Java 8 - Collect and Shuffle List or Stream

In this short Byte - we'll take a look at how you can collect and shuffle a list in Java, using the Functional API, and paired with the collectingAndThen() collector.

Advice: If you'd like to read more about collectingAndThen() - read our in-depth "Guide to Java 8 Collectors: collectingAndThen()"!

Collect and Shuffle a List in Java

When collecting a stream back into a list (streamed from a Java Collection) - you may decide to shuffle it. Using Java 8's Functional API - working with streams is efficient and simple.

An intuitive method to make use of here is collectingAndThen() which allows you to collect() a Stream and then run an anonymous function on the result:

public class Shuffle {
    private final Stream<?> stream;
    
    public List<?> getList() {
        return stream.collect(
            collectingAndThen(
                toList(),
                l -> {
                    Collections.shuffle(l);
                    return l;
                }
            )
        );
    }    
}

Here, we've collected the stream into a List, which is a terminal operation (ending the stream). Then, that list is streamed again, utilizing the central Collections.shuffle() method, which accepts any valid Collection and shuffles it in-place.

You can test the method and assert the correct output with:

@Test
public void shouldShuffleList() {
    Shuffle shuffle = new Shuffle(Stream.of(1,2,3));
    List<?> list = shuffle.getList();
    
    assertNotEquals("[1, 2, 3]", list.toString());
}

Conclusion

In this short Byte - we took a look at how you can collect and shuffle a list or stream in Java 8+.