How to join two Lists in Java

In this article, we show you 2 examples to join two lists in Java.

1. JDK – List.addAll()
2. Apache Common – ListUtils.union()

1. List.addAll() example

Just combine two lists with List.addAll().

package com.mkyong.example;

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

public class JoinListsExample {

	public static void main(String[] args) {
	
		List<String> listA = new ArrayList<String>();
		listA.add("A");
		
		List<String> listB = new ArrayList<String>();
		listB.add("B");
		
		List<String> listFinal = new ArrayList<String>();
		listFinal.addAll(listA);
		listFinal.addAll(listB);
		
		//same result
		//List<String> listFinal = new ArrayList<String>(listA);
		//listFinal.addAll(listB);
		
		System.out.println("listA : " + listA);
		System.out.println("listB : " + listB);
		System.out.println("listFinal : " + listFinal);
		
	}

}

Output

listA : [A]
listB : [B]
listFinal : [A, B]

listA.addAll(listB);

2. ListUtils.union example

Apache common library – ListUtils.union().

package com.mkyong.example;

import java.util.ArrayList;
import java.util.List;
import org.apache.commons.collections.ListUtils;

public class JoinListsExample2 {

	public static void main(String[] args) {
	
		List<String> listA = new ArrayList<String>();
		listA.add("A");
		
		List<String> listB = new ArrayList<String>();
		listB.add("B");

		List<String> listFinal = ListUtils.union(listA, listB);
		
		System.out.println("listA : " + listA);
		System.out.println("listB : " + listB);
		System.out.println("listFinal : " + listFinal);
		
	}

}

Output

listA : [A]
listB : [B]
listFinal : [A, B]

Dig into the source code, the ListUtils.union is using the same List.addAll() to combine lists.

    public static List union(final List list1, final List list2) {
        final ArrayList result = new ArrayList(list1);
        result.addAll(list2);
        return result;
    }

References

1. How To Count Duplicated Items In Java List
2. Apache common ListUtils JavaDoc
3. List.addAll() JavaDoc