Generics type parameters and method calling

Question

I have I_class with type parameters I and D:

class I_class<I extends I_class<I,D>, D extends D_class<I,D>>

I also have X_class with the same type parameters:

class X_class<I extends I_class<I,D>, D extends D_class<I,D>>

X_class has the method:

public D produceDeliver(I item) {...}

But when I try to call this method from I_class:

X_class<I, D> x_object = ...    

public D produceDeliver(){ 
         D deliver = x_object.produceDeliver(this);
         ...

I get this error message:

The method produceDeliver(I) in the type X_class< I,D > is not applicable for the arguments (I_class< I,D >)

I don't understand the non-equivalence between I and I_class< I,D >, since I extends I_class< I,D >

`I` extends `I_class` but `I_class` does not extend `I`. What you're trying to do is equivalent to assigning an `Object` to a variable of type `String`. — Jorn Vernee, Mar 14 '17 at 21:51
That's right @Jorn and if he is sure they're compatible he needs to cast. — Jordão, Mar 14 '17 at 21:53
The types are not compatible. Check out this [similar question](http://stackoverflow.com/questions/26275665/java-generics-b-extends-baseb-does-not-match-extends-baseb) — Jordão, Mar 14 '17 at 21:54

Chris Owens · Accepted Answer · 2017-03-14T22:01:07.420

An example with more concrete names might be helpful:

class Building<B extends Building<B, X>, X extends Thing<B, X>> {
  Shop<B,X> s = new Shop<>();
  X makesError = s.getLocation(this);
}
class Shop<S extends Building<S, X>, X extends Thing<S, X>> {
   public X get_location(S shop) {...}
}

The problem is that when you are invoking getLocation, you are passing an argument of type Building. Not every instance of type Building is of type Shop.

Generics type parameters and method calling

1 Answers1