T copy()

Hi,

This seems contrary to Java's design, but I will ask the question
anyway.

Suppose I want to have

public interface Element {

  public Element copy();

}

and

public class MyClass implements Element {

  public MyClass copy() {
    return new MyClass();
}

}

This is invalid Java but can something like this be accomplished with
Java 5 features?

Here is another question of the same kind. Suppose I am implementing
some kind of mathematical operator theory package. I have to concepts:
LinearOperator and Sum with the property that LO(a+b) = LO(a) + LO(b)

I believe this logic belongs at the LinearOperator level, rather than
at the level of a derived class, such as (TimeDerivative extends
LinearOperator).

So here's the type of method I'd like to write in LinearOperator (fake
code):

public class LinearOperator {
  Element argument;

  public Sum distribute() {
    if (argument.isASum()) {
      Sum sum = (Sum) argument;
        return new Sum(new LinearOperator(sum.a), new LinearOperator
(sum.b));
    }

    throw new RuntimeException();
  }
}

However, rather than creating an instance of LinearOperator, I need to
create an instance of the derived class, such as TimeDerivative.

So is there a good way to keep this logic at the LinearOperator level?