Cuga Cuga - 3 months ago 19
Java Question

How to properly override clone method?

I need to implement a deep clone in one of my objects which has no superclass.

What is the best way to handle the checked

CloneNotSupportedException
thrown by the superclass (which is
Object
)?

A coworker advised me to handle it the following way:

@Override
public MyObject clone()
{
MyObject foo;
try
{
foo = (MyObject) super.clone();
}
catch (CloneNotSupportedException e)
{
throw new Error();
}

// Deep clone member fields here

return foo;
}


This seems like a good solution to me, but I wanted to throw it out to the StackOverflow community to see if there are any other insights I can include. Thanks!

Answer

Do you absolutely have to use clone? Most people agree that Java's clone is broken.

Josh Bloch on Design - Copy Constructor versus Cloning

If you've read the item about cloning in my book, especially if you read between the lines, you will know that I think clone is deeply broken. [...] It's a shame that Cloneable is broken, but it happens.

You may read more discussion on the topic in his book Effective Java 2nd Edition, Item 11: Override clone judiciously. He recommends instead to use a copy constructor or copy factory.

He went on to write pages of pages on how, if you feel you must, you should implement clone. But he closed with this:

Is all this complexities really necessary? Rarely. If you extend a class that implements Cloneable, you have little choice but to implement a well-behaved clone method. Otherwise, you are better off providing alternative means of object copying, or simply not providing the capability.

The emphasis was his, not mine.


Since you made it clear that you have little choice but to implement clone, here's what you can do in this case: make sure that MyObject extends java.lang.Object implements java.lang.Cloneable. If that's the case, then you can guarantee that you will NEVER catch a CloneNotSupportedException. Throwing AssertionError as some have suggested seems reasonable, but you can also add a comment that explains why the catch block will never be entered in this particular case.


Alternatively, as others have also suggested, you can perhaps implement clone without calling super.clone.

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