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

Question

why does

public interface ArrayOfONEITEMInterface <T extends ONEITEMInterface>{
    public List<T> getONEITEM();
}

compile, but not

public interface ArrayOfONEITEMInterface <? extends ONEITEMInterface>{
    public List<?> getONEITEM();
}

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

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<T extends Bar> {
     List<T> get();
}

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

interface Foo {
     List<? extends Bar> get()
}