Apache Commons Collections - Union

Checking union

union() method of CollectionUtils can be used to get the union between two collections.

Usage

List list3 = CollectionUtils.union(list1, list2);

Declaration

Following is the declaration for org.apache.commons.collections4.CollectionUtils.union() method −

public static <O> Collection<O> union(Iterable<? extends O> a, Iterable<? extends O> b)

Parameters

• a − The first collection, must not be null.

• b − The second collection, must not be null.

Return Value

The union of the two collections.

Example - Getting Union of Two Lists with some common elements

The following example shows the usage of union() method. We'll get the union of two lists.

CommonCollectionsTester.java

package com.tutorialspoint;

import java.util.Arrays;
import java.util.List;
import org.apache.commons.collections4.CollectionUtils;

public class CommonCollectionsTester {
   public static void main(String[] args) {
      //checking inclusion
      List<String> list1 = Arrays.asList("A","A","A","C","B","B");
      List<String> list2 = Arrays.asList("A","A","B","B");
      System.out.println("List 1: " + list1);
      System.out.println("List 2: " + list2);
      System.out.println("Union of List 1 and List 2: " + CollectionUtils.union(list1, list2));
   }
}

Output

When you run the code, you will see the following output −

List 1: [A, A, A, C, B, B]
List 2: [A, A, B, B]
Union of List 1 and List 2: [A, A, A, B, B, C]

Example - Getting Union of Two Lists with no common elements

The following example shows the usage of union() method. We'll get the union of two lists.

CommonCollectionsTester.java

package com.tutorialspoint;

import java.util.Arrays;
import java.util.List;
import org.apache.commons.collections4.CollectionUtils;

public class CommonCollectionsTester {
   public static void main(String[] args) {
      //checking inclusion
      List<String> list1 = Arrays.asList("A","A","A","C","B","B");
      List<String> list2 = Arrays.asList("D","E","F","F");
      System.out.println("List 1: " + list1);
      System.out.println("List 2: " + list2);
      System.out.println("Union of List 1 and List 2: " + CollectionUtils.union(list1, list2));
   }
}

Output

When you run the code, you will see the following output −

List 1: [A, A, A, C, B, B]
List 2: [D, E, F, F]
Union of List 1 and List 2: [A, A, A, B, B, C, D, E, F, F]