What is the point in letting my class implement Cloneable?

Question

I came across some class code that implements Clonable, the documentation states:

A class implements the Cloneable interface to indicate to the Object.clone() method that it is legal for that method to make a field-for-field copy of instances of that class. Invoking Object's clone method on an instance that does not implement the Cloneable interface results in the exception CloneNotSupportedException being thrown. By convention, classes that implement this interface should override Object.clone (which is protected) with a public method. See Object.clone() for details on overriding this method. Note that this interface does not contain the clone 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.

I can't understand the point in implementing this class, as said in the docs the .clone method is not implemented in the interface, and I have to implement it. So why use this class? Why won't I just write a method copyClass in my class to make the object copy without the implementation of this class?

You have your class implement Cloneable so that you can use the built-in cloning mechanisms, so other classes can clone yours without having to know it's unique clone methods. Yes, it is a bit of a strange way to manage it, but partly it has to do with backwards compatibility. — Hot Licks

JB Nizet JB Nizet · Accepted Answer · 2013-05-12T13:10:48

To implement the clone method, you simply do:

public Object clone() throws CloneNotSupportedException {
    return super.clone();
}

You can of course customize the method to make a deeper copy if needed.

Calling super.clone() is almost mandatory because, unless the class is final and thus can't be overridden, the clone() method must return an instance of the same class as the object on which it's called. So simply creating a new instance and copy the state will work for this class, but not for all the subclasses. Moreover, you don't always have access to all the state contained in superclasses.

In short, you make the protected clone method of Object public. And the first thing that the Object.clone() method does is (this is not the real code, but this is what the method does):

if (!(this instanceof Cloneable)) {
    throw new CloneNotSupportedException();
}

So, Cloneable is just a marker interface to let the Object.clone() method know that it must not throw an exception when called.

This is one of the most badly-designed parts of Java. Usually, you should prefer using a copy contructor instead of using clone().

What is the point in letting my class implement Cloneable?

3 Answers