Abstract
Software design patterns have roots in the architectural patterns
of Christopher Alexander, and in the object movement. According
to Alexander, patterns repeat themselves, since they are a generic
solution to a given system of forces. The object movement looks
to the real world for insights into modeling software relationships.
With these dual roots, it seems reasonable that software design
patterns should be repeated in real world objects. This paper
presents a real world, non software instance of each design pattern
from the book, Design Patterns - Elements of Reusable Object-Oriented
Software [13]. The paper also discusses the implications
of non-software examples on the communicative power of a pattern
language, and on design pattern training.
1. Introduction
Within the software industry, a growing community of patterns
proponents exists. The roots of the patterns movement are found
in the writings of architect Christopher Alexander, who describes
a pattern as a generic solution to a given system of forces in
the world [1]. Alexander's patterns can be observed in everyday
structures. Each pattern in A Pattern Language[2] includes
a picture of an archetypal example of the pattern.
Since objects were the predominate world view at the time that
patterns were embraced by the software world, patterns also have
roots in the object movement [9]. Unfortunately examples of software
design patterns are not as abundant as Alexandrian patterns, since
they represent elegant designs, rather than the designs that people
generate initially [13]. Access to elegant designs is often limited
due to the proprietary nature of much of the software being developed
today.
According to Alexander, real world patterns always repeat themselves,
because under a given set of circumstances, there are always certain
fields of relationships which are most nearly well adapted to
the forces which exist [1]. In software, real world problems
are either modeled entirely, or real world objects are transformed
into hardware and software to produce real world results [5].
Since software design patterns have roots in both Alexandrian
patterns, and in the object movement, it seems logical that software
design patterns can be found in real world objects. This is not
to say that software design patterns are necessarily models of
the real world objects, but the relationships between objects
that have been adapted to deal with certain forces can be observed
both in the "real world" and in software objects. To
test this hypothesis, a real world example was sought for each
of the 23 Gang of Four Patterns [13]. The examples follow in
sections 2 through 4.
2. Creational Patterns
Five creational patterns have been documented by the Gang of Four.
Examples of these creational patterns can be found in manufacturing,
fast food, biology and political institutions.
2.1 Abstract Factory Example
The purpose of the Abstract Factory is to provide
an interface for creating families of related objects, without
specifying concrete classes. This pattern is found in the sheet
metal stamping equipment used in the manufacture of Japanese automobiles.
The stamping equipment is an Abstract Factory which creates
auto body parts. The same machinery is used to stamp right hand
doors, left hand doors, right front fenders, left front fenders,
hoods etc. for different models of cars. Through the use of rollers
to change the stamping dies, the concrete classes produced by
the machinery can be changed within three minutes [16].
2.2 Builder Example
The Builder pattern separates the construction of a complex
object from its representation, so that the same construction
process can create different representation. This pattern is
used by fast food restaurants to construct children's meals.
Children's meals typically consist of a main item, a side item,
a drink, and a toy (e.g., a hamburger, fries, coke, and toy car).
Note that there can be variation in the contents of the children's
meal, but the construction process is the same. Whether a customer
orders a hamburger, cheeseburger, or chicken, the process is the
same. The employee at the counter directs the crew to assemble
a main item, side item, and toy. These items are then placed
in a bag. The drink is placed in a cup and remains outside of
the bag. This same process is used at competing restaurants.
2.3 Factory Method Example
The Factory Method defines an interface for creating objects,
but lets subclasses decide which classes to instantiate. Injection
molding presses demonstrate this pattern. Manufacturers of plastic
toys process plastic molding powder, and inject the plastic into
molds of the desired shapes [15]. The class of toy (car, action
figure, etc.) is determined by the mold.
2.4 Prototype Example
The Prototype pattern specifies the kind of objects to
create using a prototypical instance. Prototypes of new products
are often built prior to full production, but in this example,
the prototype is passive, and does not participate in copying
itself. The mitotic division of a cell, resulting in two identical
cells, is an example of a prototype that plays an active role
in copying itself and thus, demonstrates the Prototype pattern.
When a cell splits, two cells of identical genotype result.
In other words, the cell clones itself.
2.5 Singleton Example
The Singleton pattern ensures that a class has only one
instance, and provides a global point of access to that instance.
The Singleton pattern is named after the singleton set,
which is defined to be a set containing one element. The office
of the President of the United States is a Singleton. The
United States Constitution specifies the means by which a president
is elected, limits the term of office, and defines the order of
succession. As a result, there can be at most one active president
at any given time. Regardless of the personal identity of the
active president, the title, "The President of the United
States" is a global point of access that identifies the person
in the office.
3. Structural Patterns
Seven structural patterns have been documented by the Gang of
Four. Examples of these patterns can be found in hand tools,
residential wiring, mathematics, holiday tradition, catalog retail,
and banking.
3.1 Adapter Example
The Adapter pattern allows otherwise incompatible classes
to work together by converting the interface of one class into
an interface expected by the clients. Socket wrenches provide
an example of the Adapter. A socket attaches to a ratchet,
provided that the size of the drive is the same. Typical drive
sizes in the United States are 1/2" and 1/4". Obviously
a 1/2" drive ratchet will not fit into a 1/4" drive
socket unless an adapter is used. A 1/2" to 1/4" adapter
has a 1/2" female connection to fit on the 1/2" drive
ratchet, and a 1/4" male connection to fit in the 1/4"
drive socket.
3.2 Bridge Example
The Bridge pattern decouples an abstraction from its implementation,
so that the two can vary independently. A household switch controlling
lights, ceiling fans, etc. is an example of the Bridge.
The purpose of the switch is to turn a device on or off. The
actual switch can be implemented as a pull chain, a simple two
position switch, or a variety of dimmer switches.
3.3 Composite Example
The Composite composes objects into tree structures, and
lets clients treat individual objects and compositions uniformly.
Although the example is abstract, arithmetic expressions are
Composites. An arithmetic expression consists of an operand,
an operator (+ - * /), and another operand. The operand can be
a number, or another arithmetic expression. Thus, 2 + 3 and (2
+ 3) + (4 * 6) are both valid expressions.
3.4 Decorator Example
The Decorator attaches additional responsibilities to an
object dynamically. Although paintings can be hung on a wall
with or without frames, frames are often added, and it is the
frame which is actually hung on the wall. Prior to hanging, the
paintings may be matted and framed, with the painting, matting,
and frame forming a single visual component.
3.5 Facade Example
The Facade defines a unified, higher level interface to
a subsystem, that makes it easier to use. Consumers encounter
a Facade when ordering from a catalog. The consumer calls
one number and speaks with a customer service representative.
The customer service representative acts as a Facade,
providing an interface to the order fulfillment department, the
billing department, and the shipping department.
3.6 Flyweight Example
The Flyweight uses sharing to support large numbers of
objects efficiently. The public switched telephone network is
an example of a Flyweight. There are several resources
such as dial tone generators, ringing generators, and digit receivers
that must be shared between all subscribers. A subscriber is
unaware of how many resources are in the pool when he or she lifts
the hand set to make a call. All that matters to subscribers
is that dial tone is provided, digits are received, and the call
is completed.
3.7 Proxy Example
The Proxy provides a surrogate or place holder to provide
access to an object. A check or bank draft is a proxy for funds
in an account. A check can be used in place of cash for making
purchases and ultimately controls access to cash in the issuer's
account.
4. Behavioral Patterns
Eleven behavioral patterns have been documented by the Gang of
Four. Examples of these patterns can be found in coin sorting
banks, restaurant orders, music, transportation, auto repair,
vending machines, and home construction.
4.1 Chain of Responsibility Example
The Chain of Responsibility pattern avoids coupling the
sender of a request to the receiver, by giving more than one object
a chance to handle the request. Mechanical coin sorting banks
use the Chain of Responsibility. Rather than having a
separate slot for each coin denomination coupled with receptacle
for the denomination, a single slot is used. When the coin is
dropped, the coin is routed to the appropriate receptacle by the
mechanical mechanisms within the bank.
4.2 Command Example
The Command pattern allows requests to be encapsulated
as objects, thereby allowing clients to be paramaterized with
different requests. The "check" at a diner is an example
of a Command pattern. The waiter or waitress takes an
order, or command from a customer, and encapsulates that order
by writing it on the check. The order is then queued for a short
order cook. Note that the pad of "checks" used by
different diners is not dependent on the menu, and therefore they
can support commands to cook many different items.
4.3 Interpreter Example
The Interpreter pattern defines a grammatical representation
for a language and an interpreter to interpret the grammar.
Musicians are examples of Interpreters. The pitch of a
sound and its duration can be represented in musical notation
on a staff. This notation provides the language of music [14].
Musicians playing the music from the score are able to reproduce
the original pitch and duration of each sound represented.
4.4 Iterator Example
The Iterator provides ways to access elements of an aggregate
object sequentially without exposing the underlying structure
of the object. On early television sets, a dial was used to
change channels. When channel surfing, the viewer was required
to move the dial through each channel position, regardless of
whether or not that channel had reception. On modern television
sets, a next and previous button are used. When the viewer selects
the "next" button, the next tuned channel will be displayed.
Consider watching television in a hotel room in a strange city.
When surfing through channels, the channel number is not important,
but the programming is. If the programming on one channel is
not of interest, the viewer can request the next channel, without
knowing its number.
4.5 Mediator Example
The Mediator defines an object that controls how a set
of objects interact. Loose coupling between colleague objects
is achieved by having colleagues communicate with the Mediator,
rather than with each other. The control tower at a controlled
airport demonstrates this pattern very well. The pilots of the
planes approaching or departing the terminal area communicate
with the tower, rather than explicitly communicating with one
another. The constraints on who can take off or land are enforced
by the tower. It is important to note that the tower does not
control the whole flight. It exists only to enforce constraints
in the terminal area.
4.6 Memento Example
The Memento captures and externalizes an object's internal
state, so the object can be restored to that state later. This
pattern is common among do-it-yourself mechanics repairing drum
brakes on their cars. The drums are removed from both sides,
exposing both the right and left brakes. Only one side is disassembled,
and the other side serves as a Memento of how the brake
parts fit together [8]. Only after the job has been completed
on one side is the other side disassembled. When the second side
is disassembled, the first side acts as the Memento.
4.7 Observer Example
The Observer defines a one to many relationship, so that
when one object changes state, the others are notified and updated
automatically. Some auctions demonstrate this pattern. Each
bidder possesses a numbered paddle that is used to indicate a
bid. The auctioneer starts the bidding, and "observes"
when a paddle is raised to accept the bid. The acceptance of
the bid changes the bid price, which is broadcast to all of the
bidders in the form of a new bid.
4.8 State Example
The State pattern allows an object to change its behavior
when its internal state changes. This pattern can be observed
in a vending machine. Vending machines have states based on the
inventory, amount of currency deposited, the ability to make change,
the item selected, etc. When currency is deposited and a selection
is made, a vending machine will either deliver a product and no
change, deliver a product and change, deliver no product due to
insufficient currency on deposit, or deliver no product due to
inventory depletion.
4.9 Strategy Example
A Strategy defines a set of algorithms that can be used
interchangeably. Modes of transportation to an airport is an
example of a Strategy. Several options exist, such as driving
one's own car, taking a taxi, an airport shuttle, a city bus,
or a limousine service. For some airports, subways and helicopters
are also available as a mode of transportation to the airport.
Any of these modes of transportation will get a traveler to the
airport, and they can be used interchangeably. The traveler must
chose the Strategy based on tradeoffs between cost, convenience,
and time.
4.10 Template Method Example
The Template Method defines a skeleton of an algorithm
in an operation, and defers some steps to subclasses. Home builders
use the Template Method when developing a new subdivision.
A typical subdivision consists of a limited number of floor plans,
with different variations available for each floor plan. Within
a floor plan, the foundation, framing, plumbing, and wiring will
be identical for each house. Variation is introduced in the latter
stages of construction to produce a wider variety of models.
4.11 Visitor Example
The Visitor pattern represents an operation to be performed
on the elements of an object structure, without changing the classes
on which it operates. This pattern can be observed in the operation
of a taxi company. When a person calls a taxi company he or
she becomes part of the company's list of customers. The company
then dispatches a cab to the customer (accepting a visitor).
Upon entering the taxi, or Visitor, the customer is no
longer in control of his or her own transportation, the taxi (driver)
is.
5. Implications
Non-software examples of each of the software design patterns
cataloged by the Gang of Four have been shown to exist. One might
now wonder about the practical implications of these examples.
These non-software examples are useful in increasing the communicative
power of the pattern language and as aids to learning the patterns.
5.1 Increasing the Communicative Power of the Pattern Language
Alexander had hoped that true patterns would enter a common language
that all could share [2]. Within the software design community,
patterns are seen as a way to develop a set of languages to streamline
communication between colleagues [4,17]. Patterns are expected
to provide a vocabulary for discussing structures larger than
modules, procedures, or objects [10].
One crucial element of a language is the mental imagery associated
with symbols of the language. In a language, a given configuration
of symbols has meaning only if one can grasp its content, which
involves mentally representing it [7]. The importance of mental
imagery to pattern languages was not lost on Alexander, who stated
that a language was not morphologically complete until the types
of buildings that it generates could be visualized concretely
[1]. In software design, Richle and Z¸llighoven recognized
the importance of concrete examples in guiding our perception
of the application domain [18].
If software design patterns are to become a common language among
programmers, shared meaning is essential. If design decisions
are communicated, but not understood, designers are forced to
make missing assumptions to complete the job [19]. Commonplace
examples facilitate understanding, because in order to understand
anything, people must find the closest item in memory to which
it relates [20]. The projects at AG Communication Systems that
make extensive use of patterns often use non-software examples
to illustrate the relationships at work in the patterns. The
examples help provide a common understanding between designers.
By establishing common understanding early in the design process,
communication between designers is facilitated throughout the
project life cycle.
5.2 Non-Software Examples as Aids to Learning Patterns
Students require examples whenever new concepts are presented.
This was evident in the evaluation of a course in patterns offered
at AG Communication Systems and has also been documented by others
[12]. When learning something new, the student naturally tends
to exploit prior knowledge in an effort to understand the new
concepts [6]. For this reason, many examples should be included
when students are first exposed to software design patterns [12].
Specific examples should be ones with which the student is acquainted,
but not expert [3]. Providing acquaintance examples does not
increase the new material that must be learned. At the same time,
choosing an example outside of a student's expertise keeps the
student from getting so involved in the example that the point
of the new material is missed. Since patterns must ultimately
reside in one's own mind [11], using examples that are common
to a large cross section of people, training material can build
on examples already committed to memory.
6. Conclusion
The repetition of software design patterns in non-software examples
is evidence that patterns are not limited to a specific domain.
Instances of these patterns in everyday objects can benefit software
designers, even though the examples are not expressed in a programming
language. The examples presented in this paper are intended to
be ones that are familiar to a large cross section of people (although
some may be culturally biased towards North Americans). By drawing
on common experience, such examples facilitate understanding of
specific design patterns, and thus improve communication and serve
as an aid to learning the patterns.
