Transcript Chapter 9: Object-Oriented Software Development

Chapter 11 Object-Oriented Design and
Patterns
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Motivations
The preceding chapters introduced objects, classes,
class inheritance, and interfaces. You learned the
concepts of object-oriented programming. This
chapter focuses on the analysis and design of
software systems using the object-oriented
approach. You will learn class-design guidelines,
and the techniques and patterns for designing
reusable classes.
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Objectives









To become familiar with the process of program development (§12.2).
To learn the relationship types: association, aggregation, composition,
dependency, strong inheritance, and weak inheritance (§12.3).
To discover classes and determine responsibilities of each classes (§12.3).
To declare classes to represent the relationships among them (§12.4).
To design systems by identifying the classes and discovering the
relationships among these classes (§12.5).
To implement the Rational class and process rational numbers using this
class (§12.6).
To design classes that follow the class-design guidelines (§12.7).
To know the concept of framework-based programming using the Java
API (§12.8).
To introduce design patterns for developing sound software systems
(§12.9).
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Software Development Process
Requirement
Specification
System
Analysis
System
Design
Implementation
Testing
Deployment
Maintenance
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Requirement Specification
A formal process that seeks to understand
the problem and document in detail what
the software system needs to do. This
phase involves close interaction between
users and designers.
Requirement
Specification
System
Analysis
System
Design
Implementation
Testing
Most of the examples in this book are simple,
and their requirements are clearly stated. In
the real world, however, problems are not
well defined. You need to study a problem
carefully to identify its requirements.
Deployment
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Maintenance
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System Analysis
Requirement
Specification
Seeks to analyze the business
process in terms of data flow, and
to identify the system’s input and
output.
System
Analysis
System
Design
Implementation
Part of the analysis entails modeling
the system’s behavior. The model is
intended to capture the essential
elements of the system and to define
services to the system.
Testing
Deployment
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Maintenance
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System Design
The process of designing the
system’s components.
Requirement
Specification
System
Analysis
System
Design
Implementation
Testing
This phase involves the use of many levels
of abstraction to decompose the problem into
manageable components, identify classes and
interfaces, and establish relationships among
the classes and interfaces.
Deployment
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Maintenance
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Implementation
The process of translating the
system design into programs.
Separate programs are written for
each component and put to work
together.
Requirement
Specification
System
Analysis
System
Design
Implementation
This phase requires the use of a
programming language like Java.
The implementation involves
coding, testing, and debugging.
Testing
Deployment
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Maintenance
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Testing
Requirement
Specification
Ensures that the code meets the
requirements specification and
weeds out bugs.
System
Analysis
System
Design
Implementation
An independent team of software
engineers not involved in the design
and implementation of the project
usually conducts such testing.
Testing
Deployment
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Maintenance
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Deployment
Requirement
Specification
Deployment makes the project
available for use.
System
Analysis
System
Design
Implementation
For a Java applet, this means
installing it on a Web server; for a
Java application, installing it on the
client's computer.
Testing
Deployment
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Maintenance
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Maintenance
Requirement
Specification
Maintenance is concerned with
changing and improving the
product.
System
Analysis
System
Design
Implementation
Testing
A software product must continue to
perform and improve in a changing
environment. This requires periodic
upgrades of the product to fix newly
discovered bugs and incorporate changes.
Deployment
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Discovering Classes
One popular way for facilitating the discovery process is by
creating CRC cards. CRC stands for classes, responsibilities,
and collaborators. Use an index card for each class, as shown
in Figure 12.2.
Class name
Student
Register a course
Course
Responsibilities
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Collaborators
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Discovering Class Relationships
 Association
 Aggregation
and Composition
 Dependency
 Inheritance
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Association
Association represents a general binary relationship that describes
an activity between two classes.
Student
5..60
public class Student {
/** Data fields */
private Course[]
courseList;
Take
* Course
0..3
public class Course {
/** Data fields */
private Student[]
classList;
private Faculty faculty;
/** Constructors */
/** Methods */
/** Constructors */
/** Methods */
}
Teach
1
Teacher
Faculty
public class Faculty {
/** Data fields */
private Course[]
courseList;
/** Constructors */
/** Methods */
}
}
An association is usually represented as a data field in the class.
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Translation is not Unique
NOTE: If you don’t need to know the courses a student
takes or a faculty teaches, the data field coureList in Student
or Faculty can be omitted.
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Association Between Same Class
Association may exist between objects of the same class.
For example, a person may have a supervisor.
1
Person
Supervisor
1
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Aggregation and Composition
Aggregation is a special form of association, which
represents an ownership relationship between two classes.
Aggregation models the has-a relationship. If an object is
exclusively owned by an aggregated object, the
relationship between the object and its aggregated object is
referred to as composition.
Composition
Name
Aggregation
Person
Address
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Dependency
A dependency describes a relationship between two classes
where one (called client) uses the other (called supplier). In
UML, draw a dashed line with an arrow from the client class
to the supplier class. For example, the ArrayList class uses
Object because you can add objects to an ArrayList. The
relationship between ArrayList and Object can be described
using dependency, as shown in Figure 12.7(a). The
relationship between Calendar and Date can be described
using dependency, as shown in Figure 12.7(b).
ArrayList
Object
Calendar
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Date
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Dependency vs. Association
Both association and dependency describe one class as
depending on another. Association is stronger than
dependency. In association, the state of the object changes
when its associated object changes. In dependency, the client
object and the supplier object are loosely coupled. The
association relationship is implemented using data fields and
methods. There is a strong connection between two classes.
The dependency relationship is implemented using methods.
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Coupling
Dependency, association, aggregation, and
composition all describe coupling relationships
between two classes. The difference is the degree of
coupling with composition being the strongest,
followed by aggregation, association, and
dependency in this order.
coupling increases
dependency, association, aggregation, composition
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Representing Aggregation in Classes
An aggregation relationship is usually represented as a data
field in the aggregated class.
public class Name {
/** Data fields */
/** Constructors */
/** Methods */
}
public class Person {
/** Data fields */
private Name name;
private Address address;
public class Address {
/** Data fields */
/** Constructors */
/** Methods */
}
/** Constructors */
/** Methods */
}
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Inner Classes Translation
If Name or Address is used in the Person class only, they can
be declared as an inner class in Person. For example,
public class Person {
private Name name;
private Address address;
...
class Name {
...
}
class Address {
...
}
}
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Inheritance
Inheritance models the is-an-extension-of
relationship between two classes.
public class Faculty extends Person {
Person
/** Data fields */
/** Constructors */
/** Methods */
Faculty
}
(A)
(B)
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Weak Inheritance Relationship
A weak is-an-extension-of relationship can be represented using
interfaces. For example, the weak is-an-extension-of relationship
“students are comparable based on their grades” can be represented
by implementing the Comparable interface, as follows:
public class Student extends Person
implements Comparable {
Person
/** Data fields, Constructors, and */
/** Methods */
Student
Comparable
/** Implement the compareTo method */
public int compareTo(Object object) {
// ...
}
}
(A)
(B)
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Class Design
1. Identify classes for the system.
2. Describe attributes and methods in each
class.
3. Establish relationships among classes.
4. Create classes.
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Borrowing Loans
Name
Person
-firstName: String
-mi: char
-lastName: String
-name: Name
-address: Address
+Person()
+Person(name: Name, address: Address)
+getName(): Name
+seName(name: Name): void
+getAddress(): Address
+setAddress(address: Address): void
+toString(): String
+Name()
+Name(firstName: String,
mi: char, lastName: String)
+getFirstName(): String
+getMi(): char
+getLastName(): String
+setFirstName(firstName:
String): void
+setMi(mi: char): void
+setLastName(lastName:
String): void
+getFullName(): String
Borrower
-loan: Loan
Loan
Defined in
Example 6.7
Name
Address
-street: String
-city: String
-state: String
-zip: String
+Address()
+Address(street: String, city: String,
state: String, zip: String)
+getStreet(): String
+getCity(): String
+getState(): String
+getZip(): String
+setStreet(street: String): void
+setCity(city: String): void
+setState(state: String): void
+setZip(zip: String): void
+getFullAddress(): String
+Borrower()
+Borrower(name: Name, address: Address)
+getLoan(): Loan
+setLoan(loan: Loan): void
+toString(): String
Loan
Person
Borrower Address
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Borrowing Loans, cont.
The following is a test program that uses the
classes Name, Person, Address,
Borrower, and Loan.
BorrowLoan
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Run
27
The Rational Class
1
java.lang.Number
+byteValue(): byte
+shortValue(): short
+intValue(): int
+longVlaue(): long
+floatValue(): float
+doubleValue():double
java.lang.Comparable
compareTo(Object): int
Rational
-numerator: long
-denominator: long
+Rational()
+Rational(numerator: long, denominator: long)
+getNumerator(): long
+getDenominator(): long
+add(secondRational: Rational): Rational
+multiply(secondRational: Rational): Rational
+subtract(secondRational: Rational): Rational
+divide(secondRational: Rational): Rational
+toString(): String
-gcd(n: long, d: long): long
1
Rational
TestRationalClass
Add, Subtract, Multiply, Divide
Run
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Class Design Guidelines
 Designing
a Single Class.
 Using
Modifiers public, protected, private
and static
 Using
Inheritance or Aggregation
 Using
Interfaces or Abstract Classes
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Designing a Class
 A class
should describe a single entity or a set of
similar operations. A single entity with too many
responsibilities can be broken into several classes
to separate responsibilities. The String class,
StringBuffer class, and StringTokenizer class all
deal with strings, for example, but have different
responsibilities.
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Designing a Class, cont.
 Classes
are usually designed for use by many
different customers. To make a class useful in a
wide range of applications, the class should
provide a variety of ways for customization
through properties and methods.
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Designing a Class, cont.
 Classes
are designed for reuse. Users can
incorporate classes in many different combinations,
orders, and environments. Therefore, you should
design a class that imposes no restrictions on what
or when the user can do with it, design the properties
to ensure that the user can set properties in any
order, with any combination of values, and design
methods to function independently of their order of
occurrence.
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Designing a Class, cont.
 Provide
a public no-arg constructor and override the
equals method and the toString method defined in
the Object class whenever possible.
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Designing a Class, cont.
 Follow
standard Java programming style and
naming conventions. Choose informative
names for classes, data fields, and methods.
Always place the data declaration before the
constructor, and place constructors before
methods. Always provide a constructor and
initialize variables to avoid programming
errors.
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Using Visibility Modifiers

Each class can present two contracts – one for the users
of the class and one for the extenders of the class. Make
the fields private and accessor methods public if they are
intended for the users of the class. Make the fields or
method protected if they are intended for extenders of the
class. The contract for the extenders encompasses the
contract for the users. The extended class may increase
the visibility of an instance method from protected to
public, or change its implementation, but you should
never change the implementation in a way that violates
that contract.
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Using Visibility Modifiers, cont.
 A class
should use the private modifier to hide its
data from direct access by clients. You can use get
methods and set methods to provide users with
access to the private data, but only to private data
you want the user to see or to modify. A class should
also hide methods not intended for client use. The
gcd method in the Rational class in Example 11.2,
“The Rational Class,” is private, for example,
because it is only for internal use within the class.
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Using the static Modifier
 A property
that is shared by all the
instances of the class should be declared
as a static property.
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Using Inheritance or Aggregation
In general, the difference between inheritance
and aggregation is the difference between the
is-an-extension-of relationship and the has-a
relationship. For example, an apple is fruit;
thus, you would use inheritance to model the
relationship between the classes Apple and
Fruit. A person has a name; thus, you would
use aggregation to model the relationship
between the classes Person and Name.
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Using Inheritance or
Aggregation, cont.
Sometimes, the choice between inheritance
and aggregation is not obvious. For example,
you have used inheritance to model the
relationship between the classes Circle and
Cylinder. One could argue that a cylinder
consists of circles; thus, you might use
aggregation to define the Cylinder class as
follows:
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Using Inheritance or
Composition, cont.
public class Cylinder {
private Circle circle;
/** Constructors */
/** Methods */
}
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Using Inheritance or
Aggregation, cont.
Both designs are fine. Which one is
preferred? If polymorphism is desirable, you
need to use the inheritance design. If you
don’t care about polymorphism, the
aggregation design gives more flexibility
because the classes are less dependent using
aggregation than using inheritance.
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Using Interfaces or Abstract
Classes
Both interfaces and abstract classes can be
used to generalize common features. How do
you decide whether to use an interface or a
class? In general, a strong is-an-extension-of
relationship that clearly describes a parentchild relationship should be modeled using
classes.
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Using Interfaces or Abstract
Classes, cont.
For example, since an orange is a fruit, their relationship
should be modeled using class inheritance. A weak is-anextension-of relationship, also known as an is-kind-of
relationship, indicates that an object possesses a certain
property. A weak is-an-extension-of relationship can be
modeled using interfaces. For example, all strings are
comparable, so the String class implements the
Comparable interface. A circle or a rectangle is a
geometric object, for example, so Circle can be designed
as a subclass of GeometricObject. Circles are different
and comparable based on their radius, for example, so
Circle can implement the Comparable interface.
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Using Interfaces or Abstract
Classes, cont.
Interfaces are more flexible than abstract classes,
because a subclass can extend only one superclass,
but implement any number of interfaces. However,
interfaces cannot contain concrete methods. You
can combine the virtues of interfaces and abstract
classes by creating an interface with a companion
abstract class that implements the interface. So you
can use the interface or its companion class
whichever is more convenient.
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The Java API
The Java API (Application Program
Interface, Application Programming
Interface, or Application Programmer
interface) consists of numerous classes
and interfaces grouped into more than a
dozen of packages. You have used
classes and interfaces in the java.lang,
javax.swing, and java.util packages.
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Framework-Based Programming
To create comprehensive projects, you have to
use more classes and interfaces in the Java API.
The classes and interfaces in the Java API
establish a framework for programmers to
develop applications using Java. For example, the
classes and interfaces in the Java GUI API
establish a framework for developing GUI
programs. You have to use these classes and
interfaces and follow their conventions and rules
to create applications. This is referred to as
framework-based programming.
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Framework-Based Programming, cont.
Once you understand the concept of Java and
object-orient programming, the most important
lesson from now on is learning how to use the
API to develop useful programs. The most
effective way to achieve it is to imitate good
examples. The book provides many carefully
designed examples to demonstrate the concept of
the framework-based programming using the
Java API.
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Framework-Based Programming, cont.
Package
Description
java.lang
contains core Java classes and interfaces (e.g., System,
Math, Object, String, StringBuffer, Number, Character,
Boolean, Byte, Short, Integer, Long, Float, Double,
Comparable, and Cloneable). This package is implicitly
imported to every Java program.
javax.swing
contains the lightweight graphical user interface
components (e.g., JOptionPane, JFrame, JButton) for
developing Swing GUI programs.
java.util
contains utility classes and interfaces, such as Scanner,
Arrays, ArrayList, Date, Calendar, and GregorianCalendar.
java.io
contains classes and interfaces for IO, such as File and
PrintWriter.
java.io
contains classes and interfaces for IO, such as File and
PrintWriter.
java.math
contains classes BigInteger for performing arbitraryprecision integer arithmetic and BigDecimal for performing
arbitrary-precision decimal arithmetic.
java.net
contains classes for networking programming.
java.applet
contains classes for supporting applets.
java.awt
contains classes for supporting GUI programming.
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Design Patterns
One important benefit of object-oriented programming is
to reuse code. For example, to create a button, you
simply use the JButton class to create an instance of
JButton. The JButton class is already defined in the Java
API so you can use it without having to reinvent the
wheel. Design patterns are proven sound software
strategies for designing classes. Applying design patterns
is like reusing experience. You can apply successful
patterns to develop new software without reinventing
new solution strategies.
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