Java - downcast in clone

Question

Suppose Class B extends class A and class A is Cloneable as follows:

 public class A implements Cloneable { public Object clone() throws CloneNotSupportedException { A ac = (A) super.clone(); return ac; } } public class B extends A { public Object clone() throws CloneNotSupportedException { B a = (B) super.clone(); return a; } } 

Why is it permissible to drop from A to B in the next line:

 B a = (B) super.clone(); // (super of B is A) 

and the next shot is a runtime error?

 A a = new A(); B b = (B) a.clone(); 

Thanks in advance!

Answer 1

Ultimately, this uses Object.clone() to create the object - and this guarantees the creation of a new object of the same type of runtime as the object that it named on:

The clone method for the Object class performs a specific clone operation. First, if the class of this object does not implement the Cloneable interface, then a CloneNotSupportedException . Note that all arrays are thought to implement the Cloneable interface and that the return type of the clone method of the 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 the exact contents of the corresponding fields of this object, as if it were assigned; the contents of the fields are not cloned per se. Thus, this method performs a “shallow copy” of this object, and not a “deep copy” operation.

So, if we get the clone() call executed on instance B , then super.clone() will return B (or a subclass) - so the action will be valid.

In your second case, you only create an instance of A , which is not an instance of B , so the failure is executed.