Eclipse External Null Annotation for AbstractStreamEx.nonNull()

Question

Consider the following example code. This code uses Eclipse's @NonNull and @Nullable annotations to check for nulls. Unfortunately, Eclipse flags an error on the line map(toNonNull).

import java.util.function.Function;
import org.eclipse.jdt.annotation.NonNull;
import org.eclipse.jdt.annotation.Nullable;
import one.util.streamex.StreamEx;

Function<@NonNull Integer, @Nullable Integer> toNull;
Function<@NonNull Integer, @NonNull Integer> toNonNull;

toNull    = value -> value;
toNonNull = value -> value;

StreamEx.
   of(1, 2, 3, 4, 5, 6).
   map(toNull).
   nonNull().
   map(toNonNull).   // Error is here
   count();

Here's the error message:

Null type mismatch (type annotations): required 'Function' but this expression has type 'Function<@NonNull Integer, @NonNull Integer>'

I would like to create an Eclipse External Annotation on StreamEx's nonNull() (or more precisely AbstractStreamEx's nonNull()) so that Eclipse will know that the values in the stream can not be null.

Here's the start of the Eclipse External Annotation.

class one/util/streamex/AbstractStreamEx<TS>

nonNull
 ()TS;
 ()???;

What do I put for ????

Answer 1

For StreamEx 0.6.4 or newer, StreamEx overrides nonNull() so that one can use the following External Annotation:

nonNull
 ()Lone/util/streamex/StreamEx<TT;>;
 ()L1one/util/streamex/StreamEx<T1T;>;

For StreamEx 0.6.3 or older, one ugly solution is to change the Java code. This works around the compiler error.

StreamEx.
   of(1, 2, 3, 4, 5, 6).
   map(toNull).
   nonNull().
   map(item -> item != null ? toNonNull.apply(item) : null).
   count();