Java Program to Merge two lists

Example 1: Merge two lists using addAll()

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

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

    // create first list
    List<Integer> prime = new ArrayList<>();
    prime.add(2);
    prime.add(3);
    prime.add(5);
    System.out.println("First List: " + prime);

    // create second list
    List<Integer> even = new ArrayList<>();
    even.add(4);
    even.add(6);
    System.out.println("Second List: " + even);

    // create merged list
    List<Integer> numbers = new ArrayList<>();
    numbers.addAll(prime);
    numbers.addAll(even);

    System.out.println("Merged List: " + numbers);

  }
}

Output

First List: [2, 3, 5]
Second List: [4, 6]
Merged List: [2, 3, 5, 4, 6]

In the above example, have two lists named prime and even. Here, we have used the Java ArrayList addAll() method to add all elements from prime and even to the new list named numbers.

Example 2: Merge two lists using the Stream class

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

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

    // create first list
    // asList() converts the array into the arraylist
    List<Integer> prime = new ArrayList<>(Arrays.asList(2, 3, 5));
    System.out.println("First List: " + prime);

    // create second list
    List<Integer> even = new ArrayList<>(Arrays.asList(4, 6));
    System.out.println("Second List: " + even);

    // convert both lists into stream
    Stream<Integer> list1 = prime.stream();
    Stream<Integer> list2 = even.stream();

    // merge two streams
    Stream<Integer> merged = Stream.concat(list1, list2);

    // convert the merged stream into list
    List<Integer> numbers = merged.collect(Collectors.toList());

    System.out.println("Merged List: " + numbers);

  }
}

Output

First List: [2, 3, 5]
Second List: [4, 6]
Merged List: [2, 3, 5, 4, 6]

In the above example, we have used the Stream class to merge the two lists. Here,

• stream() - converts the list into a stream
• concat() - merges the two streams
• collect(Collectors.toList()) - convert the stream into a list

To learn more about stream, visit Java Stream Class.