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Thread: Generics problem

  1. #1
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    Default Generics problem

    abstract class ABS<K extends Number>
    {
    	public abstract <K> K useMe(Object k);// understood ..  
    	public abstract <K> ABS<? extends Number> useMe(ABS<? super K> k)	; //1
    	public abstract <K> ABS<? super Number> useMe(ABS<? extends K> k); //2
    	public abstract <K> ABS<K> useMe(ABS<K> k);// understood ..  
    }

    1 and 2 this both should not work because K can be anything here....

    can anyone please explain???

    Thanks in advance...


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: Generics problem

    I'm not entirely sure what you're asking for,

    #2-4 should all fail to compile (numbering 1 is the top-most method, 4 is the bottom-most)

    abstract class ABS<K extends Number>
    {
    	public abstract <K> K useMe(Object k);// compiles fine...
    	public abstract <K> ABS<? extends Number> useMe(ABS<? super K> k)	; // fails to compile because ABS<? super K> could be declared to be something like ABS<Object>, which is always impossible. Also fails for the same reason #3 fails (generic type hiding)
    	public abstract <K> ABS<? super Number> useMe(ABS<? extends K> k); // fails to compile because K here is hidden by the method generic typing, and isn't the class generic typing. Will also fail because it must return ABS<? super Number>, which fails for the same reason #2 fails.
    	public abstract <K> ABS<K> useMe(ABS<K> k);// fails to compile by the same reason as #3.
     
    	public abstract <V> ABS<? extends Number> useMe(ABS<? super V> k); // equivalent statement for #2.
     
    	public abstract <V> ABS<? super Number> useMe(ABS<? extends V> k); // equivalent statement for #3.
     
    	public abstract <V> ABS<? extends Number> useMe(ABS<V> k); // equivalent statement for #4.
    }
    Last edited by helloworld922; January 22nd, 2011 at 02:05 PM.

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