Using Java 8’s Optional with Stream::flatMap

by

Java 9

Optional.stream has been added to JDK 9. This enables you to do the following, without the need of any helper method:

Optional<Other> result =
    things.stream()
          .map(this::resolve)
          .flatMap(Optional::stream)
          .findFirst();

Java 8

Yes, this was a small hole in the API, in that it’s somewhat inconvenient to turn an Optional<T> into a zero-or-one length Stream<T>. You could do this:

Optional<Other> result =
    things.stream()
          .map(this::resolve)
          .flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
          .findFirst();

Having the ternary operator inside the flatMap is a bit cumbersome, though, so it might be better to write a little helper function to do this:

/**
 * Turns an Optional<T> into a Stream<T> of length zero or one depending upon
 * whether a value is present.
 */
static <T> Stream<T> streamopt(Optional<T> opt) {
    if (opt.isPresent())
        return Stream.of(opt.get());
    else
        return Stream.empty();
}

Optional<Other> result =
    things.stream()
          .flatMap(t -> streamopt(resolve(t)))
          .findFirst();

Here, I’ve inlined the call to resolve() instead of having a separate map() operation, but this is a matter of taste.

More Related Contents:

  1. Java 8 List into Map
  2. Why filter() after flatMap() is “not completely” lazy in Java streams?
  3. How can I throw CHECKED exceptions from inside Java 8 streams?
  4. How to force max to return ALL maximum values in a Java Stream?
  5. Java 8 lambdas, Function.identity() or t->t
  6. Copy a stream to avoid “stream has already been operated upon or closed”
  7. Java 8 Nested (Multi level) group by
  8. Java 8 Streams: multiple filters vs. complex condition
  9. Group by and sum objects like in SQL with Java lambdas?
  10. Retrieving a List from a java.util.stream.Stream in Java 8
  11. Filter Java Stream to 1 and only 1 element
  12. Java 8, Streams to find the duplicate elements
  13. Why doesn’t java.util.Collection implement the new Stream interface?
  14. Find maximum, minimum, sum and average of a list in Java 8
  15. Does Java SE 8 have Pairs or Tuples?
  16. How to use a Java8 lambda to sort a stream in reverse order?
  17. Java 8 Collectors.toMap SortedMap
  18. Why does Stream.allMatch() return true for an empty stream?
  19. Java 8 grouping using custom collector?
  20. How can I throw CHECKED exceptions from inside Java 8 lambdas/streams?
  21. How to apply multiple predicates to a java.util.Stream?
  22. Parallel stream from a HashSet doesn’t run in parallel
  23. Picking elements of a list until condition is met with Java 8 Lambdas
  24. Non-interference exact meaning in Java 8 streams
  25. In Java 8 how do I transform a Map to another Map using a lambda?
  26. Convert String array to Map using Java 8 Lambda expressions
  27. Group sequences of values
  28. Java 8 stream map to list of keys sorted by values
  29. Java 8: How do I work with exception throwing methods in streams?
  30. Cannot use Java 8 method with lambda arguments without specifying type arguments

Leave a Comment