Bdtyktl

The Optional.or method was added in Java 9. This is the method signature

public Optional<T> or​(Supplier<? extends Optional<? extends T>> supplier)

Why is the type parameter of the Supplier taking ? extends Optional rather than just Optional since Optional is a final class.

The same is true for Optional.flatMap method. This is a change from Java 8.

In Java 8, it was Function<? super T, Optional<U>> mapper wherease it was changed to Function<? super T,​? extends Optional<? extends U>> in Java 9.

I found the reasoning behind this from Stuart Marks himself

http://mail.openjdk.java.net/pipermail/core-libs-dev/2016-October/044026.html

This has to do with nested generics (Optional is nested within Function).
From the mail thread

  Function<..., Optional<StringBuilder>>



is not a subtype of



  Function<..., Optional<? extends CharSequence>>



To get around this, we have to add the outer wildcard as well, so that



  Function<..., Optional<StringBuilder>>



is a subtype of



  Function<..., ? extends Optional<? extends CharSequence>>