Question

I have below class hierarchy.

First class

class First<T> {
}

Second class

class Second<T> extends First<T> {

}

Third Class

class Third<T extends First<T>> {
}

Error line:

Third<Second<String>> name = new Third<Second<String>>();//Compilation error

Bound mismatch: The type Second<String> is not a valid substitute for the
bounded parameter <T extends First<T>> of the type Third<T>

I am really confused with above error. Can you please explain Why this compilation error occurs?

Answer 1

T extends First<T> is not the same as Second<String>, because T is bound both to String and something that extends First<T>.

I have the feeling that you want to use different parameters in Third, doing something like

class Third<T extends First<?>> {
}