Home > Articles > Software Development & Management

  • Print
  • + Share This
This chapter is from the book Learning the Bridge Pattern: An Example

Learning the Bridge Pattern: An Example

To help you understand the thinking behind the Bridge pattern and what it is trying to do, I will work through an example from scratch. Starting with requirements, I will derive the pattern and then see how to apply it.

Perhaps this example will seem basic. But look at the concepts discussed in this example and then try to think of situations that you have encountered that are similar, having

  • Variations in abstractions of a concept.
  • Variations in how these concepts are implemented.

You will see that this example has many similarities to the CAD/CAM problem discussed earlier. Rather than give you all the requirements up front, however, I am going to give them a little at a time, just as they were given to me. You can’t always see the variations at the beginning of the problem.

Bottom line: During requirements definition, explore for variations early and often!

Suppose I have been given the task of writing a program that will draw rectangles with either of two drawing programs. I have been told that when I instantiate a rectangle, I will know whether I should use drawing program 1 (DP1) or drawing program 2 (DP2).

The rectangles are defined as two pairs of points, as represented in Figure 10-1. The differences between the drawing programs are summarized in Table 10-1.

Figure 10-1 Positioning the rectangle.

Table 10-1 Different Drawing Programs


DP1

DP2

Used to draw a line

draw_a_line( x1, y1, x2, y2)
drawline( x1, x2, y1, y2)

Used to draw a circle

draw_a_circle( x, y, r)
drawcircle( x, y, r)

Our analysis specifies that we don’t want the code that draws the rectangles to worry about what type of drawing program it should use. It occurs to me that because the rectangles are told what drawing program to use when instantiated, I can have two different kinds of rectangle objects: one that uses DP1 and one that uses DP2. Each would have a draw method but would implement it differently. Figure 10-2 shows this.

Figure 10-2 Design for rectangles and drawing programs (DP1 and DP2).

By having an abstract class Rectangle, I take advantage of the fact that the only difference between the different types of Rectangles are how they implement the drawLine method. The V1Rectangle is implemented by having a reference to a DP1 object and using that object’s draw_a_line method. The V2Rectangle is implemented by having a reference to a DP2 object and using that object’s drawline method. However, by instantiating the right type of Rectangle, I no longer have to worry about this difference.

Example 10-1 Java Code Fragments

  abstract public class Rectangle {
   private double _x1, _y1, _x2, _y2;
   public Rectangle 
      (double x1, double y1, double x2, double y2) {
      x1= x1; _y1= y1; _x2= x2; _y2= y2;
   }
   public void draw() {
      drawLine( _x1, _y1, _x2, _y1);
      drawLine( _x2, _y1, _x2, _y2);
      drawLine( _x2, _y2, _x1, _y2);
      drawLine( _x1, _y2, _x1, _y1);
   }
   abstract protected void drawLine
      (double x1, double y1, double x2, double y2);
}  

Now suppose that after completing this code, one of the inevitable three (death, taxes, and changing requirements) comes my way. I am asked to support another kind of shape—this time, a circle. However, I am also given the mandate that the collection object does not want to know the difference between Rectangles and Circles.

It occurs to me that I can just extend the approach I’ve already started by adding another level to my class hierarchy. I only need to add a new class, called Shape, from which I will derive the Rectangle and Circle classes. This way, the Client object can just refer to Shape objects without worrying about what kind of Shape it has been given.

As a beginning object-oriented analyst, it might seem natural to implement these requirements using only inheritance. For example, I could start out with something like Figure 10-2, and then, for each kind of Shape, implement the shape with each drawing program, deriving a version of DP1 and a version of DP2 for Rectangle and deriving a version of DP1 and a version of DP2 one for Circle. I would end up with Figure 10-3.

Figure 10-3 A straightforward approach: implementing two shapes and two drawing programs.

I implement the Circle class the same way that I implemented the Rectangle class. However, this time, I implement draw by using drawCircle instead of drawLine.

Example 10-2 Java Code Fragments

  abstract class Shape {
  abstract public void draw();
}
// the only change to Rectangle is
abstract class Rectangle extends Shape {  
//
// V1Rectangle and V2Rectangle don’t change

abstract public class Circle extends Shape {
   protected double _x, _y, _r;
   public Circle (double x, double y, double r) {
      _x= x; _y= y; _r= r;
   }
   public void draw() {
      drawCircle();
   }
   abstract protected void drawCircle();
}
public class V1Circle extends Circle {
   public V1Circle 
      (double x, double y, double r) {
      super(x,y,r);
   }
   protected void drawCircle () {
      DP1.draw_a_circle( _x, _y, _r);
   }
}
public class V2Circle extends Circle {
   public V2Circle(double x, double y, double r) {
      super( x, y, r);
   }
   protected void drawCircle () {
      DP2.drawCircle( _x, _y, _r);
   }
}

To understand this design, let’s walk through an example. Consider what the draw method of a V1Rectangle does.

  • Rectangle’s draw method is the same as before (calling drawLine four times as needed).
  • drawLine is implemented by calling DP1’s draw_a_line.

In action, this looks like Figure 10-4.

Figure 10-4 Sequence Diagram when have a V1Rectangle.

Even though the class diagram makes it look as if there are many objects, in reality I am only dealing with three objects (see Figure 10-5):

  • The Client using the rectangle
  • The V1Rectangle object
  • The DP1 drawing program

When the Client object sends a message to the V1Rectangle object (called myRectangle) to perform draw, it calls Rectangle’s draw method resulting in Steps 2 through 9.

Figure 10-5 The objects present.

Unfortunately, this approach introduces new problems. Look back at Figure 10-3 and pay attention to the third row of classes. Consider the following:

  • The classes in this row represent the four specific types of Shapes that I have.
  • What happens if I get another drawing program—that is, another variation in implementation? I will have six different kinds of Shapes (two Shape concepts times three drawing programs).
  • Imagine what happens if I then get another type of Shape, another variation in concept. I will have nine different types of Shapes (three Shape concepts times three drawing programs).

The class explosion problem arises because in this solution the abstraction (the kinds of Shapes) and the implementation (the drawing programs) are tightly coupled. Each type of shape must know what type of drawing program it is using. I need a way to separate the variations in abstraction from the variations in implementation so that the number of classes only grows linearly (see Figure 10-6).

Figure 10-6 The Bridge pattern separates variations in abstraction and implementation.

This is exactly the intent of the Bridge pattern: “[to] decouple an abstraction from its implementation so that the two can vary independently.”2

Before showing a solution and deriving the Bridge pattern, I want to mention a few other problems (beyond the combinatorial explosion).

Looking at Figure 10-3, ask yourself what else is poor about this design.

  • Does there appear to be redundancy?
  • Would you say things have strong cohesion or weak cohesion?
  • Are things tightly or loosely coupled?

Would you want to have to maintain this code?

When I first looked at these problems, I thought that part of the difficulty might have been that I simply was using the wrong kind of inheritance hierarchy. Therefore, I tried the alternative hierarchy shown in Figure 10-7.

Figure 10-7 An alternative implementation.

I still have the same four classes representing all of my possible combinations. However, by first deriving versions for the different drawing programs, I eliminated the redundancy between the DP1 and DP2 classes.

Unfortunately, I am unable to eliminate the redundancy between the two types of Rectangles and the two types of Circles, each pair of which has the same draw method.

In any event, the class explosion that was present before is still present here.

The sequence diagram for this solution is shown in Figure 10-8.

Figure 10-8 Sequence diagram for new approach.

Although this may be an improvement over the original solution, it still has a problem with scaling. It also still has some of the original cohesion and coupling problems.

Bottom line: I do not want to have to maintain this version either! There must be a better way.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020