java generic method signature

Question

I'm a bit confused about the syntax of generic methods. My understanding after reading this post was that generic method should be declared like this:

public static <E> void printArray( E[] inputArray )

with <E> being a placeholder that informs that E is a generic type

So why do I find in the javadoc things like this:

Stream<T> filter(Predicate<? super T> predicate)

No placeholder? I would have expect

<T> Stream<T> filter(Predicate<? super T> predicate)

And why

<R> Stream<R> map(Function<? super T,? extends R> mapper)

This time there is a placeholder, but only for R and not for T. Why?

Most likely, the method is in a class that has a type parameter T, like interface Stream<T>. docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html, so the method doesn't accept a new type parameter, you're using one from the outer scope — user
– user, Commented May 28, 2020 at 15:32

Louis Wasserman · Accepted Answer · 2020-05-28 16:51:30Z

These methods are instance methods defined on the interface Stream, which defines the type parameter on the type:

public interface Stream<T> { Stream<T> filter(Predicate<? super T>) }

...so the type parameter is defined on the interface, not the method.

As you noticed, type parameters that aren't defined on the interface are defined on a method-by-method basis, in the case of map.

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