convert from Stream<@Nullable T> to Stream<@NonNull T> using stream api

Question

I use CheckerFramework

How could I remove nulls from Stream and got collection of @NonNull objects?

Stream<@Nullable T> -> (remove nulls) -> Stream<@NonNull T>

Answer 1

Here is code that does it.

import java.util.Objects;
import java.util.stream.Stream;
import org.checkerframework.checker.nullness.qual.NonNull;
import org.checkerframework.checker.nullness.qual.Nullable;

class RemoveNullsFromStream {

  @SuppressWarnings("nullness") // Nullness Checker is not hard-coded with
                  // implementation details of filter and Objects::nonNull
  <T>
  Stream<@NonNull T> removeNullsFromStream(Stream<@Nullable T> arg) {
    return arg.filter(Objects::nonNull);
  }

}

Note the use of @SuppressWarnings, because the Nullness Checker is conservative: it issues a warning whenever it cannot definitively prove the code is safe. In general, the output of filter is the same as its input; when the argument to filter is Objects::nonNull is a special case.

That special-case could be hard-coded into the Nullness Checker, and that would make the Nullness Checker more precise. This behavior is not currently special-cased, so you use @SuppressWarnings instead.