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Programs and Classes
A program is made up from classes
Classes may be grouped into packages
A class has two parts
static parts exist independently
Non-static parts define what objects in the class
look like.
Every class is automatically in existence when the
program runs.
Classes and Objects
An object is an instance of a class, and is created
using the new operator.
The non-static part of the class defines what each
object looks like.
Many instances (objects) can be created from a
class … no limit except reality
An object contains information and functionality of a
“thing”, e.g., Account, Vehicle, Employee, etc.
Classes’ and Objects’ Components
Classes (and thus also objects) are composed of
methods and data values
Data values store information
Methods do things, and also have their own local data
Graphical Representation
The class name
appears on top of the
icon.
Account
An icon for a class is the
rectangle.
The object’s name
appears on top of the
icon.
SV129
Account
The class name is
placed inside the
object icon.
An icon for an object is
the rounded rectangle.
Instance-of Relationship
Employee
Before you can create
instances of a class, the
class must be defined.
The dotted line shows
the instance-of
relationship.
The class name can be
omitted since it is clear
which class these
objects belong to .
Bill
Steve
Andy
Employee
Employee
Employee
Visibility Modifiers: public and private
The modifiers public and private designate the
accessibility of objects’ and class’ data values and
methods
If a component is declared private, nothing outside
the class can access it.
If a component is declared public, anything outside
the class can access it.
In general, be private (military demotion)
Make class components private whenever you can
This supports the notion of encapsulation, which
makes for more robust software development
Class and Instance Data Values
A class data value (indicated by the static
modifier) is used to maintain information shared by
all instances or aggregate information about the
instances.
An instance data value is used to maintain
information specific to individual instances.
Make instance data values private always
Sample Data Values
Account
minimum balance
100.00
There is one copy of
minimum balance for
the whole class and
shared by all instances.
All three Account objects
possess the same instance
data value current balance.
SV129
SV506
SV008
Account
Account
Account
current balance
908.55
current balance
1304.98
current balance
354.00
Primitive and Reference Data Values
Data Type
reference
primitive
long
byte
int
String
short
char
MessageBox Applet
double
HiLo
float
boolean
Primitive variables contain values
Reference variables point at objects
InputBox
etc.
Methods
Methods have code (to do stuff) and data
A method defined for a class is called a class method
(indicated by the static modifier) and a method
defined for an object is called an instance method.
Messages
To instruct a class or an object to do something, we a
message to one of its methods
Values passed to a method when sending a message
are called arguments or parameters of the message.
The (formal) parameters of a method are local
variables that receive the message parameters
Methods can return one data value to the calling
method
Sending a Message
Message deposit with
the argument 250.00 is
sent to chk-008.
chk-008
Account
deposit 250.00
deposit
Message name is usually
omitted in the diagram.
250.00
deposit
Getting an Answer
This message has no
argument.
chk-008
Account
getMonthlyFee
monthly fee
The method returns the
value monthly fee back
to the message sender.
Calling a Class Method
Account
getAverageBalance
average balance
The average balance of
all accounts is returned.
Program Components
A Java file is composed of
comments,
import statements, and
class declarations.
Files and Classes
A Java program file ends with .java
There must be one public class per file
It must have the same name as the file
One public class (i.e., one file) must have
the main method
Simple Java Programs
Simple Java programs can be written in just
the one file, containing
One public class (with the main method)
Other class methods and final data values
as required
Such programs do not create any objects, but
simply run class methods (starting with the
main method) and use primitive data.