Full Code Examples - Applied Java Patterns - page 267

Strategy
For many of the collections in the Personal Information Manager, it would be useful to be able to organize and
summarize individual entries. This demonstration uses the Strategy pattern to summarize entries in a
ContactList , a collection used to store Contact objects.
Example A.108 ContactList.java
1. import java.io.Serializable;
2. import java.util.ArrayList;
3. public class ContactList implements Serializable{
4. private ArrayList contacts = new ArrayList();
5. private SummarizingStrategy summarizer;
6.
7. public ArrayList getContacts(){ return contacts; }
8. public Contact [] getContactsAsArray(){ return (Contact [])(contacts. toArray(new
Contact [1])); }
9.
10. public void setSummarizer(SummarizingStrategy newSummarizer){ summarizer =
newSummarizer; }
11. public void setContacts(ArrayList newContacts){ contacts = newContacts; }
12.
13. public void addContact(Contact element){
14. if (!contacts.contains(element)){
15. contacts.add(element);
16. }
17. }
18. public void removeContact(Contact element){
19. contacts.remove(element);
20. }
21.
22. public String summarize(){
23. return summarizer.summarize(getContactsAsArray());
24. }
25.
26. public String [] makeSummarizedList(){
27. return summarizer.makeSummarizedList(getContactsAsArray());
28. }
29. }
The ContactList has two methods, which can be used to provide summary information for the Contact objects
in the collection— summarize and make-SummarizedList . Both methods delegate to a SummarizingStrategy ,
which can be set for the ContactList with the setSummarizer method.
Example A.109 SummarizingStrategy.java
1. public interface SummarizingStrategy{
2. public static final String EOL_STRING = System.getProperty("line.separator");
3. public static final String DELIMITER = ":";
4. public static final String COMMA = ",";
5. public static final String SPACE = " ";
6.
7. public String summarize(Contact [] contactList);
8. public String [] makeSummarizedList(Contact [] contactList);
9. }
SummarizingStrategy is an interface that defines the two delegate methods summarize and
makeSummarizedList . The interface represents the Strategy in the design pattern. In this example, two classes
represent ConcreteStrategy objects: NameSummarizer and OrganizationSummarizer . Both classes summarize
the list of contacts; however, each provides a different set of information and groups the data differently.
The NameSummarizer class returns only the names of the contacts with the last name first. The class uses an inner
class as a comparator ( NameComparator ) to ensure that all of the Contact entries are grouped in ascending order
by both last and first name.
Example A.110 NameSummarizer.java
1. import java.text.Collator;
2. import java.util.Arrays;
3. import java.util.Comparator;
4. public class NameSummarizer implements SummarizingStrategy{
5. private Comparator comparator = new NameComparator();
6.