The trap in clone

To my horror, I found out today that I’d missed something basic about how to override clone(). I’d say even the best carpenters sometimes slip, except I hardly think of myself as one of the best. Let’s see where the problem lies.

public class A implements Cloneable{
 @Override
 protected Object clone() throws CloneNotSupportedException {  
  return new A();
 } 
}

public class B extends A implements Cloneable {
 @Override
 protected Object clone() throws CloneNotSupportedException {  
  return (B) super.clone();
 } 
}

The problem is in the overridden clone method of class A, which fails to use the superclass’s clone method (java.lang.Object#clone). The java.lang.Object method guarantees that the result is a copy of the very object it was called on.

Here’s the correct version.

@Override
 protected Object clone() throws CloneNotSupportedException {  
  Object clone = super.clone();
                assert clone instanceof A;
                return clone;
 }

If we fail to follow this convention in every class, we’ll get a ClassCastException. Good to know! I’m almost embarrassed to tag this post “for beginners.”