Working with Design Pattern: Decorator, Page 2

Creating Additional Decorators

I can quickly bang out more decorators. How about the capability to send a duplicate (not a CC or BCC) of the email to another recipient? Here's a test:

import static org.junit.Assert.*;
import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;

import org.junit.*;

public class DuplicatingEmailTest {
   @Test
   public void create() throws AddressException,
      MessagingException {
      final Set<String> sentEmails = new HashSet<String>();
      Sendable wrappedEmail = new Email() {
         @Override
         public void send(String to, String from, String subject,
            String content) {
            sentEmails.add(to);
         }
      };
      DuplicatingEmail email =
         new DuplicatingEmail(wrappedEmail, "jeff@langrsoft.com");
      email.send("joe@x.com", "from", "subject", "content");
      assertEquals(2, sentEmails.size());
      assertTrue(sentEmails.contains("joe@x.com"));
      assertTrue(sentEmails.contains("jeff@langrsoft.com"));
   }
}

And the implementation:

import javax.mail.*;
import javax.mail.internet.*;

public class DuplicatingEmail implements Sendable {
   private Sendable wrappedEmail;
   private final String additionalAddress;

   public DuplicatingEmail(Sendable wrappedEmail,
      String additionalAddress) {
      this.wrappedEmail = wrappedEmail;
      this.additionalAddress = additionalAddress;
   }

   public void send(
         String toAddress, String fromAddress, String subject,
         String content)
         throws AddressException, MessagingException {
      wrappedEmail.send(toAddress, fromAddress, subject, content);
      wrappedEmail.send(additionalAddress, fromAddress, subject,
                        content);
   }
}

With two decorators defined, the client code can choose how to wrap Sendable implementations:

Sendable sendable =
   new DuplicatingEmail(new CensoringEmail(new Email()), "Me@z.com");
sendable.send("jane@x.com", "Joe@z.com", "yo", "the text");

The decorator pattern allows for considerable flexibility. As demonstrated here, its most useful application is when you have many potential subclasses that would create a very large number of potential combinations: DuplicatingCensoringEmail, CensoringAttachmentVerifyingEmail, and so on.

Click here for a larger image.

Figure 1: The decorator pattern.

Reference

[Gamma] Gamma, E., et. al. Design Patterns: Elements of Reusable Object-Oriented Software. Addison-Wesley Professional, 1995.

About the Author

Jeff Langr is a veteran software developer celebrating his 25th year of professional software development. He's authored two books and dozens of published articles on software development, including Agile Java: Crafting Code With Test-Driven Development (Prentice Hall) in 2005. You can find out more about Jeff at his site,or you can contact him via email at jeff at langrsoft.com.