Full Code Examples - Applied Java Patterns - page 260

State
Inner classes are most appropriate for States. They are very closely coupled with their enclosing class and have
direct access to its attributes. The following example shows how this works in practice.
A standard feature of applications is that they only save files when necessary: when changes have been made.
When changes have been made but a file has not been saved, its state is referred to as dirty. The content might be
different from the persistent, saved version. When the file has been saved and no further changes have been made,
the content is considered clean. For a clean state, the content and the file will be identical if no one else edits the
file.
This example shows the State pattern being used to update Appointments for the PIM, saving them to a file as necessary.
The State transition diagram for a file is shown in Figure A.1 .
Figure A.1. State transition diagram for a file
Two states ( CleanState and DirtyState ) implement the State interface. The states are responsible for
determining the next state, which in this case is reasonably easy, as there are only two.
The State interface defines two methods, save and edit . These methods are called by the CalendarEditor
when appropriate.
Example A.97 State.java
1. public interface State{
2. public void save();
3. public void edit();
4. }
The CalendarEditor class manages a collection of Appointment objects.
Example A.98 CalendarEditor.java
1. import java.io.File;
2. import java.util.ArrayList;
3. public class CalendarEditor{
4. private State currentState;
5. private File appointmentFile;
6. private ArrayList appointments = new ArrayList();
7. private static final String DEFAULT_APPOINTMENT_FILE = "appointments.ser";
8.
9. public CalendarEditor(){
10. this(DEFAULT_APPOINTMENT_FILE);
11. }
12. public CalendarEditor(String appointmentFileName){
13. appointmentFile = new File(appointmentFileName);
14. try{
15. appointments = (ArrayList)FileLoader.loadData(appointmentFile);
16. }
17. catch (ClassCastException exc){
18. System.err.println("Unable to load information. The file does not contain a list
of appointments.");