Description Danny Yates 2004-11-09 16:56:55 EST

Hi folks,

I've just started to play with generics, and I have a warning that I don't 
understand. I've reported it on the Sun Forums, and am assured it's an Eclipse 
bug! I'm using 3.1M3.

I use the Visitor pattern extensively in my work, and I like to have my accept
() methods take an additional object which gets passed to the visitor 
implementation as an additional argument along with the visited object. 
Typically, I make this parameter type Object, and so I figured I could use 
generics to tighten this up a bit.

Here are my efforts (sorry if the formatting is bad - this is my first forum 
post):


public interface Visitor<R, T> {
  public R visitBase(AbstractBase ab, T obj);
  public R visitDerived(Derived d, T obj);
}

public interface Visitable {
  public <R, T> R accept(Visitor<R, T> v, T obj);
}

public class AbstractBase implements Visitable {
  public <R, T> R accept(Visitor<R, T> v, T obj) {
    return v.visitBase(this, obj);
  }
}

public class Derived extends AbstractBase implements Visitable {
  public <R, T> R accept(Visitor<R, T> v, T obj) {
    return v.visitDerived(this, obj);
  }
}

public class Test {
  public static void main(String[] args) {
    AbstractBase ab = new AbstractBase();
    Derived d = new Derived();
    AbstractBase ab2 = new Derived();
    
    Visitor<String, String> v = new MyVisitor();
    System.out.println(ab.accept(v, ab.getClass().getName()));  // "Visited 
base: AbstractBase"
    System.out.println(d.accept(v, d.getClass().getName()));  // "Visited 
derived: Derived"
    System.out.println(ab2.accept(v, ab2.getClass().getName()));  // "Visited 
derived: Derived"
    
    // ab2.accept(v, new Object());  // Correctly fails to compile
  }
  
  public static class MyVisitor implements Visitor<String, String> {
    public String visitBase(AbstractBase ab, String obj) {
      return "Visited base: " + obj;
    }

    public String visitDerived(Derived d, String obj) {
      return "Visited derived: " + obj;
    }
  }
}


The warning Eclipse gives is on both the AbstractBase and Derived classes. It 
highlights the return type R on the accept() method 

public <R, T> __R__ accept(...)

and says:

"Type safety: The return type R of the method accept(Visitor<R, T>, T) of type 
AbstractBase [or Derived] needs unchecked conversion to conform to the return 
type R of inherited method"

Many thanks,

Dan.