Java 8 - Collect and Slice List or Stream

In this short Byte - we'll take a look at how you can collect and slice 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()"!

Slice a List in Java

When collecting a stream back into a list (streamed from a Java Collection) - you may decide to slice it, given some start and end, effectively returning a sublist of the original one. 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 Slice {
    private final Stream<Integer> s;
    private final int from;
    private final int to;
    
    public List<Integer> getList() {
        return s.collect(
            Collectors.collectingAndThen(
                toList(),
                l -> {
                    return l.stream()
                        .skip(from)
                        .limit(to - (from - 1))
                        .collect(toList());
                }
            )
        );
    }
}

Here, we've collected the stream into a List, which is a terminal operation (ending the stream). Then, that list is streamed again, utilizing skip() and limit() to sublist the original list, collecting back into a List.

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

@Test
public void shouldSliceList() {
    Stream<Integer> stream = IntStream
        .rangeClosed(0, 10)
        .boxed();
    Slice slice = new Slice(stream, 3, 7);
    List<Integer> list = slice.getList();
    
    assertEquals(
        "[3, 4, 5, 6, 7]",
        list.toString()
    );
}

Conclusion

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