可以在子类的对象上调用受保护的方法而不会覆盖它们。 为什么java.lang.Object中的clone方法不同？

Question

If I have a class A

    public class A {
        protected void show() {
        }
    }

Another class B

    public class B extends A {
    }

Driver class:

    public class Driver {
          public static void main(String[] args) {
                  B b = new B();
                  b.show();
          }
    }

This thing works fine. But why is it different for clone method in Object class? If I don't override the clone method with a public modifier I get an error like: "clone has protected access in Object". We know Object is by default parent of all classes. So, this should also work the same as for the classes Driver, A and B. Why is it different?

Answer 1

From javadoc

the clone() method in a class must respect some conventions :

By convention, the returned object should be obtained by calling super.clone. If a class and all of its superclasses (except Object) obey this convention, it will be the case that x.clone().getClass() == x.getClass().

By convention, the object returned by this method should be independent of this object (which is being cloned). To achieve this independence, it may be necessary to modify one or more fields of the object returned by super.clone before returning it. Typically, this means copying any mutable objects that comprise the internal "deep structure" of the object being cloned and replacing the references to these objects with references to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually the case that no fields in the object returned by super.clone need to be modified.

The method clone for class Object performs a specific cloning operation. First, if the class of this object does not implement the interface Cloneable, then a CloneNotSupportedException is thrown. Note that all arrays are considered to implement the interface Cloneable and that the return type of the clone method of an array type T[] is T[] where T is any reference or primitive type. Otherwise, this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.

The class Object does not itself implement the interface Cloneable, so calling the clone method on an object whose class is Object will result in throwing an exception at run time.

It means that either you have to override the clone() method and implements Cloneable OR use clone() directly with handling of a thrown CloneNotSupportedException