Roedy Green coughed up:

http://java.sun.com/developer/JDCTechTips/2005/tt0104.html#2

Take my example, but modify the clone to return a simple string.  Now the
clone() method is returning something that is a subtype of Object (the
return type used in Object.clone()).  Note that it does not have to be a
subtype of Thing.  This works, though is a little silly:

public class Clone2
{
   public static class Thing implements Cloneable
   {
       public String clone()
       {
           return "hello";
       }
   }

   public static void main(String[] args)
   {
       Thing thing = new Thing();
       System.out.println(thing.clone());
   }
}

So let's make a Thing and a sub-Thing (a Car):

public class Clone3
{
   public static class Thing
   {
       public List method()
       {
           return new LinkedList();
       }
   }

   public static class Car extends Thing
   {
       public ArrayList method()
       {
           return new ArrayList();
       }
   }

   public static void main(String[] args)
   {
       Car car = new Car();
       System.out.println(car.method());
   }
}

This works because the (Car).method() returns an ArrayList, a sub-type of
List.

If I modify this to return some non-sub type:

public class Clone4
{
   public static class Thing
   {
       public List method()
       {
           return new LinkedList();
       }
   }

   public static class Car extends Thing
   {
       public String method()
       {
           return "hello";
       }
   }

   public static void main(String[] args)
   {
       Car car = new Car();
       System.out.println(car.method());
   }
}

I get this (same as from 1.4) error message:

$ com Clone4.java
javac 1.5.0-beta2
Clone4.java:22: method() in experiments.simple.Clone4.Car cannot override
method
() in experiments.simple.Clone4.Thing; attempting to use incompatible return
typ
e
found   : java.lang.String
required: java.util.List
       public String method()
                     ^
1 error