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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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class definitions
instance data
encapsulation and Java modifiers
method declaration and parameter passing
constructors
graphical objects
events and listeners
buttons and text fields
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4-2
Outline
Structure of Class Definitions
Encapsulation
Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields
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4-3
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
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4-4
Classes and Objects
• Recall from our overview of objects in Chapter 1
that an object has state and behavior
• Consider a “counter” like those operated by
museum attendants
 It’s state can be defined as the number counted so far
 It’s primary behavior is that it can be incremented
• We can represent a counter in software by
designing a class called Counter that models this
state and behavior
 The class serves as the blueprint for a counter object
• We can then instantiate as many counter objects
as we need for any particular program
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4-5
Classes
• A class can contain data declarations and method
declarations
int size, weight;
char category;
Data declarations
“Instance data,” or
“Instance variables”
Method declarations
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4-6
Classes
• The values of the data define the state of an object
created from the class
• The functionality of the methods define the
behaviors of the object
• For our Counter class, we might declare an
integer that represents the current value of the
count
• One of the methods increments that count by 1
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4-7
Classes
• Let’s first consider how we might use the Counter
class inside the main method of another class:
Counter c = new Counter();
c.count = 0;
c.increment();
System.out.println(c.count);
• Design Counter so that this gives 1 as output
• Warning: We won’t start by writing this class
“correctly”; let’s start simply, and work up to
what’s right gradually
• So…how do we write Counter so that it can do
this?
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4-8
Class Definition: First Try
• Before writing down the class definition, here’s a
picture:
int count;
void increment()
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Data declarations
“Instance data,” or
“Instance variables”
Method declaration
(only one for now)
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Class Definition: First Try
• Now let’s write down the class definition:
public class Counter
{
public int count;
public void increment()
{
count = count + 1;
}
Instance variable
declaration
Method header
}
• When we invoke the increment() method, it
changes the state of the Counter accordingly
• Let’s code this and run it.
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4-10
Information Hiding
• There is a problem with doing things like this:
c.count = 0;
• For example, suppose count is 2 and the user of
the class tries to do this:
c.count = 7;
which makes no sense for a counter object!
• For this reason, we want to prohibit users of the
class from directly accessing count
• This is an important concept in OOP called
information hiding or encapsulation
• We hide the state of an object, and change it only
via methods
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4-11
Class Definition: Second Try
• State information is hidden inside objects by
making the variables private, not public
• Private instance data cannot be directly accessed
outside the class
• That is, if we made count private, trying to get the
value as in c.count is syntactically illegal
• So let’s make count private!
public class Counter
{
private int count;
//(the rest is as before)
}
Information hiding
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4-12
Class Definition: Access Method
• Well, darn it, there’s still another problem: How do
we “see” the value of count?
• Answer: Write a method that returns it!
int count;
Data declarations
“Instance data,” or
“Instance variables”
void increment()
Method declarations
int getCount()
Method to return the
value in count
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4-13
Class Definition: Access Method
• Well, darn it, there’s still another problem: How do
we “see” the value of count?
• Answer: Write a method that returns it!
• That method is quite simple to write:
public int getCount ()
{
return count;
}
• And you can easily invoke it:
Counter c = new Counter();
System.out.println(“count = “ + c.getCount());
c.increment();
System.out.println(“count = “ + c.getCount());
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4-14
Class Definition: Mutator Method
• No, we’re not done yet
• A counter ought to be capable of being reset to 0
• But since count is private, we can’t set it to 0
directly
• So (again) we write a method to set it to 0
• Add the following method to the Counter class
definition:
public void reset ()
{
count = 0;
}
• Easy to invoke this too:
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c.reset();
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Class Definition: Constructors
• Yes, we’re almost done
• When a counter is created, the count should be 0
• As it happens, it is already 0, because int-type instance
variables are automatically initialized to 0
• But we would like to guarantee that with a constructor that
we write ourselves
• For other classes, constructors will be more essential
• For the Counter class, it should look like this:
public Counter ()
{
count = 0;
}
• See Counter.java and CountDemo.java
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4-16
Another Example
• Let’s now consider the slightly more sophisticated
class defined in the text
• Consider a six-sided die (singular of dice)
 It’s state can be defined as which face is showing
 It’s primary behavior is that it can be rolled
• We can represent a die in software by designing a
class called Die that models this state and
behavior
 The class serves as the blueprint for a die object
• We can then instantiate as many die objects as we
need for any particular program
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4-17
Classes
• The picture is similar to before, but we now have
different instance variables and methods:
int faceValue;
final int MAX;
Data declarations
int roll()
int
getFaceValue()
Method declarations
(not all of them)
String
toString()
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4-18
Classes
• The values of the data define the state of an object
created from the class
• The functionality of the methods define the
behaviors of the object
• For our Die class, we might declare an integer that
represents the current value showing on the face
• One of the methods “rolls” the die by setting that
value to a random number between one and six
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4-19
Classes
• We’ll want to design the Die class with other data
and methods to make it a versatile and reusable
resource
• Any given program will not necessarily use all
aspects of a given class
• See RollingDice.java (page 157)
• See Die.java (page 158)
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4-20
The Die Class
• The Die class contains two data values
 a constant MAX that represents the maximum face value
 an integer faceValue that represents the current face
value
• The roll method uses the random method of the
Math class to determine a new face value
• There are also methods to explicitly set and
retrieve the current face value at any time
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4-21
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
• We could have defined one for Counter:
public String toString ()
{
return Integer.toString(count);
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}
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Constructors
• As mentioned previously, a constructor is a
special method that is used to set up an object
when it is initially created
• A constructor has the same name as the class,
and NO return type
• It is the only type of method like this
• The Die constructor is used to set the initial face
value of each new die object to one
• We examine constructors in more detail later in
this chapter
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4-23
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
• In the Die class, the variable result is declared
inside the toString() method -- it is local to that
method and cannot be referenced anywhere else
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Instance Data
• The faceValue variable in the Die class is called
instance data because each instance (object) that
is created has its own version of it
• A class declares the type of the data, but it does
not reserve any memory space for it
• Every time a Die object is created, a new
faceValue variable is created as well
• The objects of a class share the method
definitions, but each object has its own data space
• That's the only way two objects can have different
states
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4-25
Instance Data
• We can depict the two Die objects from the
RollingDice program as follows:
die1
faceValue
5
die2
faceValue
2
Each object maintains its own faceValue
variable, and thus its own state
Invoking roll() for die1 (as in die1.roll())
has no effect on die2
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4-26
A Convenient Notation: UML
• 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)
• Lines between classes represent associations
• A dotted arrow shows that one class uses the
other (calls its methods)
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4-27
UML Class Diagrams
• A UML class diagram for the CountDemo
program:
CountDemo
Counter
count : int
main (args : String[]) : void
increment() : void
reset () : void
getCount() : int
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4-28
UML Class Diagrams
• A UML class diagram for the RollingDice
program:
RollingDice
Die
faceValue : int
main (args : String[]) : void
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roll() : int
setFaceValue (int value) : void
getFaceValue() : int
toString() : String
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UML Class Diagrams
• UML is “unified” because the notation applies to
any object-oriented programming language
• You don’t even have to know a class definition to
use the notation, and that’s part of the point
StringMutation
main (args : String[]) : void
String
toUppercase() : String
replace (char, char) : String
substring(int, int) : String
length() : int
• UML allows us to think about object/class
relationships abstractly
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4-30
Outline
Structure of Class Definitions
Encapsulation
Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields
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4-31
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 other objects
• From the external view, an object is an
encapsulated entity, providing a set of specific
services, whose structure we can’t “see”
• These services comprise the interface to the
object, that dictates how other objects interact
with it
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4-32
Encapsulation
• One object (called the client) may use another object for the
services it provides (example: CountDemo is a client of a
Counter object)
• The client of an object may request its services (call its
methods), but it should not have to be aware of how those
services are accomplished
• Any changes to the object's state (its variables) should be
made by that object's methods
• We should make it difficult, if not impossible, for a client to
access an object’s variables directly
• Often “should” becomes “must”; e.g., CountDemo could
violate the rules of a Counter by incorrectly setting the
instance variable count, as we saw earlier
• That is, an object must be self-governing
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4-33
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
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4-34
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
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4-35
Visibility Modifiers
• Members of a class that are declared with public
visibility can be referenced anywhere
• Members of a class that are declared with private
visibility can be referenced only within that class
• Members declared without a visibility modifier
have default visibility and can be referenced by
any class in the same package
• An overview of all Java modifiers is presented in
Appendix E
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4-36
Visibility Modifiers
• Public variables violate encapsulation because
they allow the client to “reach in” and modify the
values directly
• Therefore instance variables should not be
declared with public visibility
• It is acceptable to give a constant public visibility,
which allows it to be used outside of the class
• Public constants do not violate encapsulation
because, although the client can access it, its
value cannot be changed
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4-37
Visibility Modifiers
• Methods that provide the object's services are
declared with public visibility so that they can be
invoked by clients
• Public methods are also called service methods
• A method created simply to assist a service
method is called a support or helper method
• Since a support method is not intended to be
called by a client, it should not be declared with
public visibility
• Note that most classes are declared as public
since they are intended to be used elsewhere
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4-38
Support Method: Example
• It’s a bit contrived, but suppose in Die we want to design a
separate method to change the state of the object when we
roll the die, and invoke it in the method roll()
• It might be done like this:
public int roll ()
{
//invoke support method:
rollTheDie();
return faceValue;
}
//declare support method:
private void rollTheDie ()
{
faceValue = (int)(Math.random()*MAX) + 1;
}
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4-39
Visibility Modifiers
Variables
Methods
public
private
Violate
encapsulation
Enforce
encapsulation
Provide services
to clients
Support other
methods in the
class
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4-40
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”
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4-41
Mutator Restrictions
• The use of mutators gives the class designer the
ability to restrict a client’s options to modify an
object’s state
• A mutator is often designed so that the values of
variables can be set only within particular limits
• For example, the setFaceValue mutator of the
Die class should have restricted the value to the
valid range (1 to MAX)
• We’ll see in Chapter 5 how such restrictions can
be implemented
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4-42
Outline
Anatomy of a Class
Encapsulation
Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields
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4-43
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
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4-44
Method Control Flow
• If the called method is in the same class, only the
method name is needed. Recalling slide 39:
roll()
rollTheDie()
rollTheDie();
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4-45
Method Control Flow
• The called method is often part of another class or
object. Again recalling slide 39:
main
die1.roll();
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roll()
rollTheDie()
rollTheDie();
4-46
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
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4-47
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
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sum and result
are local data
They are created
each time the
method is called, and
are destroyed when
it finishes executing
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The return Statement
• The return type of a method indicates the type of
value that the method sends back to the calling
location
• A method that does not return a value has a void
return type
• A return statement specifies the value that will be
returned
return expression;
• Its expression must conform to the return type
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4-49
Parameters
• When a method is called, the actual parameters or
arguments 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;
}
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4-50
Local Data
• As we’ve seen, local variables can be declared
inside a method
• Formal parameters of a method are created as
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, exist as long as the object exists
• Do not confuse instance variables (created when
an object is created) with local variables and
parameters (created when a method is invoked)
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4-51
Bank Account Example
• Let’s look at another example that demonstrates
the implementation details of classes and methods
• We’ll represent a bank account by a class named
Account
• It s state can include the account number, the
current balance, and the name of the owner
• An account’s behaviors (or services) include
deposits and withdrawals, and adding interest
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4-52
Driver Programs
• A driver program drives the use of other, more
interesting parts of a program
• Driver programs are often used to test other parts
of the software
• The Transactions class contains a main method
that drives the use of the Account class,
exercising its services
• See Transactions.java (page 172)
• See Account.java (page 173)
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4-53
Bank Account Example
acct1
acctNumber
72354
balance 102.56
“Ted Murphy”
name
acct2
acctNumber
69713
balance
40.00
name
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“Jane Smith”
4-54
Bank Account Example
• There are some improvements that can be made to
the Account class
• Formal getters and setters could have been
defined for all data
• The design of some methods could also be more
robust, such as verifying that the amount
parameter to the withdraw method is positive
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4-55
Constructors Revisited
• Note that a constructor has no return type
specified in the method header, not even void
• A common error is to put a return type on a
constructor, which makes it a “regular” method
that happens to have the same name as the class
• The programmer does not have to define a
constructor for a class
• Each class has a default constructor that accepts
no parameters, and that initializes variables to
“obvious” default values (e.g., 0 for int)
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4-56
Problems with Existing Examples
• In a Die object, the setFaceValue() method can set
faceValue to anything, even a value not between 1 and MAX
• In an Account object, the withdraw() method can cause a
negative balance
• We need some way of preventing clients from violating the
internal laws of these objects (a Die should only have values
from 1 to MAX; an Account should have a non-negative
balance)
• We will find out how to do this in Chapter 5
• Meanwhile, there is quite a bit we can do just with objects
and graphics.
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4-57
Outline
Anatomy of a Class
Encapsulation
Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields
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4-58
Graphical Objects
• Some objects contain information that determines
how the object should be represented visually
• Most GUI components are graphical objects
• We can have some effect on how components get
drawn
• We did this in Chapter 2 when we defined the
paint method of an applet
• Let's look at some other examples of graphical
objects
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4-59
Smiling Face Example
• The SmilingFace program draws a face by
defining the paintComponent method of a panel
• See SmilingFace.java (page 177)
• See SmilingFacePanel.java (page 178)
• The main method of the SmilingFace class
instantiates a SmilingFacePanel and displays it
• The SmilingFacePanel class is derived from the
JPanel class using inheritance
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4-60
Smiling Face Example
• Every Swing component has a paintComponent
method
• The paintComponent method accepts a Graphics
object that represents the graphics context for the
panel
• We define the paintComponent method to draw
the face with appropriate calls to the Graphics
methods
• Note the difference between drawing on a panel
and adding other GUI components to a panel
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4-61
Splat Example
• The Splat example is structured a bit differently
• It draws a set of colored circles on a panel, but
each circle is represented as a separate object that
maintains its own graphical information
• The paintComponent method of the panel "asks"
each circle to draw itself
• See Splat.java (page 180)
• See SplatPanel.java (page 181)
• See Circle.java (page 182)
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4-62
Outline
Anatomy of a Class
Encapsulation
Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields
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4-63
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, panels, buttons, text fields, menus, etc.
• Some components are containers that hold and
organize other components
 frames, panels, applets, dialog boxes
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4-64
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
• Events often correspond to user actions, but not
always
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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 when the system (not the programmer!)
invokes an appropriate method of that object
• We (the programmer) design the method bodies of
listener objects to take the actions we want to take
when an event occurs
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Events and Listeners
Event
Component
Listener is “attached” to (in Java
terminology, “registered with”)
the component
A component object
may generate an event
Listener
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
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4-67
GUI Development
• Generally we use components and events that are
predefined by classes in the Java class library
• Therefore, to create a Java program that uses a
GUI we must:
 instantiate and set up the necessary components
(establish the “look” of the GUI)
 implement listener classes for any events we care about,
and register instantiations of those classes with the
components (establish the “feel” of the GUI)
• Let's now explore some new components and see
how this all comes together
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4-68
Outline
Anatomy of a Class
Encapsulation
Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields
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4-69
Buttons
• A push button (or just plain “button”) is a
component that allows the user to initiate an
action by pressing a graphical button using the
mouse
• A button is defined by the JButton class
• It generates an action event
• The PushCounter example displays a button that
increments a counter each time it is pushed
• See PushCounter.java (page 186)
• See PushCounterPanel.java (page 187)
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4-70
Push Counter Example
• The components of the GUI are the button, a label
to display the counter, a panel to organize the
components, and the main frame
• The PushCounterPanel class represents the
panel used to display the button and label
• The PushCounterPanel class is derived from
JPanel using inheritance
• The constructor of PushCounterPanel sets up the
elements of the GUI and initializes the counter to
zero
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Push Counter Example
• The ButtonListener class is the listener for the
action event generated by the button
• It is implemented as an inner class, which means it
is defined within the body of another class
• That facilitates the communication between the
listener and the GUI components - inner class
objects can access the instance variables
• Inner classes should only be used in situations
where there is an intimate relationship between the
two classes and the inner class is not needed in
any other context
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Push Counter Example
• Listener classes are written by implementing a
listener interface
• The ButtonListener class implements the
ActionListener interface
• An interface is a list of methods that the
implementing class must define
• The only method in the ActionListener interface
is the actionPerformed method
• The Java class library contains interfaces for
many types of events
• See Chapter 6 for more about interfaces
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Push Counter Example
• The PushCounterPanel constructor:
 instantiates the ButtonListener object
 registers the listener with the button 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
• Pushing the button invokes actionPerformed,
not the programmer!
• The actionPerformed method increments the
counter and resets the text of the label
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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)
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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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Adding Machine Example
• Let’s do something more useful
• See Adder.java and AdderPanel.java
• In AdderPanel there are two buttons (one to add,
one to reset) and one text field
• The buttons are both placed in a sub-panel, and so
is the text field
• In this example we explicitly use layout managers
(BorderLayout and FlowLayout) - see Chapter 6
• Each button needs its own listener, hence there is
an inner class for each button
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Summary
• Chapter 4 focused on:




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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
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