Question

In the following:

public interface SomeInteface<A, B> {  
    public B doSomething(A a);  
} 

I want to implement a version where the method doSomething returns the parameter a back.
I tried a Holder class;

class Holder<A> {  
     public A value;  
     public(A a){this.value = a;}  
}

and return Holder. However, I am not sure how to define an implementation class of SomeInterface so that I am able to do this.

The following does not even compile:

public class SomeImplementation<X> implements SomeInterface<T> {  

  private class Holder<A> {  
    public A value;  

    public class Holder<A>{  
      public A value;  
      public(A a){this.value = a;}  
    } 
  }   

  class Implementation<A, Holder<A>> implements SomeInterface<A, Holder<A>>{    
    public Holder<A> doSomething(A a){  
      //do stuff      
      return new Holder(a);  
    }   
  }      
}       

What am I messing up here?

Answer 1

It needs to be

class Implementation<A> implements SomeInteface<A, Holder<A>>{    
          public Holder<A> doSomething(A a){  
              //do stuff  
              return new Holder<A>(a);  
          }     

   } 

In the classname you define the generic variables and their constraints. You don’t need a Holder variable.