what is the difference between ? and T in class and method signatures?

Question

why does

public interface ArrayOfONEITEMInterface {
    public List getONEITEM();
}

compile, bu

Answer 1

? is a wildcard and means any subclass of ONEITEMInterface including itself.

T is a specific implementation of ONEITEMInterface in this case.

Since ? is a wildcard, there is no relation between your ? in the class declaration and the ? in your method declaration hence it won't compile. Just List getONEITEM(); will compile though.

---

The first scenario means the entire class can handle exactly one type of Bar per instance.

interface Foo {
     List get();
}

The second scenario allows each instance to operate on any subtype of Bar

interface Foo {
     List get()
}