when would CloneNotSupportedException be thrown?

Question

I'm going through some old code and found the following:

public class MyClass implements Cloneable {

    public Object clone() {
        Object o = null;
        try {
            o = super.clone();
        } catch (CloneNotSupportedException ex) {
        }
        return o;
    }

}

I've read the javadocs on Object.clone(), and I'm trying to figure out why this catch is even there. I mean, I understand it has to be there because Object.clone() throws it, but when would it ever get there, if I'm only extending Object by default, and this class is implmenting Cloneable? If this class was extended and the sub-class didn't implement Cloneable, is that what it's there for?

So is it OK to leave that catch block empty?

Answer 1

No, don't leave it empty. Log and throw a RuntimeException. Always do this for things that you think are impossible - that way, if the impossible eventually happens, it's treated as an unexpected error (which it is) rather than just returning null as if nothing bad had happened.

Admittedly I really don't expect you to ever see it, but the above is a generally good way to handle errors you shouldn't see...

Answer 2

there is no problem if you re-throw or not the exception. Your object MyClass will never throw CloneNotSupportedEx because it is being thrown only when the interface is not implemented. See the javadoc for Clonable

By convention, classes that implement this interface should override Object.clone (which is protected) with a public method. See {@link java.lang.Object#clone()} for details on overriding this method.

Note that this interface does not contain the clone method. Therefore, it is not possible to clone an object merely by virtue of the fact that it implements this interface. Even if the clone method is invoked reflectively, there is no guarantee that it will succeed.