Generics <?> is the same as <? extends Object>

[Sean Lim]

Hi guys,

I was reading through the K&B SCJP 6 book and maybe I read it wrong but it said that <?> was equivalent to <? extends Object>. I was wondering if that is true. I am asking because of a little inconsistency I saw when trying out some code. When running the below code it compiled just fine.

import java.util.*;

public class Temp {
	public static void main(String[] args) {
		List<?> list = new ArrayList();
	}
}

But compiling it with <? extends Object> I got a compiler warning.

Note: Temp.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

Can somebody clarify this for me. Thanks in advance.

[Jeremy James Brown]

They are not equivalent. <?> has no type parameter specified. Any object you try to add will have a type and therefor be disallowed. The exception is null because it has no type. However that is quite useless, isn't it? <? extends Object> sets Object as the upper bound and will accept only additions of type Object. Upcasting is not implicit with generics because that would ruin the whole point.

To make you code work you should pass a type to the instantiation. Like this.

List<?> list = new ArrayList<Object>();
List<? extends Object> list2 = new ArrayList<Object>();

This article is excellent, and fully explains the use of generics.

http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf

[John Stark]

<? extends Object> sets Object as the upper bound and will accept only additions of type Object.

Are you sure this is the case? If you do

void doIt(List<? extends Object> aList)
{
       aList.add(new Object());
}

then you get a compilation error. This makes sense as the list passed as parameter could be of any type and for a List of type <String> you cannot add an instance of Object.
Do I misunderstand anything?

John

[Paul Clapham]

Yes, I think you understand it correctly. To repeat what you said in different words: "List<? extends Object>" doesn't mean a List which can include anything which extends Object. It means a List of some unknown type which extends Object. For example a "List<File>" could be such a List.