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Re: Why it necessary to implement clonealbe interface?

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On 11/24/2015 11:03 AM, Eric Sosman wrote:
> On 11/24/2015 10:28 AM, Xue Feng wrote:
>> A class has to implement cloneable interface, otherwise its object can
>> not be cloned. IMO, it is enough that a class clone itself by inheriting
>> the clone method from Object class. It is somewaht reduntant to
>> implement cloneable interface. Actually, there is nothing in cloneable
>> interface. Namely, it is defined as follows:
>>
>> interface cloneable {
>> }
>>
>> I wonder the reason for this design. Any explanation? Thanks.
>
>      Object's clone() is a protected method, so it can be called only
> from a subclass of Object.  Since every class descends from Object
> this may not seem like much of a restriction, but it is: If classes
> X and Y both inherit clone() from object, then X's clone() and Y's
> clone() are also protected.  X can call its own clone() and can call
> Object's clone(), but it cannot call Y's clone -- because Y's clone()
> is protected, and X is not a subclass of Y.  Similarly, Y can call
> its own clone() and Object's clone(), but cannot call X's clone().
> So, mere inheritance of the clone() method is not enough to make it
> usable outside one branch of the inheritance tree.
>
>      Also, Object's clone() throws CloneNotSupportedException if the
> object being cloned is of a non-Cloneable class.  The Cloneable
> interface is a "marker," a notification that the class has provided
> for its own cloning.  (The "marker interface" pattern is unusual, and
> some would even call it an anti-pattern, but cloning is an unusual
> operation to begin with.)  By implementing Cloneable, a class announces
> to Java: "I have decided what it means to make a `clone' of my objects,
> and have made arrangements to create such clones."  Usually, the
> arrangements involve overriding the inherited clone() so as to do
> some class-specific things; the class might also choose to make its
> overriding clone() method public.

To this excellent explanation I would just like to add that
today (today as in last decade) instead of a marker interfaces
one would usually use an annotation. But Cloneable goes back
to Java 1.0.

Arne

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Thread

Why it necessary to implement clonealbe interface? Xue Feng <xf.lotus@yahoo.com> - 2015-11-24 23:28 +0800
  Re: Why it necessary to implement clonealbe interface? Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-11-24 11:03 -0500
    Re: Why it necessary to implement clonealbe interface? Arne Vajhøj <arne@vajhoej.dk> - 2015-11-24 19:49 -0500
      Re: Why it necessary to implement clonealbe interface? Eric Douglas <e.d.programmer@gmail.com> - 2015-11-25 11:01 -0800
        Re: Why it necessary to implement clonealbe interface? Arne Vajhøj <arne@vajhoej.dk> - 2015-11-25 16:41 -0500
  Re: Why it necessary to implement clonealbe interface? Lew <lewbloch@gmail.com> - 2015-11-29 23:00 -0800

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