This chapter is from the book
This chapter is from the book
This chapter is from the book
Prototype
Pattern Properties
Purpose
To make dynamic creation easier by defining classes whose objects can create duplicates of themselves.
Introduction
In the PIM, you want to be able to copy an address entry so that the user doesn't have to manually enter all the information when creating a new contact. One way to solve this is to perform the following steps:
- Create a new Address object.
- Copy the appropriate values from the existing Address.
While this approach solves the problem, it has one serious drawback—it violates the object-oriented principle of encapsulation. To achieve the solution mentioned above, you have to put method calls to copy the Address information, outside of the Address class. This means that it becomes harder and harder to maintain the Address code, since it exists throughout the code for the project. It is also difficult to reuse the Address class in some new project in the future.
The copy code really belongs in the Address class itself, so why not instead define a "copy" method in the class? This method produces a duplicate of the Address object with the same data as the original object—the prototype. Calling the method on an existing Address object solves the problem in a much more maintainable way, much truer to good object-oriented coding practices.
Applicability
Use the Prototype pattern when you want to create an object that is a copy of an existing object.
Description
The Prototype pattern is well named; as with other prototypes, it has an object that is used as the basis to create a new instance with the same values. Providing a "create based on existing state" behavior allows programs to perform operations like user-driven copy, and to initialize objects to a state that has been established through use of the system. This is often preferable to initializing the object to some generic set of values.
Classic examples for this pattern exist in graphic and text editors, where copy-paste features can greatly improve user productivity. Some business systems use this approach as well, producing an initial model from an existing business object. The copy can then be modified to its desired new state.
Implementation
The Prototype class diagram is shown in Figure 1.4.
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Figure 1.4 Prototype class diagram
To implement Prototype, you need:
Prototype – Provides a copy method. That method returns an instance of the same class with the same values as the original Prototype instance. The new instance can be a deep or shallow copy of the original (see the Benefits and Drawbacks section of this pattern).
Benefits and Drawbacks
The Prototype is helpful because it allows systems to produce a copy of a usable object, with variables already set to a (presumably) meaningful value, rather than depending on some base state defined in the constructor. An example of Prototype use is shown in Figure 1.5.
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Figure 1.5 Example of Prototype use
A key consideration for this pattern is copy depth.
A shallow copy duplicates only the top-level elements of a class; this provides a faster copy, but isn't suitable for all needs. Since references are copied from the original to the copy, they still refer to the same objects. The lower-level objects are shared among copies of the object, so changing one of these objects affects all of the copies.
Deep copy operations replicate not only the top-level attributes, but also the lower-level objects. This typically takes longer than shallow copy operations, and can be very costly for objects with an arbitrarily complex structure. This makes sure that changes in one copy are isolated from other copies.
By its nature, the clone method in Object supports only one form of copy. For cases where you must support multiple methods of post-creation initialization.
Pattern Variants
Pattern variants include the following:
Copy constructor – One variant of the prototype is a copy constructor. A copy constructor takes an instance of the same class as an argument and returns a new copy with the same values as the argument.
Example 1.17 Copy constructor
public class Prototype {
private int someData;
// some more data
public Prototype(Prototype original) {
super();
this.someData = original.someData;
//copy the rest of the data
}
// rest of code
}
An example is the String class, where you can create a new String instance by calling for instance: new String("text");
The benefit of this variant is that the intention of creating a new instance is very clear, but only one type of copy (deep or shallow) can be executed. It is possible to have a constructor that can use both. The constructor would take two arguments: the object to be copied and a boolean to mark whether it should apply a deep or shallow copy.
A drawback is that the copy constructor must check the incoming reference to see if it is not null. With the normal Prototype implementation, the method is certain to be called on a valid object.
clone method – The Java programming language already defines a clone method in the java.lang.Object class—the superclass of all Java classes. For the method to be usable on an instance, the class of that object has to implement the java.lang.Clonable interface to indicate that an instance of this class may be copied. Because the clone method is declared protected in Object, it has to be overridden to make it publicly available.
According to Bloch, "clone() should be used judiciously" [Bloch01]. As mentioned, a class has to implement Clonable, but that interface does not provide a guarantee that the object can be cloned. The Clonable interface does not defined the clone method, so it is possible that the clone method is not available when it is not overridden. Another drawback of the clone method is that it has a return type of Object, requiring you to cast it to the appropriate type before using it.
Related Patterns
Related patterns include the following:
Abstract Factory (page 6) – Abstract Factories can use the Prototype to create new objects based on the current use of the Factory.
Factory Method (page 21) – Factory Methods can use a Prototype to act as a template for new objects.
Example
NOTE
For a full working example of this code example, with additional supporting classes and/or a RunPattern class, see "Prototype" on page 357 of the "Full Code Examples" appendix.
The Address class in this example uses the Prototype pattern to create an address based on an existing entry. The core functionality for the pattern is defined in the interface Copyable.
Example 1.18 Copyable.java 0
1. public interface Copyable{
2. public Object copy();
3. }
The Copyable interface defines a copy method and guarantees that any classes that implement the interface will define a copy operation. This example produces a shallow copy—that is, it copies the object references from the original address to the duplicate.
The code also demonstrates an important feature of the copy operation: not all fields must necessarily be duplicated. In this case, the address type is not copied to the new object. A user would manually specify a new address type from the PIM user interface.
Example 1.19 Address.java 0
1. public class Address implements Copyable{
2. private String type;
3. private String street;
4. private String city;
5. private String state;
6. private String zipCode;
7. public static final String EOL_STRING =
8. System.getProperty("line.separator");
9. public static final String COMMA = ",";
10. public static final String HOME = "home";
11. public static final String WORK = "work";
12.
13. public Address(String initType, String initStreet,
14. String initCity, String initState, String initZip){
15. type = initType;
16. street = initStreet;
17. city = initCity;
18. state = initState;
19. zipCode = initZip;
20. }
21.
22. public Address(String initStreet, String initCity,
23. String initState, String initZip){
24. this(WORK, initStreet, initCity, initState, initZip);
25. }
26. public Address(String initType){
27. type = initType;
28. }
29. public Address(){ }
30.
31. public String getType(){ return type; }
32. public String getStreet(){ return street; }
33. public String getCity(){ return city; }
34. public String getState(){ return state; }
35. public String getZipCode(){ return zipCode; }
36.
37. public void setType(String newType){ type = newType; }
38. public void setStreet(String newStreet){ street = newStreet; }
39. public void setCity(String newCity){ city = newCity; }
40. public void setState(String newState){ state = newState; }
41. public void setZipCode(String newZip){ zipCode = newZip; }
42.
43. public Object copy(){
44. return new Address(street, city, state, zipCode);
45. }
46.
47. public String toString(){
48. return "\t" + street + COMMA + " " + EOL_STRING +
49. "\t" + city + COMMA + " " + state + " " + zipCode;
50. }
51. }