Union of Two Arrays in Java

Learn to find the union between two arrays in Java using HashSet and Stream APIs. In set theory, the union (denoted by U) of a collection of sets is the set of all elements in the collection.

For example, the union of two arrays A and B is a collection of all the elements which are either in A, or in B, or in both A and B.

1. Union of Arrays using HashSet

To get the union of two arrays, follow these steps:

• Push the first array in a HashSet instance.
• Use addAll() method to add the elements of the second array into the set.
• Similarly, add all the elements of more arrays in the set, if any.

Java program to get the union between two integer arrays and print the output.

Integer[] arr1 = {0, 2};
Integer[] arr2 = {1, 3};

HashSet<Integer> set = new HashSet<>();

set.addAll(Arrays.asList(arr1));
set.addAll(Arrays.asList(arr2));

Assertions.assertEquals(Set.of(0, 1, 2, 3), set);

//convert to array, if needed
Integer[] union = set.toArray(new Integer[0]);

Assertions.assertArrayEquals(new Integer[]{0, 1, 2, 3}, union);

2. Using Java 8 Stream

We can use the Stream APIs to iterate over the elements of both streams and use flatMap() to normalize the items of both arrays in a single stream, and then collect to a new array.

Using Streams allows us to perform additional actions on the union items in the same stream pipeline, and collect the results in a variety of target types.

Integer[] arr1 = {0, 2};
Integer[] arr2 = {1, 3};

Integer[] union = Stream.of(arr1, arr2)
		.flatMap(Stream::of)
		.toArray(Integer[]::new);

Assertions.assertArrayEquals(new Integer[]{0, 1, 2, 3}, union);

Happy Learning !!

Read More : How to get intersection of two arrays