Generic Collections---Dysfuntional Java

[beezerbutt]

Hi

Can anyone please tell me why this code does not compile
on the 3 lines where i am adding Strings to the collection?

I think Generics and Collections in Java although good in their goal of achieving type safety, confuse the heck out of the newbie!!!

Thanks in advance:

package com.psg.cert.ch3.wrapper;

import java.util.Arrays;
import java.util.List;
import java.util.ArrayList;
import java.util.Vector;

public class BoolTest {

static List<?> aList=new Vector<String>();

/**
* @param args
*/
public static void main(String[] args) {
aList.add("USA");
aList.add("Russia");
aList.add("UK");
}
}

[Fubarable]

I'm curious, why List<?> and not List<String> here?

[Singing Boyo]

It doesn't compile because the compiler does not know what is in the list, because it is declared as List<?>, and because of that, you cannot add elements to it. You have to change it to List<String> for it to work.

[beezerbutt]

Hi Guys

Thanks for the replies.
I just want to understand the usage of "?".
i.e. what context i would use it in...for method parameters only?

Also, let's say that i want a Collection that is heterogeneous...
How would i achieve that with the new Generic Collections.
I know the point is to have type safety, but for argument sakes, how would i get it done?
This should answer the question: Why "?" and not "String"....

regards
beez

[Singing Boyo]

You rarely really want to use <?>, except occasionally in method parameters. <?> is roughly equivalent to <? extends Object>.

[Fubarable]

Also, let's say that i want a Collection that is heterogeneous...

You really don't want this. First of all, it's a bad idea, one that will often cause bad bugs later; second it goes against OOP principles; lastly, why use generics here if you are trying to do this? Why even worry about "type safety" when the collection you are trying to create has no type?

How would i achieve that with the new Generic Collections.
I know the point is to have type safety, but for argument sakes, how would i get it done?

By declaring your List as a List<String>

[beezerbutt]

OKay...great thanks to you both.

I re-read the Certification book and Add is disallowed with "?"...i had somehow missed this.
Also, the cert book says that "?" and "? extends Object" are identical...just an FYI.

I will mark the thread as SOLVED...

best regards
beez