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You might suggest that, to remove clonability, the clone( ) method simply be made private, but this won't work since you cannot take a base-class method and make it more private in a derived class. So it's not that simple. And yet, it's necessary to be able to control whether an object can be cloned. There are actually a number of attitudes you can take to this in a class that you design:
Indifference. You don't do anything about cloning, which means that your class can't be cloned but a class that inherits from you can add cloning if it wants. This works only if the default Object.clone( ) will do something reasonable with all the fields in your class.
Support clone( ). Follow the standard practice of implementing Cloneable and overriding clone( ). In the overridden clone( ), you call super.clone( ) and catch all exceptions (so your overridden clone( ) doesn't throw any exceptions).
Support cloning conditionally. If your class holds handles to other objects that might or might not be cloneable (an example of this is a collection class), you can try to clone all of the objects that you have handles to as part of your cloning, and if they throw exceptions just pass them through. For example, consider a special sort of Vector that tries to clone all the objects it holds. When you write such a Vector, you don't know what sort of objects the client programmer might put into your Vector, so you don't know whether they can be cloned.
Don't implement Cloneable but override clone( ) as protected, producing the correct copying behavior for any fields. This way, anyone inheriting from this class can override clone( ) and call super.clone( ) to produce the correct copying behavior. Note that your implementation can and should invoke super.clone( ) even though that method expects a Cloneable object (it will throw an exception otherwise), because no one will directly invoke it on an object of your type. It will get invoked only through a derived class, which, if it is to work successfully, implements Cloneable.
Try to prevent cloning by not implementing Cloneable and overriding clone( ) to throw an exception. This is successful only if any class derived from this calls super.clone( ) in its redefinition of clone( ). Otherwise, a programmer may be able to get around it.
Prevent cloning by making your class final. If clone( ) has not been overridden by any of your ancestor classes, then it can't be. If it has, then override it again and throw CloneNotSupportedException. Making the class final is the only way to guarantee that cloning is prevented. In addition, when dealing with security objects or other situations in which you want to control the number of objects created you should make all constructors private and provide one or more special methods for creating objects. That way, these methods can restrict the number of objects created and the conditions in which they're created. (A particular case of this is the singleton pattern shown in Chapter 16.)
Here's an example that shows the various ways cloning can be implemented and then, later in the hierarchy, "turned off:"
//: CheckCloneable.java
// Checking to see if a handle can be cloned
// Can't clone this because it doesn't
// override clone():
class Ordinary
// Overrides clone, but doesn't implement
// Cloneable:
class WrongClone extends Ordinary
}
// Does all the right things for cloning:
class IsCloneable extends Ordinary
implements Cloneable
}
// Turn off cloning by throwing the exception:
class NoMore extends IsCloneable
}
class TryMore extends NoMore
}
class BackOn extends NoMore
public Object clone()
}
// Can't inherit from this, so can't override
// the clone method like in BackOn:
final class ReallyNoMore extends NoMore
public class CheckCloneable catch(CloneNotSupportedException e)
}
return x;
}
public static void main(String[] args) ;
Ordinary x = new Ordinary();
// This won't compile, since clone() is
// protected in Object:
//! x = (Ordinary)x.clone();
// tryToClone() checks first to see if
// a class implements Cloneable:
for(int i = 0; i < ord.length; i++)
tryToClone(ord[i]);
}
} ///:~
The first class, Ordinary, represents the kinds of classes we've seen throughout the book: no support for cloning, but as it turns out, no prevention of cloning either. But if you have a handle to an Ordinary object that might have been upcast from a more derived class, you can't tell if it can be cloned or not.
The class WrongClone shows an incorrect way to implement cloning. It does override Object.clone( ) and makes that method public, but it doesn't implement Cloneable, so when super.clone( ) is called (which results in a call to Object.clone( )), CloneNotSupportedException is thrown so the cloning doesn't work.
In IsCloneable you can see all the right actions performed for cloning: clone( ) is overridden and Cloneable is implemented. However, this clone( ) method and several others that follow in this example do not catch CloneNotSupportedException, but instead pass it through to the caller, who must then put a try-catch block around it. In your own clone( ) methods you will typically catch CloneNotSupportedException inside clone( ) rather than passing it through. As you'll see, in this example it's more informative to pass the exceptions through.
Class NoMore attempts to "turn off" cloning in the way that the Java designers intended: in the derived class clone( ), you throw CloneNotSupportedException. The clone( ) method in class TryMore properly calls super.clone( ), and this resolves to NoMore.clone( ), which throws an exception and prevents cloning.
But what if the programmer doesn't follow the "proper" path of calling super.clone( ) inside the overridden clone( ) method? In BackOn, you can see how this can happen. This class uses a separate method duplicate( ) to make a copy of the current object and calls this method inside clone( ) instead of calling super.clone( ). The exception is never thrown and the new class is cloneable. You can't rely on throwing an exception to prevent making a cloneable class. The only sure-fire solution is shown in ReallyNoMore, which is final and thus cannot be inherited. That means if clone( ) throws an exception in the final class, it cannot be modified with inheritance and the prevention of cloning is assured. (You cannot explicitly call Object.clone( ) from a class that has an arbitrary level of inheritance; you are limited to calling super.clone( ), which has access to only the direct base class.) Thus, if you make any objects that involve security issues, you'll want to make those classes final.
The first method you see in class CheckCloneable is tryToClone( ), which takes any Ordinary object and checks to see whether it's cloneable with instanceof. If so, it casts the object to an IsCloneable, calls clone( ) and casts the result back to Ordinary, catching any exceptions that are thrown. Notice the use of run-time type identification (see Chapter 11) to print out the class name so you can see what's happening.
In main( ), different types of Ordinary objects are created and upcast to Ordinary in the array definition. The first two lines of code after that create a plain Ordinary object and try to clone it. However, this code will not compile because clone( ) is a protected method in Object. The remainder of the code steps through the array and tries to clone each object, reporting the success or failure of each. The output is:
Attempting IsCloneable
Cloned IsCloneable
Attempting NoMore
Could not clone NoMore
Attempting TryMore
Could not clone TryMore
Attempting BackOn
Cloned BackOn
Attempting ReallyNoMore
Could not clone ReallyNoMore
So to summarize, if you want a class to be cloneable:
Implement the Cloneable interface.
Override clone( ).
Call super.clone( ) inside your clone( ).
Capture exceptions inside your clone( ).
This will produce the most convenient effects.
Cloning can seem to be a complicated process to set up. It might seem like there should be an alternative. One approach that might occur to you (especially if you're a C++ programmer) is to make a special constructor whose job it is to duplicate an object. In C++, this is called the copy constructor. At first, this seems like the obvious solution. Here's an example:
//: CopyConstructor.java
// A constructor for copying an object
// of the same type, as an attempt to create
// a local copy.
class FruitQualities
// Other constructors:
//
// Copy constructor:
FruitQualities(FruitQualities f)
}
class Seed
Seed(Seed s)
}
class Fruit
// Other constructors:
//
// Copy constructor:
Fruit(Fruit f)
// To allow derived constructors (or other
// methods) to put in different qualities:
protected void addQualities(FruitQualities q)
protected FruitQualities getQualities()
}
class Tomato extends Fruit
Tomato(Tomato t)
}
class ZebraQualities extends FruitQualities
ZebraQualities(ZebraQualities z)
}
class GreenZebra extends Tomato
GreenZebra(GreenZebra g)
void evaluate()
}
public class CopyConstructor
public static void slice(Fruit f)
public static void main(String[] args)
} ///:~
This seems a bit strange at first. Sure, fruit has qualities, but why not just put data members representing those qualities directly into the Fruit class? There are two potential reasons. The first is that you might want to easily insert or change the qualities. Note that Fruit has a protected addQualities( ) method to allow derived classes to do this. (You might think the logical thing to do is to have a protected constructor in Fruit that takes a FruitQualities argument, but constructors don't inherit so it wouldn't be available in second or greater level classes.) By making the fruit qualities into a separate class, you have greater flexibility, including the ability to change the qualities midway through the lifetime of a particular Fruit object.
The second reason for making FruitQualities a separate object is in case you want to add new qualities or to change the behavior via inheritance and polymorphism. Note that for GreenZebra (which really is a type of tomato - I've grown them and they're fabulous), the constructor calls addQualities( ) and passes it a ZebraQualities object, which is derived from FruitQualities so it can be attached to the FruitQualities handle in the base class. Of course, when GreenZebra uses the FruitQualities it must downcast it to the correct type (as seen in evaluate( )), but it always knows that type is ZebraQualities.
You'll also see that there's a Seed class, and that Fruit (which by definition carries its own seeds) contains an array of Seeds.
Finally, notice that each class has a copy constructor, and that each copy constructor must take care to call the copy constructors for the base class and member objects to produce a deep copy. The copy constructor is tested inside the class CopyConstructor. The method ripen( ) takes a Tomato argument and performs copy-construction on it in order to duplicate the object:
t = new Tomato(t);
while slice( ) takes a more generic Fruit object and also duplicates it:
f = new Fruit(f);
These are tested with different kinds of Fruit in main( ). Here's the output:
In ripen, t is a Tomato
In slice, f is a Fruit
In ripen, t is a Tomato
In slice, f is a Fruit
This is where the problem shows up. After the copy-construction that happens to the Tomato inside slice( ), the result is no longer a Tomato object, but just a Fruit. It has lost all of its tomato-ness. Further, when you take a GreenZebra, both ripen( ) and slice( ) turn it into a Tomato and a Fruit, respectively. Thus, unfortunately, the copy constructor scheme is no good to us in Java when attempting to make a local copy of an object.
The copy constructor is a fundamental part of C++, since it automatically makes a local copy of an object. Yet the example above proves that it does not work for Java. Why? In Java everything that we manipulate is a handle, while in C++ you can have handle-like entities and you can also pass around the objects directly. That's what the C++ copy constructor is for: when you want to take an object and pass it in by value, thus duplicating the object. So it works fine in C++, but you should keep in mind that this scheme fails in Java, so don't use it.
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