Java Code Examples Showing Problem Domain Classes

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Chapter 12

• Java Code Examples Showing Problem Domain Classes

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Chapter Twelve Objectives

• Students learn Java Programming Syntax
• Class Structure
• Sending Messages in Driver Programs
• Implementing key concepts of classes,
  objects,attributes, methods, encapsulation,
  messages, inheritance, polymorphism and
  association relationships

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Introduction to Java

• Two slashes indicate the beginning of a comment
  that extends to the end of the line.
• Java is case sensitive (i.e. John is different
  JOHN).
• Each java statement ends with a semicolon but
  statements can continue from line to line without
  a continuation character.
• Each block of code is enclosed with in curly
  braces . Blocks of code are nested within each
  other. Example

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A block of Java code

• Java provides standard programming constructs,
  including if..thanelse, dowhile, dountil and
  standard numeric operations.

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Class Structure

• In java each program must be a class that can
  have attributes and methods.
• One source code file is created for each class,
  with the filename the same as the class name the
  class Person is Person.java. When the file is
  compiled, it is named Person.class.
• Internal documentation is included at the top of
  the file as comments beginning with two slashes.
• The first line in the example declares the name
  of the class public class Person. The key word
  public means that objects can be accessed by
  anything in the system. By convention names of
  the classes are capitalized.

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Class Structure (contd)

• After the class name there is a curly brace to
  indicate the beginning of the class and
  everything inside the block belongs to the class.
  The end curly brace ends the class.
• Attributes of the person class are declared next
  name and dateOfBirth are simply variables
  declared in the class.
• A constructor is a special method used to create
  new object of the class.

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Class Structure

• Standard methods are often called accessor
  methods because they allow access of the
  attributes of the class.
• Accessor methods that set the value of an
  attribute begin with the word set, as in setName,
  setDateOfBirth. Methods that get the value of an
  attribute begin with get, as in getName,
  getDateOfBirth. Method names begin with lower
  case.

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Sending messages in Driver Programs

• New classes must be tested.
• One way to test classes is to create a driver
  program. That creates objects of the class and
  sends messages to the objects. This concept is
  the same as a driver program in structured
  program.

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Sending messages in Driver Programs

• Person person1, declares a variable that the
  driver program can assign as a reference to the
  new object
• The statement person1.getName() is a message to
  the object referenced by the variable person1.
  The object is person1 and the message is
  getName(). The statement person1.getDateOfBirth()
  is another message to the person1 object.
• The next line sends messages to the second person
  object again asking for its name and date of
  birth.

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Sending messages in Driver Programs

• Note that these two objects exist as long as the
  driver program is running, which is for an
  instant.
• If a Java application or applet created these
  objects they would be running as long as the
  applet or application is running. To exist
  beyond that it would be necessary to store the
  new objects in a file or a database.

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Sending messages in Driver Programs
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Inheritance and Polymorphism

• It is relatively easy to create a subclass that
  inherits its attributes and methods.
• PatientPerson class code is shown in Figure 12.7.
  To inherit, or extend the Person class, it is
  only necessary to add the phrase extends Person,
  as in public class PatientPerson extends Person.
  The attributes declared are the two additional
  attributes employer and insuranceCompany.
• You dont have to redeclare the attributes of
  Person. Get and set methods included only apply
  to the additional attributes. When patient is
  declared it will automatically have all four
  attributes and all eight get and set methods.

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Inheritance and Polymorphism
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PatientPerson Class

• A name, a date, an employer and an insurance are
  the four arguments in the constructor
• The constructor then invokes the constructor of
  the Person class, the superclass using the
  keyword super followed by the two string
  arguments the Person constructor expects.
• All of the code in Person class constructor get
  executed, additional statements in the Person
  class constructor get executed. The result is
  that all four values are assigned to the
  attributes of the new PatientPerson.

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DoctorPerson subclass

• Also adds two attributes and four get and set
  methods, but they are different attributes and
  methods than those in the PatientPerson class.
• The constructor expects four strings as arguments
  and the superclass constructor of person is also
  invoked.

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Polymorphism

• When two different types of objects respond in
  their own special way to the same massage.
• The tellAboutSelf() method in DoctorPerson has
  the same name as the method in PatientPerson and
  note that the statements in both methods are
  similar.
• However when a DoctorPerson and PatientPerson is
  asked to tell abut itself, it responds in its own
  way.
• Note that \n inserts a new line.

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• The system in medical clinic is only interested
  in persons if they are either doctor or patients.
  
• To make the Person class abstract with Java the
  key word abstract is inserted before the class
  name.

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Association Relationships

• In association relationship is to include an
  attribute in each class to hold a reference to
  and object of the other class.
• The reference point is actual objects not foreign
  key.

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• The last statement in the constructor literally
  asks the PatientPerson object to associate with
  this new treatemt, by sending a message
  patientPerson.associateWithTreatemt(this).
• The key word this is a reference to the
  treatment being created.

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PatientPerson2

• The PatientPerson class has to be revised to
  allow to associate with many treatments.
• The array named treatments will contain object
  reference of the Treatment class declared as
  private Treatment treatments.

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PatientPerson2

• The expanded constructor adds statements to
  create the actual array of up to ten Treatment
  reference and initialize the treatmentCount to
  zero.
• A standard method named associatedWithTreatment
  is added for PatientPerson.
• The single statement in the method first adds one
  to the counter as treatmentCount using the
  increment operator.

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PatientPerson2

• public void associatedWithTreatment
• (Treatment aTreatment)
• treatmentstreatmentCount aTreatment

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Person

• In this sequence diagram shown a scenario in
  which the user asks a PatientPerson to get
  information about all of its Treatments.
• To get this job done an additional method is
  added named getAllTreatments to PatientPerson2.
• A driver program to test the treatment class and
  expanded PatientPerson2 class.
• Three objects are created (Brian, Kevin and Ida).
  Next 7 treatment objects are created. Note that
  the new treatments are not assigned to object
  reference in the driver program.
• After that the driver program asks each person to
  tellAboutSelf(). Then each person is asked to
  getAllTreatments().

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PatientPerson2

• public String getAllTreatments()
• String allTreatments
• allTreatments Treatment for Patient
  getName() include \n
• for(int i 0 ilttreatmentCount i)
• // Append to string here

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Sequence Diagram
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PersonDriver2 Program
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Output produced by the PersonDriver2 program