How to understand a strange generic?

Question

Java

Hello. Please explain what the following generic entry means

public static <I, O> List<O> map(Stream<I> stream, Function<I, O> mapper)

What does '<'I, O'>' mean and how is it used before List? What is the name of this phenomenon?

Answer 1

The phenomenon is called data type. In this case, this function returns a hash table with key-value pairs: keys are "I", values ​​are "O". The weird thing here is that it's just a hash table, so you can access its element by calling its key of type I, rather than just by element number as in a normal array.

Answer 2

is used to declare that a method is a generic method, the enumerated types will be used in the signature or in the body of the method.

Answer 3

this means that the function takes as arguments a Stream of type I and a Function of type I and O, returns a List of type O

Answer 4

I think you should pay attention to Alexander Matorin's video about Generics in Java.
Tyts!