Generic Declaration

[Vierda Mila]

Dear All,

please find the one below from devaka coorey simulator

abstract class Fractal <K extends Number> {
    //public abstract <K> K useMe(Object k); ok
    //public abstract K <K>  useMe(Object k); return type before generic declaration
    //public abstract <K> Fractal <? extends Number> useMe(Fractal<? super K>k);ok
    //public abstract <K> Fractal <? super Number> useMe(Fractal<? extends K>k);ok
    //public abstract <K> Fractal <K> useMe(Fractal<K>k); error type parameter K is not within its bound 
    //public abstract <V extends K> Fractal <V> useMe(Fractal<V>k);ok
    //public abstract <V super K> Fractal <V> useMe(Fractal<V>k); error illegal start of type
    //public abstract <V extends Character> Fractal <? super V> useMe(Fractal<K>k);type parameter ? super U is not within its bound 
    //public abstract <V super Character> Fractal <? super V> useMe(Fractal<K>k);illegal start of type 
}    

I have compiled above code to see what happened. I don't understand compiler error that said

type parameter K is not within its bound and type parameter ? super V is not within its bound

what is error exactly mean and I'm not clear enough how to deal with question like this.

really appreciate for any kind help. thank you.

[Henry Wong]

what is error exactly mean and I'm not clear enough how to deal with question like this.

This option...

public abstract <K> Fractal <K> useMe(Fractal<K> k);

is just too weird. Hopefully, this will never be encountered in real code. My main issue is this generic method declares a generic type K, but the method is in a class that also has a generic type K. It's just too confusing to figure out which K is which...

But to answer your question...

This generic method is defining a new type K (which is unbounded), and then declares a method that takes a Fractal of type K, and returns a Fractal of type K. Unfortunately, Fractual can't take an unbounded type. It can only work with types that extend Number.

This is what the compiler meant by "type parameter K is not within its bound". The unbounded type K can't be used as part of Fractal.

Henry

[Devaka Cooray]

Henry Wong wrote:My main issue is this generic method declares a generic type K, but the method is in a class that also has a generic type K. It's just too confusing to figure out which K is which...

The Explanation for this question says:

When you have declare a generic type for a method, as same as to the generic type declare for the class, the class-generic-type will be shadowed by the, method's-generic-type. Therefore, for this method, K is just a new generic type.

Hmmm,.... Is this question too much tough for SCJP? Let me know, if it is, I'll remove this question from the next version of ExamLab.

Devaka.

[Vierda Mila]

Hmmm,.... Is this question too much tough for SCJP?

dear Devaka,

I don't know about others thinks but I have to reschedule my exam after trying your simulator . in another mock I can gained 75%-80% but in yours I never get beyond 55% thats mean I'm fail in exam

but really appreciate for this simulator.it makes me to learn a lot. and thanks for make it free. keep a great job man