Generics

[Omni Muss]

In the book I am reading there is written:

Wildcards can be bounded. For example, <? extends Runnable> specifies that ? can match
any type as long as it is Runnable or any of its derived types. Note that extends is inclusive, so
you can replace X in ? extends X. However, in <? super Runnable> , ? would match only the
super types of Runnable, and Runnable itself will not match (i.e., it is an exclusive clause).

I just want to confirm if the bold part is is wrong or else I am understanding it wrongly. Given the below:

import java.util.*;

public class Example1 {

	static class A {}
	static class B extends A {}
	static class C extends B {}
	
	public static void main(String... args){
	
		List<A> lA = new ArrayList<>();
		List<B> lB = new ArrayList<>();
		List<C> lC = new ArrayList<>();
		
		List<? super B> lSupB1 = lA;	// 1
		List<? super B> lSupB2 = lB;	// 2
	}
}

Line 2 compiles successfully. Therefore is the statement "and Runnable itself will not match" correct within the given context?

[Matthew Brown]

No - the book is incorrect. Which book is it? That's not a good sign!

[Omni Muss]

Matthew Brown wrote:No - the book is incorrect. Which book is it? That's not a good sign!

Thanks you for your answer. The book is Oracle Certified Professional Java SE 7 Programmer Exam.
To be honest this is the one error I found so far.

[Omni Muss]

I just want to confirm I understand <? super B> correctly.

Am I right to assume that List<? super B> ONLY make sense to use when you want to add elements to that List which are of type B or a class that extends B? I am making this assumption as a List<? super B> cannot return anything other then Object; i.e. - the following would fail:

		List<? super B> l = new ArrayList<>();
		l.add(new B());	// OK
		l.add(new C());	// OK
		B b = l.get(0); // Not OK

I understand why the line marked as "Not OK" doesn't work, as a List<? super B> can be a List<A> and therefore "B b = l.get(0)" would fail. I just want to confirm my understanding / assumption of where it is right to use <? super X> is right or wrong.

?

[Paul Clapham]

Omni Muss wrote:I just want to confirm I understand <? super B> correctly.

Am I right to assume that List<? super B> ONLY make sense to use when you want to add elements to that List which are of type B or a class that extends B?

No, that's wrong. List<? super B> means you have a List whose elements are of some class X which is a superclass of B. Not a class which extends B, that would be a subclass of B. And remember that B is classified as a superclass of B when you're talking about generic wildcards.

Edit:

As for adding elements to such a list, remember that the compiler doesn't know anything about the class X except that it's a superclass of B. So you can add an object to the list provided that it's a X, i.e. it's any object which is a subclass of X. In the example we know that B is a subclass of X, so we can add a B element to the list. Likewise if C is a subclass of B then it's a subclass of X, so we can add a C element to the list.

[Omni Muss]

But from a practical point of view the only thing you can do with a List<? super B> is add objects of type B or extend from B right? What other uses are there for <? super B>?

Paul Clapham wrote:

Omni Muss wrote:I just want to confirm I understand <? super B> correctly.

Am I right to assume that List<? super B> ONLY make sense to use when you want to add elements to that List which are of type B or a class that extends B?

No, that's wrong. List<? super B> means you have a List whose elements are of some class X which is a superclass of B. Not a class which extends B, that would be a subclass of B. And remember that B is classified as a superclass of B when you're talking about generic wildcards.

Edit:

As for adding elements to such a list, remember that the compiler doesn't know anything about the class X except that it's a superclass of B. So you can add an object to the list provided that it's a X, i.e. it's any object which is a subclass of X. In the example we know that B is a subclass of X, so we can add a B element to the list. Likewise if C is a subclass of B then it's a subclass of X, so we can add a C element to the list.