Question

Hullo all,

Wondering if there are any Java hackers who can clue me in at to why the following doesn’t work:

public class Parent {     public Parent copy() {        Parent aCopy = new Parent();        ...        return aCopy;     } }  public class ChildN extends Parent {     ... }  public class Driver {      public static void main(String[] args) {          ChildN orig = new ChildN();          ...          ChildN copy = orig.getClass().cast(orig.copy());      } } 

The code is quite happy to compile, but decides to throw a ClassCastException at runtime D=

Edit: Whoah, really quick replies. Thanks guys! So it seems I cannot downcast using this method… is there any other way to do downcasting in Java? I did think about having each ChildN class overwrite copy(), but wasn’t enthusiastic about adding the extra boilerplate code.

Answer 1

(Can’t add code in a comment, so I’ll add here)

Regarding Cloneable: if you’re implementing Cloneable, implement it as follows; much cleaner to call…

public class Foo implements Cloneable {     public Foo clone() {         try {             return (Foo) super.clone();         } catch (CloneNotSupportedException e) {             return null; // can never happen!     } } 

[EDIT: I’ve also seen other folks use

throw new AssertionError('This should never happen! I'm Cloneable!'); 

in the catch block.]