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Generics and use with Vector

 
Francesco Sisca
Greenhorn
Posts: 3
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i have a simple question...

this is a simple class implements a simple interface


this is a simple interface


this is the main

give me this error


who tells me because i get this error and how i can solve?
 
Matthew Brown
Bartender
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Hi Francesco. Welcome to the Ranch!

You can't assign a Vector<PallinoNome> to a Vector<PincoPallino>. The problem is that if you did, you'd then be able to add instances of other classes that implement PincoPallino into a collection that was originally supposed to be a Vector<PallinoNome>. So you'd lose the type safety that generics are supposed to give you.

The general rule is, even if A is-a B, a GenericClass<A> is-not-a GenericClass<B>.

Your options to get round this are:

- Use either PincoPallino or PallinoNome as your generic type everywhere
- Make your method generic, and use a bounded type parameter
- Use a wildcard type parameter

Which is best depends on what you're trying to achieve, but the second and third options are more complicated. There's much more information about how they work in the Java tutorials at http://docs.oracle.com/javase/tutorial/java/generics/index.html

By the way, Vector is considered a legacy class nowadays. I'd strongly recommend using ArrayList instead.
 
Francesco Sisca
Greenhorn
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i use this solution.

thanks a lot for suggest
 
It is sorta covered in the JavaRanch Style Guide.
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