Transcript Sending messages in Driver Programs

Chapter 12
Java Code Examples Showing
Problem Domain Classes
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
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:
A block of Java code
Java provides
standard
programming
constructs, including
if..than…else,
do…while, do…until
and standard numeric
operations.
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
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.
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
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.
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.
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.
Sending messages in Driver
Programs
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 don’t 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
Inheritance and Polymorphism
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.
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.
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.
•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.
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.
The last
statement in the
constructor
literally asks the
PatientPerson
object to
associate with
this new
treatemt, by
sending a
message
patientPerson.ass
ociateWithTreate
mt(this).
The key word
“this” is a
reference to the
treatment being
created.
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.
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.
PatientPerson2
public void associatedWithTreatment
(Treatment aTreatment)
{
treatments[treatmentCount++] = aTreatment;
}
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().
PatientPerson2
public String getAllTreatments()
{
String allTreatments;
allTreatments = “Treatment for Patient” +
getName() + “ include \n”;
for(int i = 0; i<treatmentCount; i++)
{
// Append to string here
}
}
Sequence Diagram
PersonDriver2 Program
Output produced by the
PersonDriver2 program