Concatenating lists in Java

Example 1: Join Two Lists using addAll[]

import java.util.ArrayList; import java.util.List; public class JoinLists { public static void main[String[] args] { List list1 = new ArrayList[]; list1.add["a"]; List list2 = new ArrayList[]; list2.add["b"]; List joined = new ArrayList[]; joined.addAll[list1]; joined.addAll[list2]; System.out.println["list1: " + list1]; System.out.println["list2: " + list2]; System.out.println["joined: " + joined]; } }

Output

list1: [a] list2: [b] joined: [a, b]

In the above program, we used List's addAll[] method to join lists list1 and list2 to the joined list.

Example 2: Join Two Lists using union[]

import java.util.ArrayList; import java.util.List; import org.apache.commons.collections.ListUtils; public class JoinLists { public static void main[String[] args] { List list1 = new ArrayList[]; list1.add["a"]; List list2 = new ArrayList[]; list2.add["b"]; List joined = ListUtils.union[list1, list2]; System.out.println["list1: " + list1]; System.out.println["list2: " + list2]; System.out.println["joined: " + joined]; } }

The output of this program is the same as Example 1.

In the above program, we used union[] method to join the given lists to joined.

Example 3: Join Two Lists using stream

import java.util.ArrayList; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class JoinLists { public static void main[String[] args] { List list1 = new ArrayList[]; list1.add["a"]; List list2 = new ArrayList[]; list2.add["b"]; List joined = Stream.concat[list1.stream[], list2.stream[]] .collect[Collectors.toList[]]; System.out.println["list1: " + list1]; System.out.println["list2: " + list2]; System.out.println["joined: " + joined]; } }

The output of this program is the same as Example 1.

In the above program, we used Stream's concat[] method to join two lists converted to streams. Then, we convert them back to List using toList[].