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Chapter 4
Writing Classes
Writing Classes
• We've been using predefined classes. Now we will
learn to write our own classes to define objects
• Chapter 4 focuses on:

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
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class definitions
instance data
encapsulation and Java modifiers
method declaration and parameter passing
constructors
graphical objects
events and listeners
buttons and text fields
Writing Classes
• The programs we’ve written in previous examples
have used classes defined in the Java standard
class library
• Now we will begin to design programs that rely on
classes that we write ourselves
• The class that contains the main method is just
the starting point of a program
• True object-oriented programming is based on
defining classes that represent objects with welldefined characteristics and functionality
Program design
Java standard bibliotek
class String
class Random
class System
class Program
{
....main(....)
{
Random gen= new Random..
String text=....
System.out....
Konto pers=new ...
class Konto
class Bil
A ”Konto” class
Konto.java
class Konto{
private String kundnamn;
Konto
private String kontonummer;
kundnamn
instans
variabler
private int saldo;
kontonummer
public Konto(String namn, String nr ){
kundnamn = namn;
kontonummer=nr;
saldo
sättin
konstruktor
saldo=0;
}
public void sättin (int belopp){
tagut
}
}
saldo=saldo+belopp;
metoder
public class Account
{
private long acctNumber;
private double balance;
private String name;
public Account (String owner, long account, double initial)
{
name = owner;
acctNumber = account;
balance = initial;
}
}
public class TransactionsProgram
{
//----------------------------------------------------------------// Creates some bank accounts and requests various services.
//----------------------------------------------------------------public static void main (String[] args)
{
Account acct1 = new Account ("Ted Murphy", 72354, 102.56);
Account acct2 = new Account ("Jane Smith", 69713, 40.00);
Account acct3 = new Account ("Edward Demsey", 93757, 759.32);
}
}
Bank Account Example
acct1
acctNumber
72354
balance 102.56
“Ted Murphy”
name
acct2
acctNumber
69713
balance
40.00
name
“Jane Smith”
class Konto{
private String kundnamn;
private String kontonummer;
Using class Konto
class Progam{
public static void main( String [] a)
{
private int saldo;
”Marie Svensson” ”451215-2345”
Konto spar;
public Konto(String namn, String nr ){
spar=new Konto( ”Marie Svensson” , ”451215-2345”)
kundnamn = namn;
kontonummer=nr;
saldo=0;
0
}
public void sättin (int belopp){
}
}
}
saldo=saldo+belopp;
public int uttag (int belopp){
saldo=saldo-belopp
retrun saldo;
}
}
spar
class Konto{
private String kundnamn;
class Progam{
public static void main( String [] a)
{
private String kontonummer;
private int saldo;
Konto spar;
spar=new Konto( ”Marie Svensson” , ”451215-2345”);
spar.sättin( 5000 );
spar.uttag( 3000 );
int svar=
}
public Konto(String namn, String nr ){
kundnamn = namn;
kontonummer=nr;
saldo=0;
0
}
public void sättin (int belopp){
}
}
saldo=saldo+belopp;
5000
private String kundnamn ”Marie Svensson”
private String kontonummer
public int uttag (int belopp){
saldo=saldo-belopp
2000
retrun saldo;
}}
private int saldo 0
spar
2000
”451215-2345”
//----------------------------------------------------------------// Deposits the specified amount into the account. Returns the
// new balance.
//-----------------------------------------------------------------
public double deposit (double amount)
{
balance = balance + amount;
return balance;
}
//----------------------------------------------------------------// Withdraws the specified amount from the account and applies
// the fee. Returns the new balance.
//-----------------------------------------------------------------
public double withdraw (double amount, double fee)
{
balance = balance - amount - fee;
return balance;
}
public class TransactionsProgram
{
//----------------------------------------------------------------// Creates some bank accounts and requests various services.
//----------------------------------------------------------------public static void main (String[] args)
{
Account acct1 = new Account ("Ted Murphy", 72354, 102.56);
Account acct2 = new Account ("Jane Smith", 69713, 40.00);
Account acct3 = new Account ("Edward Demsey", 93757, 759.32);
// metod anrop
acct1.deposit (25.85);
double smithBalance = acct2.deposit (500.00);
System.out.println ("Smith balance after deposit: " +
smithBalance);
System.out.println ("Murphy balance after withdrawal: " +
acct1.withdraw (430.75, 1.50));
}}
The toString Method
• All classes that represent objects should define a toString
method
• The toString method returns a character string that
represents the object in some way
• It is called automatically when an object is concatenated to a
string or when it is passed to the println method
public String toString ()
{
NumberFormat fmt = NumberFormat.getCurrencyInstance();
return (acctNumber + "\t" + name + "\t" + fmt.format(balance));
}
Data Scope
• The scope of data is the area in a program in
which that data can be referenced (used)
• Data declared at the class level can be referenced
by all methods in that class
• Data declared within a method can be used only in
that method
• Data declared within a method is called local data
UML Diagrams
• UML stands for the Unified Modeling Language
• UML diagrams show relationships among classes
and objects
• A UML class diagram consists of one or more
classes, each with sections for the class name,
attributes (data), and operations (methods)
Encapsulation
• We can take one of two views of an object:
 internal - the details of the variables and methods of the
class that defines it
 external - the services that an object provides and how
the object interacts with the rest of the system
• From the external view, an object is an
encapsulated entity, providing a set of specific
services
• These services define the interface to the object
Encapsulation
• An encapsulated object can be thought of as a
black box -- its inner workings are hidden from the
client
• The client invokes the interface methods of the
object, which manages the instance data
Client
Methods
Data
Visibility Modifiers
• In Java, we accomplish encapsulation through the
appropriate use of visibility modifiers
• A modifier is a Java reserved word that specifies
particular characteristics of a method or data
• We've used the final modifier to define constants
• Java has three visibility modifiers: public,
protected, and private
• The protected modifier involves inheritance,
which we will discuss later
Visibility Modifiers
Variables
Methods
public
private
Violate
encapsulation
Enforce
encapsulation
Provide services
to clients
Support other
methods in the
class
Accessors and Mutators
• Because instance data is private, a class usually
provides services to access and modify data
values
• An accessor method returns the current value of a
variable
• A mutator method changes the value of a variable
• The names of accessor and mutator methods take
the form getX and setX, respectively, where X is
the name of the value
• They are sometimes called “getters” and “setters”
Method Declarations
• Let’s now examine method declarations in more
detail
• A method declaration specifies the code that will
be executed when the method is invoked (called)
• When a method is invoked, the flow of control
jumps to the method and executes its code
• When complete, the flow returns to the place
where the method was called and continues
• The invocation may or may not return a value,
depending on how the method is defined
Method Control Flow
• If the called method is in the same class, only the
method name is needed
compute
myMethod();
myMethod
Method Control Flow
• The called method is often part of another class or
object
main
obj.doIt();
doIt
helpMe();
helpMe
Method Header
• A method declaration begins with a method header
char calc (int num1, int num2, String message)
method
name
return
type
parameter list
The parameter list specifies the type
and name of each parameter
The name of a parameter in the method
declaration is called a formal parameter
Method Body
• The method header is followed by the method
body
char calc (int num1, int num2, String message)
{
int sum = num1 + num2;
char result = message.charAt (sum);
return result;
}
The return expression
must be consistent with
the return type
sum and result
are local data
They are created
each time the
method is called, and
are destroyed when
it finishes executing
Parameters
• When a method is called, the actual parameters in
the invocation are copied into the formal
parameters in the method header
ch = obj.calc (25, count, "Hello");
char calc (int num1, int num2, String message)
{
int sum = num1 + num2;
char result = message.charAt (sum);
return result;
}
Local Data
• As we’ve seen, local variables can be declared
inside a method
• The formal parameters of a method create
automatic local variables when the method is
invoked
• When the method finishes, all local variables are
destroyed (including the formal parameters)
• Keep in mind that instance variables, declared at
the class level, exists as long as the object exists
Graphical User Interfaces
• A Graphical User Interface (GUI) in Java is created
with at least three kinds of objects:
 components
 events
 listeners
• We've previously discussed components, which
are objects that represent screen elements
 labels, buttons, text fields, menus, etc.
• Some components are containers that hold and
organize other components
 frames, panels, applets, dialog boxes
Events
• An event is an object that represents some activity
to which we may want to respond
• For example, we may want our program to perform
some action when the following occurs:






the mouse is moved
the mouse is dragged
a mouse button is clicked
a graphical button is clicked
a keyboard key is pressed
a timer expires
• Events often correspond to user actions, but not
always
Events and Listeners
• The Java standard class library contains several
classes that represent typical events
• Components, such as a graphical button, generate
(or fire) an event when it occurs
• A listener object "waits" for an event to occur and
responds accordingly
• We can design listener objects to take whatever
actions are appropriate when an event occurs
Events and Listeners
Event
Component
Listener
A component object
may generate an event
A corresponding listener
object is designed to
respond to the event
When the event occurs, the component calls
the appropriate method of the listener,
passing an object that describes the event
Push Counter Example
• The PushCounterPanel constructor:
 instantiates the ButtonListener object
 establishes the relationship between the button and the
listener by the call to addActionListener
• When the user presses the button, the button
component creates an ActionEvent object and
calls the actionPerformed method of the listener
• The actionPerformed method increments the
counter and resets the text of the label
Text Fields
• Let's look at another GUI example that uses
another type of component
• A text field allows the user to enter one line of
input
• If the cursor is in the text field, the text field
component generates an action event when the
enter key is pressed
• See Fahrenheit.java (page 190)
• See FahrenheitPanel.java (page 191)
Fahrenheit Example
• Like the PushCounter example, the GUI is set up
in a separate panel class
• The TempListener inner class defines the listener
for the action event generated by the text field
• The FahrenheitPanel constructor instantiates
the listener and adds it to the text field
• When the user types a temperature and presses
enter, the text field generates the action event and
calls the actionPerformed method of the listener
• The actionPerformed method computes the
conversion and updates the result label
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