Re: Generics ?

From:

Daniel Pitts <newsgroup.nospam@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Sat, 17 Dec 2011 21:20:47 -0800

Message-ID:

<PKeHq.20143$c27.3382@newsfe22.iad>

On 12/17/11 11:25 AM, Knute Johnson wrote:

On 12/17/2011 10:54 AM, markspace wrote:

On 12/17/2011 10:20 AM, Knute Johnson wrote:

Which brings me back to my real question, can you
extend a generic class and still be generic?

Yes of course.

model.addElement("test");
^
required: E#1
found: String
reason: actual argument String cannot be converted to E#1 by method
invocation conversion

The problem is that you don't *have* a generic type. You have a
parametrized type of String. Otherwise, you can't stick a String in that
thing. Which is why the compiler is complaining.

Roedy's example compiles because it matches what you did. It
parametrizes the type to String because that's what you have.

No, you cannot have a "generic" class, then assume that you can also use
"string" as a type. Generics specifically prevent that.

You could put a bound on the type of E, like <E super String> and I
think that would work (I didn't test it). But since that is basically
String, CharSequence, or Object, it's really kinda limiting while being
baroque at the same time. I'd just use "String" or "CharSequence" or
"Object" unless you're really sure that the ability to parametrize that
type is really important.

Thanks for that and to John for giving me direction to follow.

What I have learned about generics:

You can't have a generic class and then attempt to use non-generic
parameters. You can extend a parameterized(?) generic class (eg.
JList<String>) and do what I was trying to do to it but then you are
limited to String types, which is what I really needed anyway.

Incorrect. You can extend a generic class and narrow (or keep) the
generic parameters..

public class KList<E extends Serializable> extends JList<E> {
}

KList<String> foo;
KList<Integer> bar;
KList<MySerializable> baz;

You can also make concrete the parameter:

public class KListOfStrings extends JList<String> {
}

FWIW, you probably shouldn't be extending JList anyway. You should be
implementing ListModel and/or adding listeners to a JList. Pretty much
the only Swing "J*" class you ever really need to extend is JComponent.

I did learn an interesting thing about JList however and that is that
the no-arg constructor creates an anonymous class for it's read-only
ListModel. I've got to look at the source and see what's up with that.

This has only come up because I'm trying to compile some old code that
was originally created in 1.5 and 1.6 with the new 1.7 compiler.

Thanks again everybody for the pointers.