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Generics in Action

 
Greenhorn
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Can anyone point out to me what exactly is the difference between AnyClass<T> and AnyClass<? extends T>.

The scenario is I am creating a dynamic factory for one of my application, which shall create the instances of those class objects which is implementing my Parent Interface. The Use Case here is that I am storing all those Classes which has to come from this Factory in a Map<String, Class> (not typesafe) which I am instantiating using reflection.

Assuming my Parent Interface is 'Ainterface' can I have a type safe Map of above as Map<String, Class<Ainterface>> OR Map<String, Class<? extends Ainterface>> OR Both are fine?

Detailed Explanation shall be really helpful!!!

If am not clear, kindly let me know.
 
Ranch Hand
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What have you tried so far? Did you write class with both approaches?

<A> and <? extends A> and <? super A> are different.

You can try declaring List<Number> and List<? extends Number>
and put some Integer, Double into it.
 
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If you have a class Aclass that implements Ainterface, Aclass.class is not compatible with Class<Ainterface> but it is compatible with Class<? extends Ainterface>. If you use Class<Ainterface> there is only one class object you can add - Ainterface.class.
 
Ankur Kapoor
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Rob Spoor wrote:If you have a class Aclass that implements Ainterface, Aclass.class is not compatible with Class<Ainterface> but it is compatible with Class<? extends Ainterface>. If you use Class<Ainterface> there is only one class object you can add - Ainterface.class.



That is the whole point of Confusion.

Assuming we have Aclass and Bclass which is implementing Ainterface.

Now UC1:


-------------------------------------------------
And UC2:



How come UC1 will work fine and UC2 won't ?? Or Java.lang.Class implementation is bit different from the rest of the Classes ??


 
Raymond Tong
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Ankur Kapoor wrote:UC2:


See the code change above for UC2.
 
Ankur Kapoor
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Thanks for pointing that to me Raymond!!!

the corrected UC2 is :



Now according to Rob, this code wont work, But we all know UC1 will work perfectly fine!!!
so still have my doubts on the same!!!

 
Raymond Tong
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You can't compare UC1 and UC2 unless UC1 is, which both failed

To make these work, you have to use ? extends Ainterface
 
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I presume you have been through the standard tutorials (use ctrl-f for "generics" and "wildcards") and Google for Angelika Langer Java Generics FAQ.
 
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