This chapter is from the book
This chapter is from the book
This chapter is from the book
Object Cloning
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.
)
Figure
6–1: Copying and cloning
Employee original = new Employee("John Public", 50000);
Employee copy = original;
copy.raiseSalary(10); // oops--also changed original
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 = (Employee)original.clone(); // must cast—clone returns an Object 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.
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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 happens in two situations. The subobject may belong 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 or not
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. 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) . . .
Even if the default (shallow copy) implementation of clone is adequate, you still need to implement the Cloneable interface, redefine clone to be public, call super.clone() and catch the CloneNotSupportedException. Here is an example:
class Employee implements Cloneable
{
public Object clone() // raise visibility level to public
{
try
{
return super.clone();
}
catch (CloneNotSupportedException e) { return null; }
// this won't happen, since we are Cloneable
}
. . .
}
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 class implements the Cloneable interface, so the exception won't be thrown. However, the compiler does not know that. Therefore, you still need to catch the exception and return a dummy value.
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()
{
try
{
// call Object.clone()
Employee cloned = (Employee)super.clone();
// clone mutable fields
cloned.hireDay = (Date)hireDay.clone()
return cloned;
}
catch (CloneNotSupportedException e) { return null; }
}
}
NOTE
Users of your clone method still have to cast the result. The clone method always has return type Object.
As you can see, cloning is a subtle business, and it makes sense that it is defined as protected in the Object class. (See Chapter 12 for an elegant method for cloning objects, using the object serialization feature of Java.)
NOTE
When you define a public clone method, you have lost a safety mechanism. Your clone method is inherited by the subclasses, whether or not it makes sense for them. For example, once you have defined the clone method for the Employee class, anyone can also 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 in general, you need to make sure to check the clone method of any class that you extend.
The program in Example 6–3 clones an Employee object, then invokes two mutators. The raiseSalary method changes the value of the salary field, while 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.
Example 6–3: CloneTest.java
1. import java.util.*;
2.
3. public class CloneTest
4. {
5. public static void main(String[] args)
6. {
7. Employee original = new Employee("John Q. Public", 50000);
8. original.setHireDay(2000, 1, 1);
9. Employee copy = (Employee)original.clone();
10. copy.raiseSalary(10);
11. copy.setHireDay(2002, 12, 31);
12. System.out.println("original=" + original);
13. System.out.println("copy=" + copy);
14. }
15. }
16.
17. class Employee implements Cloneable
18. {
19. public Employee(String n, double s)
20. {
21. name = n;
22. salary = s;
23. }
24.
25. public Object clone()
26. {
27. try
28. {
29. // call Object.clone()
30. Employee cloned = (Employee)super.clone();
31.
32. // clone mutable fields
33. cloned.hireDay = (Date)hireDay.clone();
34.
35. return cloned;
36. }
37. catch (CloneNotSupportedException e) { return null; }
38. }
39.
40. /**
41. Set the pay day to a given date
42. @param year the year of the pay day
43. @param month the month of the pay day
44. @param day the day of the pay day
45. */
46. public void setHireDay(int year, int month, int day)
47. {
48. hireDay = new GregorianCalendar(year,
49. month - 1, day).getTime();
50. }
51.
52. public void raiseSalary(double byPercent)
53. {
54. double raise = salary * byPercent / 100;
55. salary += raise;
56. }
57.
58. public String toString()
59. {
60. return "Employee[name=" + name
61. + ",salary=" + salary
62. + ",hireDay=" + hireDay
63. + "]";
64. }
65.
66. private String name;
67. private double salary;
68. private Date hireDay;
69. }