Java generics naming conventions

Question

According to the Java documentation:

The most commonly used type parameter names are:

E - Element (used extensively by the Java Collections Framework)

K - Key

N - Number

T - Type

V - Value

S,U,V etc. - 2nd, 3rd, 4th types

However , I have seen codes like this

public <R> Observable<R> compose(Transformer<? super T, ? extends R> transformer) {
        return ((Transformer<T, R>) transformer).call(this);
    }

Here it is using R, from what source can I find out what R means in here?

Answer 1

"Result" or "return type." The former is particularly common in the JDK 8 classes.

Consider the JavaDocs for java.util.function.Function, for instance:

R - the type of the result of the function

The patterns <R> and , ?R> don't appear in OpenJDK 7's classes (citation: I downloaded the source and did a grep), but they do appear in several classes for building the Java toolchain itself, like javac. There, the R sometimes seems to stand for "result" and sometimes for "return type."