RFR: 8015317: Optional.filter, map, and flatMap

[Jed Wesley-Smith]

> The ? extends Optional is unnecessary in flatMap as Optional is final.

interestingly enough, it actually is.

try the following test:

class OptionalTest {
  class Parent {};

  class Child extends Parent {};

  @Test public void covariantReturn() {
    Optional<Parent> some = some(new Parent());
    Function<Parent, Optional<Child>> f = new Function<Parent,
Optional<Child>>() {
      @Override public Optional<Child> apply(Parent p) {
        return some(new Child());
      }
    };
    Optional<Parent> mapped = some.<Parent> flatMap(f);
    assertThat(mapped.get(), notNullValue());
  }
}

adapted from the fugue test suite:

https://bitbucket.org/atlassian/fugue/src/96a65067fb7aaf1edae1bffa07167a5865cbebec/src/test/java/com/atlassian/fugue/OptionTest.java#cl-155

The point to remember is that Optional<Child> is a type and as such is
actually a subtype of Optional<Parent> –  and therefore requires a
covariant return.

cheers,
jed.




On 13 July 2013 04:15, Mike Duigou <mike.duigou at oracle.com> wrote:

> The ? extends Optional is unnecessary in flatMap as Optional is final.
> Otherwise this looks good.
>
> Mike
>
> On Jul 5 2013, at 14:37 , Henry Jen wrote:
>
> > Hi,
> >
> > Please review the webrev at
> >
> > http://cr.openjdk.java.net/~henryjen/ccc/8015317.0/webrev/
> >
> > Which adds following method to Optional,
> >
> > public static <T> Optional<T> ofNullable(T value) {}
> > public Optional<T> filter(Predicate<? super T> predicate) {}
> > public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {}
> > public<U> Optional<U> flatMap(Function<? super T, ? extends Optional<U>>
> > mapper) {}
> >
> > Also included is some cleanup on javadoc.
> >
> > Cheers,
> > Henry
>
>
>