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This chapter is from the book

This chapter is from the book

Factory Method

Also known as Virtual Constructor

Pattern Properties

Type: Creational
Level: Class

Purpose

To define a standard method to create an object, apart from a constructor, but the decision of what kind of an object to create is left to subclasses.

Introduction

Imagine that you're working on a Personal Information Manager (PIM) application. It will contain many pieces of information essential to your daily life: addresses, appointments, dates, books read, and so on. This information is not static; for instance, you want to be able to change an address when a contact moves, or change the details of an appointment if your lunch date needs to meet an hour later.

The PIM is responsible for changing each field. It therefore has to worry about editing (and therefore the User Interface) and validation for each field. The big disadvantage, however, is that the PIM has to be aware of all the different types of appointments and tasks that can be performed on them. Each item has different fields and the user needs to see an input screen appropriate to those fields. It will be very difficult to introduce new types of task information, because you will have to add a new editing capability to the PIM every time, suitable to update the new item type. Furthermore, every change in a specific type of task, such as adding a new field to an appointment, means you also have to update the PIM so that it is aware of this new field. You end up with a very bloated PIM that is difficult to maintain.

The solution is to let items, like appointments, be responsible for providing their own editors to manage additions and changes. The PIM only needs to know how to request an editor using the method getEditor, which is in every editable item. The method returns an object that implements the ItemEditor interface, and the PIM uses that object to request a JComponent as the GUI editor. Users can modify information for the item they want to edit, and the editor ensures that the changes are properly applied.

All the information on how to edit a specific item is contained in the editor, which is provided by the item itself. The graphical representation of the editor is also created by the editor itself. Now you can introduce new types of items without having to change PIM.

Applicability

Use Factory Method pattern when:

  • You want to create an extensible framework. This means allowing flexibility by leaving some decisions, like the specific kind of object to create, until later.

  • You want a subclass, rather than its superclass, to decide what kind of an object to create.

  • You know when to create an object, but not what kind of an object.

  • You need several overloaded constructors with the same parameter list, which is not allowed in Java. Instead, use several Factory Methods with different names.

Description

This pattern is called Factory Method because it creates (manufactures) objects when you want it to.

When you start writing an application, it's often not clear yet what kind of components you will be using. Normally you will have a general idea of the operations certain components should have, but the implementation is done at some other time and will not be of consequence at that moment.

This flexibility can be achieved by using interfaces for these components. But the problem with programming to interfaces is that you cannot create an object from an interface. You need an implementing class to get an object. Instead of coding a specific implementing class in your application, you extract the functionality of the constructor and put it in a method. That method is the factory method.

To create these objects, instead of coding a specific implementing class in your application, you extract the functionality of the constructor and put it in a method. This produces a ConcreteCreator whose responsibility it is to create the proper objects. That ConcreteCreator creates instances of an implementation (ConcreteProduct) of an interface (Product).

Implementation

The class diagram is shown in Figure 1.3.

Figure 1.3 Factory Method class diagram

To implement the Factory Method you need:

  • Product – The interface of objects created by the factory.

  • ConcreteProduct – The implementing class of Product. Objects of this class are created by the ConcreteCreator.

  • Creator – The interface that defines the factory method(s) (factoryMethod).

  • ConcreteCreator – The class that extends Creator and that provides an implementation for the factoryMethod. This can return any object that implements the Product interface.

Benefits and Drawbacks

A major benefit to this solution is that the PIM can be very generic. It only needs to know how to request an editor for an item. The information about how to edit a specific item is contained in the editor. The editor can also create the graphical user interface (GUI) for editing. This makes the PIM more modular, making it easier to add new types of information to be managed without changing the core program itself.

JDBC (Java database connectivity) uses the Factory Method pattern in many of its interfaces. You can use another JDBC driver as long as the correct driver is loaded. The rest of your application remains the same. (For more information on patterns in JDBC, see "JDBC" on page 308.)

The drawback to this pattern is the fact that to add a new type of product, you must add another new implementing class, and you must either change an existing ConcreteCreator or create a new class that implements Product.

Pattern Variants

There are several variations for this pattern:

  • Creator can provide a standard implementation for the factory method. That way Creator doesn't have to be an abstract class or interface, but can be a full-blown class. The benefit is that you aren't required to subclass the Creator.

  • Product can be implemented as an abstract class. Because the Product is a class, you can add implementations for other methods.

  • The factory method can take a parameter. It can then create more than one type of Product based on the given parameter. This decreases the number of factory methods needed.

Related Patterns

Related patterns include the following:

  • Abstract Factory (page 6) – Might use one or more factory methods.

  • Prototype (page 28) – Prevents subclassing of Creator.

  • Template Method (page 131) – Template methods usually call factory methods.

  • Data Access Object [CJ2EEP] – The Data Access Object pattern uses the Factory Method pattern to be able to create specific instances of Data Access Objects without requiring knowledge of the specific underlying database.

Example

NOTE

For a full working example of this code example, with additional supporting classes and/or a RunPattern class, see "Factory Method" on page 352 of the "Full Code Examples" appendix.

The following example uses the Factory Method pattern to produce an editor for the PIM. The PIM tracks a lot of information, and there are many cases where users need an editor to create or modify data. The example uses interfaces to improve the overall flexibility of the system.

The Editable interface defines a builder method, getEditor, which returns an ItemEditor interface. The benefit is that any item can provide an editor for itself, producing an object that knows what parts of a business object can change and how they can be changed. The only thing the user interface needs to do is use the Editable interface to get an editor.

Example 1.14 Editable.java 0

1. public interface Editable {
2.   public ItemEditor getEditor();
3. }

The ItemEditor interface provides two methods: getGUI and commitChanges. The getGUI method is another Factory Method—it returns a JComponent that provides a Swing GUI to edit the current item. This makes a very flexible system; to add a new type of item, the user interface can remain the same, because it only uses the Editable and the ItemEditor interfaces.

The JComponent returned by getGUI can have anything in it required to edit the item in the PIM. The user interface can simply the acquired JComponent in its editor window and use the JComponent functionality to edit the item. Since not everything in an application needs to be graphical, it could also be a good idea to include a getUI method that would return an Object or some other non-graphical interface.

The second method, commitChanges, allows the UI to tell the editor that the user wants to finalize the changes he or she has made.

Example 1.15 ItemEditor.java 0

1. import javax.swing.JComponent;
2. public interface ItemEditor {
3.   public JComponent getGUI();
4.   public void commitChanges();
5. }

The following code shows the implementation for one of the PIM items, Contact. The Contact class defines two attributes: the name of the person and their relationship with the user. These attributes provide a sample of some of the information, which could be included in an entry in the PIM.

Example 1.16 Contact.java 0

 1. import java.awt.GridLayout;
 2. import java.io.Serializable;
 3. import javax.swing.JComponent;
 4. import javax.swing.JLabel;
 5. import javax.swing.JPanel;
 6. import javax.swing.JTextField;
 7. 
 8. public class Contact implements Editable, Serializable {
 9.   private String name;
10.   private String relationship;
11. 
12.   public ItemEditor getEditor() {
13.     return new ContactEditor();
14.   }
15.   
16.   private class ContactEditor implements ItemEditor, Serializable {
17.     private transient JPanel panel;
18.     private transient JTextField nameField;
19.     private transient JTextField relationField;
20.     
21.     public JComponent getGUI() {
22.       if (panel == null) {
23.         panel = new JPanel();
24.         nameField = new JTextField(name);
25.         relationField = new JTextField(relationship);
26.         panel.setLayout(new GridLayout(2,2));
27.         panel.add(new JLabel("Name:"));
28.         panel.add(nameField);
29.         panel.add(new JLabel("Relationship:"));
30.         panel.add(relationField);
31.       } else {
32.         nameField.setText(name);
33.         relationField.setText(relationship);
34.       }
35.       return panel;
36.     }
37.     
38.     public void commitChanges() {
39.       if (panel != null) {
40.         name = nameField.getText();
41.         relationship = relationField.getText();
42.       }
43.     }
44.     
45.     public String toString(){
46.       return "\nContact:\n" +
47.         "  Name: " + name + "\n" +
48.         "  Relationship: " + relationship;
49.     }
50.   }
51. }

Contact implements the Editable interface, and provides its own editor. That editor only applies to the Contact class, and needs to change certain attributes of the Contact, it is best to use an inner class. The inner class has direct access to the attributes of the outer class. If you used another (non-inner) class, Contact would need to provide accessor and mutator methods, making it harder to restrict access to the object's private data.

Note that the editor itself is not a Swing component, but only an object that can serve as a factory for such a component. The greatest benefit is that you can serialize and send this object across a stream. To implement this feature, declare all Swing component attributes in ContactEditor transient—they're constructed when and where they're needed.

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