In Java type arguments, does <? extends E> mean strictly subtypes only? or would E also suffice?

问题

In Java type arguments, does mean strictly subtypes only? or would E also suffice?

回答1:

Yes, super and extends gives inclusive lower and upper bounds respectively.

Here's a quote from Angelika Langer's Generics FAQ:

What is a bounded wildcard?

A wildcard with an upper bound looks like ? extends Type and stands for the family of all types that are subtypes of Type , type Type being included. Type is called the upper bound.

A wildcard with a lower bound looks like ? super Type and stands for the family of all types that are supertypes of Type , type Type being included. Type is called the lower bound.

回答2:

It's not strict; E would suffice.

回答3:

List<? extends Animal> animalList=new List<Dog>();
List<? extends Animal> animalList=new List<Animal>();

Both the lines compile without any error. Any function taking the list as a parameter understands that the objects in the list are of type E or a subtype of E.

来源：https://stackoverflow.com/questions/3136680/in-java-type-arguments-does-extends-e-mean-strictly-subtypes-only-or-would