Extending Classes - The Java Programming Language - page 152

CloneNotSupportedException it may receive from other objects it
tries to clone. Or a class may have the ability to be cloned itself
but not require that all subclasses also have the ability to be
cloned.
• Allow subclasses to support clone but don't publicly support it.
Such a class doesn't implement Cloneable , but if the default im-
plementation of clone isn't correct, the class provides a protected
clone implementation that clones its fields correctly.
• Forbid clone . Such a class does not implement Cloneable and
provides a clone method that always throws CloneNotSupportedEx-
ception .
Object.clone checks whether the object on which it was invoked imple-
ments the Cloneable interface and throws CloneNotSupportedException if it
does not. Otherwise, Object.clone creates a new object of exactly the
same type as the original object on which clone is invoked and initial-
izes the fields of the new, cloned object to have the same values as the
fields of the original object. When Object.clone is finished, it returns a
reference to the new object.
The simplest way to make a class that can be cloned is to declare that it
implements the Cloneable interface, and override the clone method, re-
declaring it to be public:
public class MyClass extends HerClass implements Cloneable {
public MyClass clone()
throws CloneNotSupportedException {
return (MyClass) super.clone();
}
// ...
}
Any other code can now make a clone of a MyClass object. In this simple
case, all fields of MyClass will be assigned by Object.clone into the new