Tutorial :Why is the clone() method protected in java.lang.Object?



Question:

What is the specific reason that clone() is defined as protected in java.lang.Object?


Solution:1

The fact that clone is protected is extremely dubious - as is the fact that the clone method is not declared in the Cloneable interface.

It makes the method pretty useless for taking copies of data because you cannot say:

if(a instanceof Cloneable) {      copy = ((Cloneable) a).clone();  }  

I think that the design of Cloneable is now largely regarded as a mistake (citation below). I would normally want to be able to make implementations of an interface Cloneable but not necessarily make the interface Cloneable (similar to the use of Serializable). This cannot be done without reflection:

ISomething i = ...  if (i instanceof Cloneable) {     //DAMN! I Need to know about ISomethingImpl! Unless...     copy = (ISomething) i.getClass().getMethod("clone").invoke(i);  }  

Citation From Josh Bloch's Effective Java:
"The Cloneable interface was intended as a mixin interface for objects to advertise that they permit cloning. Unfortunately it fails to serve this purpose ... This is a highly atypical use of interfaces and not one to be emulated ... In order for implementing the interface to have any effect on a class, it and all of its superclasses must obey a fairly complex, unenforceable and largely undocumented protocol"


Solution:2

The Clonable interface is just a marker saying the class can support clone. The method is protected because you shouldn't call it on object, you can (and should) override it as public.

From Sun:

In class Object, the clone() method is declared protected. If all you do is implement Cloneable, only subclasses and members of the same package will be able to invoke clone() on the object. To enable any class in any package to access the clone() method, you'll have to override it and declare it public, as is done below. (When you override a method, you can make it less private, but not more private. Here, the protected clone() method in Object is being overridden as a public method.)


Solution:3

clone is protected because it is something that ought to be overridden so that it is specific to the current class. While it would be possible to create a public clone method that would clone any object at all this would not be as good as a method written specifically for the class that needs it.


Solution:4

The Clone method can't be directly used on any object, which is why it is intended to be overriden by the subclass.

Of course it could be public and just throw an appropriate exception when cloning is not possible, but i think that would be misleading.

The way clone is implemented right now makes you think about why you want to use clone, and how you want your object to be cloned.


Solution:5

It is protected because the default implementation does a shallow memberwise copy of all fields (including private), circumventing constructor. This is not something an object might be designed to handle in the first place (for example, it might keep track of created object instances in a shared list, or something similar).

For the same reason, the default implementation of clone() will throw if the object it's called on doesn't implement Cloneable. It's a potentially unsafe operation with far-reaching consequences, and therefore author of the class must explicitly opt-in.


Solution:6

From the javadoc of cloneable.

* By convention, classes that implement this interface (cloneable) should override   * <tt>Object.clone</tt> (which is protected) with a public method.  * See {@link java.lang.Object#clone()} for details on overriding this  * method.    * Note that this interface does <i>not</i> contain the <tt>clone</tt> method.  * Therefore, it is not possible to clone an object merely by virtue of the  * fact that it implements this interface.  Even if the clone method is invoked  * reflectively, there is no guarantee that it will succeed.  

So you could call clone on every object but this would give you most of the time not the results you want or an exception. But is only encouraged if you implement cloneable.


Solution:7

IMHO it's as simple as this:

  • #clone must not be called on non-cloneable objects, therefore it is not made public
  • #clone has to be called by subclasses ob Object that implement Cloneable to get the shallow copy of the right class

What's the right scope for methods that shall be callable by subclasses, but not by other classes?

It's protected.

Classes implementing Cloneable of course will make this method public so it can be called from other classes.


Solution:8

Clone() method has a check internally 'instance of Cloneable or not'.This is how Java team might thought will restrict the improper use of clone() method.clone() method is protected i.e. accessed by subclasses only. Since object is the parent class of all sub classes, so Clone() method can be used by all classes infact if we don't have above check of 'instance of Cloneable'. This is the reason Java team might have thought to restrict the improper use of clone() by having the check in the clone() method 'is it instance of Cloneable'.

Hence whatever classes implement cloneable can use clone() method of Object class.

Also since it made protected, it is available to only those sub classes that implements cloneable interface. If we want to make it public, this method has to be overridden by the sub class with their own implementation of it.


Solution:9

Yes, same problem that I met. But I solve it by implementing this code

public class Side implements Cloneable {      public Side clone() {            Side side = null;          try {              side = (Side) super.clone();          } catch (CloneNotSupportedException e) {              System.err.println(e);          }          return side;      }  }  

Just as the before someone said.


Solution:10

Well, also the sun developers are only human, and they did indeed make huge mistake to implement the clone method as protected, the same mistake as they implemented a non-functioning clone method in ArrayList! So, in general, there exist a much deeper misunderstanding of even experienced Java programmers about the clone method.

However, I've recently found a fast and easy solution to copy any object with all its content, no matter how it is built and what it contains, see my answer here: Bug in using Object.clone()


Solution:11

Again, Java JDK framework shows brilliant thinking:

Cloneable interface does not contain a "public T clone();" method because it acts more like an attribute (eg. Serializable) which allows an instance it to be cloned.

There is nothing wrong with this design because:

  1. Object.clone() will not do what you want with your custom-defined class.

  2. If you have Myclass implements Cloneable => you overwrite clone() with "public MyClass clone()"

  3. If you have MyInterface extends Cloneable and some MyClasses implementing MyInterface: simply define "public MyInterface clone();" in the interface and every method using MyInterface objects will be able to clone them, no matter their MyClass-class.


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