − Print the combined ArrayList.

Algorithm for LinkedHashSet() method

• Step 1 − Create an ArrayList join1.

• Step 2 − Add elements to the list.

• Step 3 − Print the elements of the ArrayList.

• Step 4 − Create another ArrayList join2.

• Step 5 − Add elements to the list.

• Step 6 − Print the elements of the ArrayList.

• Step 7 − Add items from lists to the set.

• Step 8 − Convert set to an arrayList.

• Step 9 − Print the combined array.

Approaches

• Approach 1 − using addAll() method.

• Approach 2 − using removeAll() / addAll() method.

• Approach 3 − using flatMap() method.

• Approach 4 − using LinkedHashSet() method.

Approach 1: addAll() method

In this approach we make two lists with different items and append elements of one list to the other. Here we use the method addAll().

Syntax

List1.addAll(List2);

Example

import java.util.ArrayList;

public class ArrayJoinAdd {
   public static void main(String[] args) {
      ArrayList<String> join1 = new ArrayList<String>();
      join1.add("Way");
      join1.add("Two");
      join1.add("Class");
      System.out.println("ArrayList 1: " + join1);

      ArrayList<String> join2 = new ArrayList<String>();
      join2.add("Education ");
      join2.add("comes");
      join2.add("first");
      System.out.println("ArrayList 2: " + join2);

      join1.addAll(join2);

      System.out.println("Joined ArrayList: " + join1);
   }
}

Output

ArrayList 1: [Way, Two, Class]
ArrayList 2: [Education , comes, first]
Joined ArrayList: [Way, Two, Class, Education , comes, first]

Approach 2: RemoveAll()/AddAll() method

In this approach we use two functions RemoveAll() and AddAll() where we delete the items of the first list that are in the second list and then add the first list to the second list.

Syntax

List3.removeAll(List1);
   List1.addAll(List3);

Example

import java.util.*;

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

      ArrayList<String> join1 = new ArrayList<String>();

      join1.add("Way");
      join1.add("Two");
      join1.add("Class");

      System.out.println("ArrayList 1:" + join1);

      ArrayList<String> join2 = new ArrayList<String>();

      join2.add("Education ");
      join2.add("comes");
      join2.add("first");

      System.out.println("ArrayList 2:" + join2);

      ArrayList<String> join3 = new ArrayList<>(join2);
      join3.removeAll(join1);
      join1.addAll(join3);


      System.out.println("Joined ArrayList" + join1);

   }
}

Output

ArrayList 1:[Way, Two, Class]
ArrayList 2:[Education , comes, first]
[Way, Two, Class, Education , comes, first]

Approach 3:Stream flatMap() method

Stream flatMap() method gets items from different lists in a single stream and then collects stream elements to an ArrayList.

Syntax

List<String> (combined array)= Stream.of(List1,List2)
   .flatMap(x -> x.stream())
   .collect(Collectors.toList());

Example

import java.util.*;
import java.util.stream.Stream;
import java.util.stream.Collectors;
import java.util.Arrays;
import java.util.List;

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

      ArrayList<String> join1 = new ArrayList<String>();

      join1.add("Way");
      join1.add("Two");
      join1.add("Class");

      System.out.println("ArrayList 1:" + join1);

      ArrayList<String> join2 = new ArrayList<String>();

      join2.add("Education ");
      join2.add("comes");
      join2.add("first");

      System.out.println("ArrayList 2:" + join2);

      List<String> finalJoin = Stream.of(join1,join2)
      .flatMap(x -> x.stream())
      .collect(Collectors.toList());
      System.out.println(finalJoin);

   }

}

Output

ArrayList 1:[Way, Two, Class]
ArrayList 2:[Education , comes, first]
[Way, Two, Class, Education , comes, first]

Approach 4: LinkedHashSet() method

In this approach we create a set using LinkedHashSet() and push the elements in the set. This set represents the combined arrayList, which is made of all the unique elements.

Syntax

Set <String> set = new LinkedHashSet<>(List1);
   set.addAll(List2);
ArrayList<String> combinedList = new ArrayList<> (set);

Example

import java.util.*;
import java.util.stream.Stream;
import java.util.stream.Collectors;
import java.util.Arrays;
import java.util.List;

public class ArrayjoinLinkedHashSet {
   public static void main (String args[]){
   
      ArrayList<String> join1 = new ArrayList<String>();

      join1.add("Way");
      join1.add("Two");
      join1.add("Class");

      System.out.println("ArrayList 1:" + join1);

      ArrayList<String> join2 = new ArrayList<String>();

      join2.add("Education ");
      join2.add("comes");
      join2.add("first");

      System.out.println("ArrayList 2:" + join2);

      Set <String> set = new LinkedHashSet<>(join1);
      set.addAll(join2);

      ArrayList<String> finalJoin = new ArrayList<> (set);
      System.out.println(finalJoin);
   }
}

Output

ArrayList 1:[Way, Two, Class]
ArrayList 2:[Education , comes, first]
[Way, Two, Class, Education , comes, first]

Conclusion

In the above article we discussed some easy methods to add two or more ArrayLists and print them. These ArrayLists can be combined with the above methods as per the requirement of the user,, that means if we do not want the duplicate items to be part of the combined lists we can eliminate them through methods like removeAll() and addAll().This simple methods makes it easier to understand how to join or merge these ArrayLists.