Why is Java's Cloneable Interface Not Generic?

Question

Java 5 introduced generics, and they were added to many interfaces in the java.lang package. However, Cloneable did not get generics. I wonder why?

---

Edit: In reply to the answers of @Jon and @litb, and the comment of @Earwicker, I was thinking Cloneable might be:

public interface Cloneable<T> {
    public T clone();
}

Here T clone(); overrides Object.clone(), giving it a covariant type. I believe this would still be backwards compatible and increase type safety. So why not?

---

Edit 2: As can be seen in the answers (and comments) below, the interface suggested above would break backwards-compatibility. Since Object.clone() is protected, rewriting it in the interface would force all implementers to provide a public implementation, which class designers might not want to (i.e. they might opt to keep it protected).

Answer 1

The Cloneable interface doesn't contain any members. What would be the point of making it generic?

(If Cloneable contained the clone() method, it would make sense - but that's declared in java.lang.Object.)

EDIT: clone() is in java.lang.Object as it has an implementation (which does a field-by-field copy). A better design would be to have something like .NET's MemberwiseClone() as a protected method in Object, and then a public clone() method within the interface itself. I don't know why that design wasn't chosen.

(In .NET, ICloneable isn't generic because it existed before generics - the different nature of .NET generics prevents a previously non-generic type from becoming generic.)

In both Java and .NET, however, the "normal" cloning API is generally regarded as a bad thing, as it says nothing about what depth of cloning should be performed.

Answer 2

If Cloneable was Cloneable<T>, then it would be impossible to do this:

class A extends B implements Cloneable<A>

class B implements Cloneable<B>

You can't implement the same interface with different parameters, and class A implements Cloneable<A> and Cloneable<B>.