Java Cloning
מבוא
The Cloneable interface in java is a marker interface. It does have any method. But if we write a class as below, it gives a message to the jvm that the class can be cloned. The following code shows a simple object cloning process –
[קוד]
package com.home.cloning;
ציבורי מעמד CloneClass implements Cloneable {
int א;
double ב;
// This method calls Object’s clone().
CloneClass getClone() {
לנסות {
// call clone in Object.
return (CloneClass) סופר.clone();
} לתפוס (CloneNotSupportedException e) {
מערכת.החוצה.println (” Cloning not allowed. ” );
return this;
}
}
}
[/קוד]
Let us check the second example below.
[קוד]
package com.home.cloning;
ציבורי מעמד TestCloneObject {
ציבורי להפר testIface() {
CloneClass x1 = חדש CloneClass();
CloneClass x2;
x1.a = 15;
x1.b = 35.05;
x2 = x1.getClone(); // clone x1
מערכת.החוצה.println(” x1: ” + x1.a + ” ” + x1.b);
מערכת.החוצה.println(” x2: ” + x2.a + ” ” + x2.b);
}
ציבורי סטטי להפר ראשי(args מחרוזת[]) {
TestCloneObject testCloneObject = חדש TestCloneObject();
// test via protected
testCloneObject.testIface();
}
}
In the above example, the method [קוד] getClone [/קוד] calls the [קוד] clone [/קוד] method in the object and returns the object. It must be noticed here that the object with is returned after the cloning mechanism has to be type casted into its appropriate type, in this case it is
[קוד] CloneClass [/קוד]
If the class is not implementing the cloneable interface, and we try to clone that object we get a
[קוד] CloneNotSupportedException [/קוד]
In the process of cloning, the constructor is not called rather an exact copy of the said object is created. But the object of which the clone is created, must implement the cloneable interface.
The מעמד Object’s clone() method creates and returns a copy of the object, with the same class and with all the fields having the same values. However, [קוד] Object.clone() [/קוד] throws a [קוד] CloneNotSupportedException [/קוד] unless the object is an instance of a class that implements the marker interface Cloneable.
The default implementation of [קוד] Object.clone()[/קוד] performs a shallow copy. If a class requires a deep copy or some other custom behavior, it must have its customized [קוד] clone()[/קוד] method after they obtain the copy from the superclass.
Advantages of cloning
Cloning mechanism saves extra task of the developer in case we need to create a copy of an object. We do not need to call the [קוד] חדש [/קוד] operator of the object. Thus cloning saves a lot of extra processing task of developer. A clone of an object is an exact copy of the object.
Disadvantages of cloning
One disadvantage of cloning is that the return type of the [קוד] clone [/קוד] method is an [קוד] Object [/קוד]. Hence a type casting is required on the created object.
Another disadvantage is that it is not possible to access the [קוד] clone [/קוד] method on an abstract type. Most interfaces and abstract classes in Java do not have the specify a public [קוד] clone [/קוד] שיטה. As a result, the [קוד] clone [/קוד] method is used only if the actual class of an object is known, which is against the abstraction principle of using the most generic type possible. For example, if one has a List reference in Java, one cannot invoke [קוד] clone [/קוד] method on that reference because List specifies no public clone() שיטה. Actual implementations of List like ArrayList and LinkedList all generally have clone() methods themselves, but it is inconvenient and bad abstraction to carry around the actual class type of an object.
Cloning is a potentially dangerous action, as it can have some unintended side effects. e.g, if the object being cloned contains a reference variable say refObject, then in the cloned object, refObject will have the reference of the same object which the original object is referring to. If the clone makes a change in the contents of the refObject, then the change will be reflected in the original object as well. Consider the following example – If an object opens an I/O stream and is then cloned, then both of the two objects will be capable of operating on the same stream. Further, if one of these objects closes the stream, then the stream is closed for both and if the second object tries to write to it, this causes an error.
Since cloning can cause some problems, the [קוד] clone [/קוד] שיטה. So the [קוד] clone [/קוד] method should be called from within a class which is implementing the [קוד] cloneable [/קוד] interface where the method calling the [קוד] clone [/קוד] method is made protected, or it must be explicitly overridden by the class which is public. In the example above we have seen cloning by making the [קוד] clone [/קוד] protected. The following example illustrates the cloning methodology via overriding –
[קוד]
package com.home.cloning;
ציבורי מעמד CloneViaOverRiding implements Cloneable {
int א;
double ב;
// clone() is now overridden and is public.
ציבורי Object clone() {
לנסות {
// call clone in Object.
return סופר.clone();
} לתפוס (CloneNotSupportedException e) {
מערכת.החוצה.println(“Cloning not allowed.”);
return this;
}
}
}
[/קוד]
Now check the next example below.
[קוד]
package com.home.cloning;
ציבורי מעמד TestCloneObject {
ציבורי להפר testPublic() {
CloneViaOverRiding x1 = חדש CloneViaOverRiding();
CloneViaOverRiding x2;
x1.a = 10;
x1.b = 20.98;
// here, clone() is called directly.
x2 = (CloneViaOverRiding) x1. clone ();
מערכת.החוצה.println(“x1: ” + x1.a + ” ” + x1.b);
מערכת.החוצה.println(“x2: ” + x2.a + ” ” + x2.b);
}
ציבורי סטטי להפר ראשי(args מחרוזת[]) {
TestCloneObject testCloneObject = חדש TestCloneObject();
// test via ציבורי
testCloneObject.testPublic();
}
}
[/קוד]
In this example the method, the method [קוד] clone [/קוד] of the object class is overridden that is why it is declared public in contrast to the earlier example, where the [קוד] getClone [/קוד] does not have any access modifiers making it accessible only at the package level.
In either of these two approaches, implementing the cloneable interface is mandatory.
The side effects caused by cloning are sometimes difficult to identify in the initial level. It is easy to think that a class is safe for cloning when it actually is not. In general, is not advised to implement the Cloneable interface for any class without having a solid business ground .
Alternative to cloning
Cloning mechanism has few alternatives –
- copy constructor – a copy constructor is a constructor which accepts another instance of the same class as a parameter.
- factory method – these methods are not always adequate when the concrete type of the cloned object is not known in advance.
- Use of serialization and deserialization is another alternative to using clone.
מסקנה
- Object cloning is the mechanism of creating a copy of an existing object
- Cloning follows the shallow copy mechanism
- Closing saves some extra tasks of the developer
- Once the cloning is done the created clone object is required to explicitly casted in to the required type
- Cloning has some side effects e.g if an object which is cloned has a reference to another object, and the new cloned object modifies the object which being referenced, then the original object also gets changed.
- Cloning mechanism has some alternatives – the copy constructor, factory method, serialization and de-serialization.
