Object Cloning | Core Java(TM) 2, Volume I--Fundamentals (7th Edition) (Core Series) (Core Series)

When you make a copy of a variable, the original and the copy are references to the same object. (See Figure 6-1.) This means a change to either variable also affects the other.

 Employee original = new Employee("John Public", 50000); Employee copy = original; copy.raiseSalary(10); // oops--also changed original

Figure 6-1. Copying and cloning

If you would like copy to be a new object that begins its life being identical to original but whose state can diverge over time, then you use the clone method.

 Employee copy = original.clone(); copy.raiseSalary(10); // OK--original unchanged

But it isn't quite so simple. The clone method is a protected method of Object, which means that your code cannot simply call it. Only the Employee class can clone Employee objects. There is a reason for this restriction. Think about the way in which the Object class can implement clone. It knows nothing about the object at all, so it can make only a field-by-field copy. If all data fields in the object are numbers or other basic types, copying the fields is just fine. But if the object contains references to subobjects, then copying the field gives you another reference to the subobject, so the original and the cloned objects still share some information.

To visualize that phenomenon, let's consider the Employee class that was introduced in Chapter 4. Figure 6-2 shows what happens when you use the clone method of the Object class to clone such an Employee object. As you can see, the default cloning operation is "shallow" it doesn't clone objects that are referenced inside other objects.

Figure 6-2. A shallow copy

Does it matter if the copy is shallow? It depends. If the subobject that is shared between the original and the shallow clone is immutable, then the sharing is safe. This certainly happens if the subobject belongs to an immutable class, such as String. Alternatively, the subobject may simply remain constant throughout the lifetime of the object, with no mutators touching it and no methods yielding a reference to it.

Quite frequently, however, subobjects are mutable, and you must redefine the clone method to make a deep copy that clones the subobjects as well. In our example, the hireDay field is a Date, which is mutable.

For every class, you need to decide whether

The default clone method is good enough;
The default clone method can be patched up by calling clone on the mutable subobjects;
clone should not be attempted.

The third option is actually the default. To choose either the first or the second option, a class must

Implement the Cloneable interface, and
Redefine the clone method with the public access modifier.

NOTE

The clone method is declared protected in the Object class so that your code can't simply call anObject.clone(). But aren't protected methods accessible from any subclass, and isn't every class a subclass of Object? Fortunately, the rules for protected access are more subtle (see Chapter 5). A subclass can call a protected clone method only to clone its own objects. You must redefine clone to be public to allow objects to be cloned by any method.

In this case, the appearance of the Cloneable interface has nothing to do with the normal use of interfaces. In particular, it does not specify the clone method that method is inherited from the Object class. The interface merely serves as a tag, indicating that the class designer understands the cloning process. Objects are so paranoid about cloning that they generate a checked exception if an object requests cloning but does not implement that interface.

NOTE

The Cloneable interface is one of a handful of tagging interfaces that Java provides. (Some programmers call them marker interfaces.) Recall that the usual purpose of an interface such as Comparable is to ensure that a class implements a particular method or set of methods. A tagging interface has no methods; its only purpose is to allow the use of instanceof in a type inquiry:

 if (obj instanceof Cloneable) . . .

We recommend that you do not use this technique in your own programs.

Even if the default (shallow copy) implementation of clone is adequate, you still need to implement the Cloneable interface, redefine clone to be public, and call super.clone(). Here is an example:

 class Employee implements Cloneable {    // raise visibility level to public, change return type    public Employee clone() throws CloneNotSupportedException    {       return super.clone();    }    . . . }

NOTE

Before JDK 5.0, the clone method always had return type Object. The covariant return types of JDK 5.0 let you specify the correct return type for your clone methods.

The clone method that you just saw adds no functionality to the shallow copy provided by Object.clone. It merely makes the method public. To make a deep copy, you have to work harder and clone the mutable instance fields.

Here is an example of a clone method that creates a deep copy:

 class Employee implements Cloneable {    . . .    public Object clone() throws CloneNotSupportedException    {       // call Object.clone()       Employee cloned = (Employee) super.clone();       // clone mutable fields       cloned.hireDay = (Date) hireDay.clone()       return cloned;    } }

The clone method of the Object class threatens to throw a CloneNotSupportedException it does that whenever clone is invoked on an object whose class does not implement the Cloneable interface. Of course, the Employee and Date class implements the Cloneable interface, so the exception won't be thrown. However, the compiler does not know that. Therefore, we declared the exception:

 public Employee clone() throws CloneNotSupportedException

Would it be better to catch the exception instead?

 public Employee clone() {    try    {       return super.clone();    }    catch (CloneNotSupportedException e) { return null; }    // this won't happen, since we are Cloneable }

This is appropriate for final classes. Otherwise, it is a good idea to leave the tHRows specifier in place. That gives subclasses the option of throwing a CloneNotSupportedException if they can't support cloning.

You have to be careful about cloning of subclasses. For example, once you have defined the clone method for the Employee class, anyone can use it to clone Manager objects. Can the Employee clone method do the job? It depends on the fields of the Manager class. In our case, there is no problem because the bonus field has primitive type. But Manager might have acquired fields that require a deep copy or that are not cloneable. There is no guarantee that the implementor of the subclass has fixed clone to do the right thing. For that reason, the clone method is declared as protected in the Object class. But you don't have that luxury if you want users of your classes to invoke clone.

Should you implement clone in your own classes? If your clients need to make deep copies, then you probably should. Some authors feel that you should avoid clone altogether and instead implement another method for the same purpose. We agree that clone is rather awkward, but you'll run into the same issues if you shift the responsibility to another method. At any rate, cloning is less common than you may think. Less than 5 percent of the classes in the standard library implement clone.

The program in Example 6-2 clones an Employee object, then invokes two mutators. The raiseSalary method changes the value of the salary field, whereas the setHireDay method changes the state of the hireDay field. Neither mutation affects the original object because clone has been defined to make a deep copy.

NOTE

Chapter 12 shows an alternate mechanism for cloning objects, using the object serialization feature of Java. That mechanism is easy to implement and safe, but it is not very efficient.

Example 6-2. CloneTest.java

  1. import java.util.*;  2.  3. public class CloneTest  4. {  5.    public static void main(String[] args)  6.    {  7.       try  8.       {  9.          Employee original = new Employee("John Q. Public", 50000); 10.          original.setHireDay(2000, 1, 1); 11.          Employee copy = original.clone(); 12.          copy.raiseSalary(10); 13.          copy.setHireDay(2002, 12, 31); 14.          System.out.println("original=" + original); 15.          System.out.println("copy=" + copy); 16.       } 17.       catch (CloneNotSupportedException e) 18.       { 19.          e.printStackTrace(); 20.       } 21.    } 22. } 23. 24. class Employee implements Cloneable 25. { 26.    public Employee(String n, double s) 27.    { 28.       name = n; 29.       salary = s; 30.    } 31. 32.    public Employee clone() throws CloneNotSupportedException 33.    { 34.       // call Object.clone() 35.       Employee cloned = (Employee)super.clone(); 36. 37.       // clone mutable fields 38.       cloned.hireDay = (Date)hireDay.clone(); 39. 40.       return cloned; 41.    } 42. 43.    /** 44.       Set the hire day to a given date 45.       @param year the year of the hire day 46.       @param month the month of the hire day 47.       @param day the day of the hire day 48.    */ 49.    public void setHireDay(int year, int month, int day) 50.    { 51.       hireDay = new GregorianCalendar(year, month - 1, day).getTime(); 52.    } 53. 54.    public void raiseSalary(double byPercent) 55.    { 56.       double raise = salary * byPercent / 100; 57.       salary += raise; 58.    } 59. 60.    public String toString() 61.    { 62.       return "Employee[name=" + name 63.          + ",salary=" + salary 64.          + ",hireDay=" + hireDay 65.          + "]"; 66.    } 67. 68.    private String name; 69.    private double salary; 70.    private Date hireDay; 71. }