Collect a Java Stream to an Immutable Collection

Collectors introduced in Java 8 provide a powerful mechanism for working with Java streams. In this short tutorial we will look at how to use a collector to collect the items from a stream to an immutable collection.

To save time we will use the excellent Google Guava library which provides support for immutable collections. We will install the Guava library as a maven dependency.

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<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>23.0</version>
</dependency>

Now let’s write some code to complete our task. From Guava version 21.0 each immutable collection in the Guava library comes with a corresponding collector. We can take advantage of this combination to complete our task.

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@Test
public void toImmutableList() {
    List<Integer> listOfIntegers =  List.of(1, 2, 3, 4, 4, 5);

    List<Integer> immutableList = listOfIntegers
        .stream()
        .collect(ImmutableList.toImmutableList());

    assertThat(listOfIntegers, is(immutableList));
}

In the above code we use the stream collect method and pass an immutable list collector from the Guava library.

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ImmutableList.toImmutableList()

This collector collects the elements of the stream into an immutable list.

Conclusion

In this tutorial we looked at how to collect the elements of a stream into an immutable list. We used the excellent Guava library that provided us with immutable collections and collectors to perform this task.