Java wildcards with super

Question

I read that using a wildcard with super like this:

public class MyClass <T extends Comparable<? super T>> {
   ...
}

instead of:

public class MyClass <T extends Comparable<T>> {
   ...
}

could make the class 'more generic', but I do not understand why.

Can someone provide some concrete examples?

Answer 1

This way you can supply a class for T, which does not for itself implements Comparable, but inherits from a class implementing Comparable.

E.g.

class Baseclass implements Comparable<Baseclass> {
...
}

class Inherited extends Baseclass {
...
}

With a specification like

public class MyClass <T extends Comparable<? super T>> {
...
}

you can use MyClass<Inherited>, and MyClass<Baseclass>, but with

public class MyClass <T extends Comparable<T>> {
...
}

you can only use MyClass<Baseclass>