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Chapter 3 - Introduction to Java Applets
Outline
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
Introduction
Sample Applets from the Java 2 Software Development Kit
3.2.1
The TicTacToe Applet
3.2.2
The DrawTest Applet
3.2.3
The Java2D Applet
A Simple Java Applet: Drawing a String
3.3.1
Compiling and Executing WelcomeApplet
Two More Simple Applets: Drawing Strings and Lines
Another Java Applet: Adding Floating-Point Numbers
Viewing Applets in a Web Browser
3.6.1
Viewing Applets in Netscape Navigator 6
3.6.2
Viewing Applets in Other Browsers Using the Java
Plug-In
Java Applet Internet and World Wide Web Resources
(Optional Case Study) Thinking About Objects: Identifying
the Classes in a Problem Statement
1
Introduction
• Applet
– Program that runs in
• appletviewer (test utility for applets)
• Web browser (IE, Communicator)
– Executes when HTML (Hypertext Markup Language) document
containing applet is opened and downloaded
– Applications run in command windows
2
Sample Applets from the Java 2 SDK
• Sample Applets
– Provided in Java 2 Software Development Kit (J2SDK)
– Source code included (.java files)
• Study and mimic source code to learn new features
• All programmers begin by mimicking existing programs
– Located in demo directory of J2SDK install
– Can download demos and J2SDK from
java.sun.com/products/jdk/1.3/
• Running applets
– In command prompt, change to demo subdirectory of applet
cd c:\jdk1.3\demo\applets
cd appletDirectoryName
– There will be an HTML file used to execute applet
– Type appletviewer example1.html
• appletviewer loads the html file specified as its command-line argument
• From the HTML file, determines which applet to load (more section 3.3)
– Applet will run, Reload and Quit commands under Applet menu
• Upcoming program: Create an applet to display
"Welcome to Java Programming!" //Fig 3.6
3
The TicTacToe Applet
• You start as player "X"
Fig. 3.2
Sample execution of the TicTacToe applet.
4
The DrawTest Applet
Fig. 3.4
Sample execution of applet DrawTest.
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The Java2D Applet
• Demonstrates 2D drawing capabilities built into Java2 (Fig 3.5)
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// Fig. 3.6: WelcomeApplet.java
// A first applet in Java.
// Java core packages
import java.awt.Graphics;
import allows us to use predefined classes
(allowing us to use applets and graphics, in this case).
// Java extension packages
import javax.swing.JApplet;
Outline
// import class Graphics
Java applet
// import class JApplet
public class WelcomeApplet extends JApplet {
extends allows us to inherit the
// draw text on applet’s background
capabilities of class JApplet.
public void paint( Graphics g )
{
// call inherited version of method paint
super.paint( g );
Method paint is guaranteed
to be called in all
applets. Its first line must be defined as above.
y-coordinate 25
// draw a String at x-coordinate 25 and
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
// end method paint
// end class WelcomeApplet
<html>
<applet code = "WelcomeApplets.class" width = "300" height = "40">
</applet>
</html>
Program Output
2002 Prentice Hall.
All rights reserved.
3.3
A Simple Java Applet: Drawing a String
5
import java.awt.Graphics;
// import class Graphics
8
import javax.swing.JApplet;
// import class JApplet
• import the Graphics class (package java.awt) to draw graphics
– Can draw lines, rectangles, ovals, strings of characters
– Applets have at least one class definition (like applications)
• Rarely create classes from scratch Use Inheritance and existing class
definitions for new class.
10
public class WelcomeApplet extends JApplet {
– Begins class definition for class WelcomeApplet
– extends Japplet //Inherit methods (over 200), do not have to
define them all
• Indicates class to inherit from (JApplet)
– JApplet : superclass (base class)
– WelcomeApplet : subclass (derived class)
• WelcomeApplet now has methods and data of Japplet
• Do not need to know every detail of class Japplet
– Keyword public required. File can only have one public class
public class name must be file name
8
A Simple Java Applet: Drawing a String
13
public void paint( Graphics g )
– Our class inherits method paint from JApplet
• By default, paint has empty body. We override (redefine) paint in our class.
– Methods paint, init, and start
• Guaranteed to be called automatically
• Our applet gets "free" version of these by inheriting from JApplet
– Free versions have empty body (do nothing)
– Every applet does not need all three methods. Override the ones you need
– Applet container “draws itself” by calling method paint
– Method paint
• Draws graphics on screen
• void indicates paint returns nothing when finishes task
• Parenthesis define parameter list - where methods receive data to perform tasks
– Normally, data passed by programmer, as in
JOptionPane.showMessageDialog
• paint gets parameters automatically
– Graphics object used by paint
• Mimic paint's first line
9
3.3
A Simple Java Applet: Drawing a String
19
g.drawString( "Welcome to Java Programming!", 25, 25 );
– Body of paint
• Method drawString (of class Graphics)
• Called using Graphics object g and dot operator (.)
• Method name, then parenthesis with arguments
– First argument: String to draw
– Second: x coordinate (in pixels) location
– Third: y coordinate (in pixels) location
– Java coordinate system
• Measured in pixels (picture elements)
• Upper left is (0,0)
– Running the applet
• javac WelcomeApplet.java
• If no errors, bytecodes stored in WelcomeApplet.class
– Create an HTML file
– To execute an applet: appletviewer WelcomeApplet.html
• Or the browser should load and execute
10
Two More Simple Applets: Drawing Strings and Lines
• More applets
– First example
• Display two lines of text
• Use drawString to simulate a new line with two drawString
statements
– Second example
• Method g.drawLine(x1, y1, x2, y2 )
– Draws a line from (x1, y1) to (x2, y2)
– Remember that (0, 0) is upper left
• Use drawLine to draw a line beneath and above a string
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// Fig. 3.8: WelcomeApplet2.java
// Displaying multiple strings in an applet.
// Java core packages
import java.awt.Graphics;
// import class Graphics
// Java extension packages
import javax.swing.JApplet;
// import class JApplet
public class WelcomeApplet2 extends JApplet {
// draw text on applet’s background
public void paint( Graphics g )
{
// call inherited version of method paint
super.paint( g );
// draw two Strings at different locations
g.drawString( "Welcome to", 25, 25 );
g.drawString( "Java Programming!", 25, 40 );
}
}
// end method paint
// end class WelcomeApplet2
12
Outline
1. import
2. Class
WelcomeApplet2
(extends
JApplet)
3. paint
3.1 drawString
The two drawString statements simulate
3.2 drawString
a newline. In fact, the concept of lines
on ofsame x
text does not exist when drawing strings.
coordinate, but
<html>
<applet code = "WelcomeApplet2.class" width = "300" height = "60">
</applet>
</html>
15 pixels down
2002 Prentice Hall.
All rights reserved.
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// Fig. 3.10: WelcomeLines.java
// Displaying text and lines
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Outline
// Java core packages
import java.awt.Graphics;
// import class Graphics
// Java extension packages
import javax.swing.JApplet;
// import class JApplet
public class WelcomeLines extends JApplet {
// draw lines and a string on applet’s background
public void paint( Graphics g )
{
// call inherited version of method paint
super.paint( g );
// draw horizontal line from (15, 10) to (210, 10)
g.drawLine( 15, 10, 210, 10 );
// draw horizontal line from (15, 30) to (210, 30)
g.drawLine( 15, 30, 210, 30 );
// draw String between lines at location (25, 25)
g.drawString( "Welcome to Java Programming!", 25, 25 );
}
}
// end method paint
// end class WelcomeLines
WelcomeLines.java
2. Class
WelcomeLines
(extends
JApplet)
3. paint
3.1 drawLine
3.2 drawLine
3.3 drawString
Draw horizontal lines with
drawLine (endpoints have same
y coordinate).
<html>
<applet code = "WelcomeLines.class" width = "300" height = "40">
</applet>
</html>
Program Output
2002 Prentice Hall.
All rights reserved.
Another Java Applet: Adding Floating-Point Numbers
• Next applet
– Mimics application for adding two integers (Fig 2.9)
• This time, use floating point numbers (numbers with a decimal point)
– Using primitive data types
• Double – double precision floating-point numbers
• Float – single precision floating-point numbers
– Show program, then discuss
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// Fig. 3.12: AdditionApplet.java
// Adding two
two floating-point
floating-point numbers
numbers.
import java.awt.Graphics;
// import class Graphics
// Java core packages
import java.awt.Graphics;
// import class Graphics
public
AdditionApplet
// Javaclass
extension
packages extends JApplet {
double
sum; // sum of the//values
the user
import
javax.swing.*;
importentered
packagebyjavax.swing
Outline
AdditionApplet.java
1. import
* allows any class in the the
2. package
Class
public
class
extends JApplet {
public
voidAdditionApplet
init()
AdditionApplet
to be used. * indicates all classes
in
double sum; // sum of values entered by user
{
(extends
javax.swing
are available
firstNumber,
// first string
by user
// String
initialize
applet by obtaining
values entered
from user
JApplet)
secondNumber;
// second string entered by user
public void
init()
{ double number1,
// first number to add
String number2;
firstNumber;
// second
first string
//
number entered
to add by user
3. Instance variable
String secondNumber; // second string entered by user
double number1;
// first number to add
// read in first number from user
init
double number2;
// second number to addInstance variable sum 4.
may
be used anywhere
firstNumber =
in the class, even in other methods.
// JOptionPane.showInputDialog(
obtain first number from user
4.1 Declare variables
"Enter =first
floating-point value" );
firstNumber
JOptionPane.showInputDialog(
Data type double can store floating point
"Enter first floating-point value" );
numbers with double precision
float with
4.2
showInputDialog
// read in second number from user
// obtain second number from user
single precision.
secondNumber =
secondNumber = JOptionPane.showInputDialog(
JOptionPane.showInputDialog(
"Enter second floating-point value" );
"Enter second floating-point value" );
// convert numbers from type String to type double
number1 = Double.parseDouble( firstNumber );
number2
= Double.parseDouble(
secondNumber
//
convert
numbers from type String
to type );
double
4.3 parseDouble
2002 Prentice Hall.
All rights reserved.
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}
}
Outline
//
// add
add the
numbers
numbers
sum
=
number1
number2;
sum = number1 ++ number2;
// draw results in a rectangle on applet’s background
paint( Graphics
Graphics gg ))
public void paint(
{
{
// call inherited version of method paint
//
draw the results
with g.drawString
super.paint(
g );
g.drawRect( 15, 10, 270, 20 );
// draw rectangle
(15,
g.drawString(
"The starting
sum is " from
+ sum,
25,10)
25 that
); is 270
// pixels wide and 20 pixels tall
} g.drawRect( 15, 10, 270, 20 );
5. Draw applet
contents
5.1 Draw a rectangle
5.2 Draw the results
}
// draw results as a String at (25, 25)
<html> g.drawString( "The sum is " + sum, 25, 25 );
<applet code="AdditionApplet.class" width=300 height=50>
} // end method paint
</applet>
drawRect takes the upper
</html>
} // end class AdditionApplet
left coordinate, width,
and height of the rectangle to draw.
<html>
<applet code = "WelcomeLines.class" width = "300" height = "40">
</applet>
</html>
HTML file
2002 Prentice Hall.
All rights reserved.
17
Outline
Program Output
2002 Prentice Hall.
All rights reserved.
Another Java Applet: Adding Floating-Point Numbers
– Lines 1-2: Comments
5
import java.awt.Graphics;
– Line 5: imports class Graphics
• import not needed if use full package and class name
public void paint ( java.awt.Graphics g )
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import javax.swing.*;
– Line 8: specify entire javax.swing package
• * indicates all classes in javax.swing are available not loaded.
– Includes JApplet and JOptionPane
– Use JOptionPane instead of javax.swing.JOptionPane
• * does not not load all classes. Compiler only loads classes it uses
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double sum;
// sum of values entered by user
– Instance variable declaration
• Each object of class gets own copy of the instance variable
• Declared in body of class, but not inside methods
– Variables declared in methods are local variables
– Can only be used in body of method
• Instance variables can be used anywhere in class. default value (0.0)
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Another Java Applet: Adding Floating-Point Numbers
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double sum;
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// sum of values entered by user
– Primitive data type double: To store floating point (decimal) numbers
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public void init()
– Method init (Returns nothing (void), takes no arguments)
• Normally initializes instance variables and applet class
• Guaranteed to be first method called in applet
• First line must always appear as above
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{
– Begins body of method init
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String
String
double
double
firstNumber;
secondNumber;
number1;
number2;
//
//
//
//
first string entered by user
second string entered by user
first number to add
second number to add
– Two types of variables
• Reference variables (called references)
– Refer to objects (contain location in memory)
• Objects defined in a class definition. Can have multiple data and methods
– paint receives a reference called g to a Graphics object
– Reference used to call methods on the Graphics object
• Primitive data types (called variables): Contain one piece of data
Another Java Applet: Adding Floating-Point Numbers
33
}
– Ends method init
• appletviewer (or browser) calls inherited method start
• start usually used with multithreading
– Advanced concept, in Chapter 15
– We do not define it, so empty definition in JApplet used
• Next, method paint called
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g.drawString( "The sum is " + sum, 25, 25 );
Sends drawString message (calls method) to Graphics object using
reference g
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Viewing Applets in a Web Browser
• Applets can execute on Java-enabled browsers
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– Many different browser version support different Java versions
• Some support for Java 1.0, many for Java 1.1 inconsistently
– For browsers that do not support Java, download the Java Plug-in
(section 3.6.2) at java.sun.com/products/plugin. The plug-in bypasses a
browser’s Java support and uses a complete version of the Java 2
Runtime Environment
– HTML files containing an applet require converting the <applet> and
</applet> tags into tags that load the Java Plug-in and execute the applet.
Sun provides a converter.)
• Netscape Navigator 6 supports Java 2
–
–
–
–
Default installation component
able to load applet HTML into browser and execute applet
Download browser at www.netscape.com
After installing, open applet HTML file using Open File… menu item in
File menu
3.6.1 Viewing Applets in Netscape Navigator 6
Fig. 3.14 Applet of Fig. 3.10 executing in Netscape Navigator 6.
applet’s upper-left corner
HTML file loaded into browser
status bar
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Viewing Applets in Other Browsers Using Java Plug-In
• Java Plug-in support from Sun
– Uses Java 2 Runtime Environment (J2RE)
• Can be downloaded and installed dynamically
– Applet HTML file must indicate use of Java Plug-in
• Convert <applet> and </applet> tags to plug-in-loading tags
• Sun provides Java Plug-in 1.3 HTML Converter for conversion
– Download and info at java.sun.com/products/plugin
– Executable in classes subdirectory of converter directory
• Batch file HTMLConverter.bat on Windows
• HTML Converter.sh shell script for Linux/UNIX
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3.6.2 Viewing Applets in Other Browsers Using
the Java Plug-In
• Java Plug-in HTML Converter process
– Select directory containing HTML files to convert
• Click Browse button in Converter to open file chooser to select directory
• Or type in the directory
– Select conversion template to support browsers
• Defaults: Netscape Navigator and Microsoft Internet Explorer
• Use Template File drop-down list
– Click Convert… button to convert
• Might need to download J2RE if not installed
• After conversion, progress and status window pops up
• Able to use applet HTML in supported browser
– (see pictorial figures)
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3.6.2 Viewing Applets in Other Browsers Using
the Java Plug-In
Fig. 3.15 Java Plug-in HTML Converter window.
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3.6.2 Viewing Applets in Other Browsers Using
the Java Plug-In
Fig. 3.16 Selecting the directory containing HTML files to convert.
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3.6.2 Viewing Applets in Other Browsers Using
the Java Plug-In
Fig. 3.17 Selecting the template used to convert the HTML files.
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3.6.2 Viewing Applets in Other Browsers Using
the Java Plug-In
Fig. 3.18 Confirmation dialog after conversion completes
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Java Applet Internet and World Wide Web Resources
• Many Java applet resources available
– Sun site developer.java.sun.com/developer
–
–
–
–
–
–
• Tech support, discussion forums, training, articles, links, etc.
• Registration required
http://java.sun.com/applets/index.html
• Many resources and free applets
• Has demo applets from J2SDK
http://java.sun.com/jdc
• Free registration
• Tech support, training, articles, resources, links
http://www.developer.com
• Java directory page
• Thousands of applets and resources
http://www.gamelan.com
• All-around Java resource
• References, games, downloads, talk to experts...
http://www.jars.com
• (jars - Java Applet Rating Services)Rates applets, top 1, 5 and 25 percent
• View best applets on web
• Another all around resource
http://java.sun.com/j2se/1.3/docs/api/
29
Thinking About Objects
• Java an object-oriented language
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– However, Java has constructs from structured programming
• First seven chapters focus on "conventional" parts of Java
– Introduce object-oriented concepts as encountered
• Object orientation
– Natural way to think about world and writing computer programs
• Object-oriented programming models the real world
–
–
–
–
Attributes - properties of objects: Size, shape, color, weight, etc.
Behaviors - actions that objects can perform: A ball rolls, bounces, inflates, deflates
Inheritance: New classes of objects absorb characteristics of existing classes
Information hiding
• Objects usually do not know how other objects are implemented
• We can drive cars without knowing how every part works internally
• Abstraction: View the big picture,
Think in terms of houses, not bricks
Thinking About Objects
31
• First step in OOD process is identifying classes in a system
–
–
–
–
–
Look for nouns in problem statement; only that perform important duties
Java focuses on nouns (classes): C focuses on verbs and is action oriented
Determine classes by grouping nouns into categories
Class - unit of Java programming
Class is "Blueprint" of objects //Objects are created from the class
• Built-in types (like ints) are variables. User-defined types are objects
– Contain methods: Implement behaviors
– Contain data: Implement attributes
– Classes are reusable: Standardized, interchangeable parts
ClassName
Attributes
Operations
UML Notation for a Class
(Optional Case Study) Thinking About Objects:
Identifying the Classes in a Problem Statement
• Identifying classes in a System
– Nouns of system to implement elevator simulation
Nouns (and noun phrases) in
the problem statement
company
elevator system
graphical user interface (GUI)
office building
elevator
software-simulator application
passenger
elevator car
person
floor (first floor; second floor)
First Floor GUI button
elevator shaft
display
model
bell inside the elevator
floor door
light on that floor
Second Floor GUI button
user of our application
energy
audio
floor button
capacity
elevator music
elevator button
Fig. 3.19
Nouns (and noun phrases) in problem statement.
32
Thinking About Objects
• Not all nouns pertain to model (not highlighted)
– Company and building not part of simulation
– Display, audio, and elevator music pertain to presentation
– GUI, user of application, First and Second Floor buttons
• How user controls model only
–
–
–
–
–
Capacity of elevator only a property
Energy preservation not modeled
Simulation is the system
Elevator and elevator car are same references
Disregard elevator system for now
• Nouns highlighted to be implemented in system
– Elevator button and floor button separate functions
– Capitalize class names
• Each separate word in class name also capitalized
• ElevatorModel, ElevatorShaft, Elevator, Person, Floor,
ElevatorDoor, FloorDoor, ElevatorButton, FloorButton,
Bell, and Light
33
Thinking About Objects
• Using UML to model elevator system
– Class diagrams models classes and relationships
• Model structure/building blocks of system
• Representing class Elevator using UML
Elev ator
– Top rectangle is class name
– Middle contains class’ attributes
– Bottom contains class’ operations
34
Thinking About Objects
35
• Second step in OOD process is to identify relationships or
associations between classes
– Numbers near lines indicate multiplicity; how many objects of a
class participate in the association
– An association can be named on the line
– Associations are directional indicated by
– Aggregation relationship represented by
• Implies whole/part relationship
• Class that has aggregation symbol on its end of an association line is
the whole; class on the other end is the part
• Third step in OOD process is to develop Class Diagram
• Fourth step in OOD process is to develop Object diagram
– Model objects at a specific time in program execution
Thinking About Objects
36
• Class associations using UML
– Elided diagram
• Class attributes and operations ignored
• Class relation among ElevatorShaft, Elevator and FloorButton
Elev atorShaft
1
1
Resets
Sig na ls
arriva l
2
FloorButto n
1
2
Re quests
1
Elev ator
• Solid line is an association, or relationship between classes
• Numbers near lines express multiplicity values
– Indicate how many objects of class participate association
• In diagram, “Requests” gives association and arrow direction of association
• Aggregation relationship: diamond end is “owner” and other end is “part”
– In diagram, elevator shaft “has an” elevator and two floor buttons
Thinking About Objects
– Diagram shows two objects of class FloorButton participate in
association with one object of ElevatorShaft
– FloorButton has two-to-one relationship with ElevatorShaft
Symbol
0
1
m
0..1
m, n
m..n
*
0..*
1..*
Fig. 3.22
Meaning
None.
One.
An integer value.
Zero or one.
m or n
At least m, but not more than n.
Zero or more.
Zero or more
One or more
Multiplicity types.
37
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Thinking About Objects
Fig. 3.23 Class diagram for the elevator model.
C re at es
1
Light
2
Eleva to rMo del
Floo r
1
1
2
1
Turns
on/ off
1
FloorDo or
Walks
ac ross
1
Elev at orShaft
2
1
1
1
O pens
Eleva to rDoor
1
1
Req uest s
1
1
FloorBut to n
2
1
Signals
a rriv al
O p ens
Re set s
1
1
Elev at or
1
1
Signa ls to
m ov e
1 Reset s
1
Presses
1
Presses
1
1
Ele va to rBut t on
1
Rid es
Bell
0. .*
Perso n
Ring s
1
1
1
Thinking About Objects
• The complete class diagram for elevator model
– Several of many and aggregates
• ElevatorModel aggregates one ElevatorShaft and two Floor
• Elevator is aggregation of ElevatorDoor, ElevatorButton and
Bell
– Several of many associations
• Person “presses” buttons
• Person also “rides” Elevator and “walks” across Floor
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Thinking About Objects
Fig. 3.24 Object diagram of an empty building in our elevator model.
: Ele va t o rM o d e l
first Flo o rLig h t : Lig h t
first Flo o r : Flo o r
se c o n d Flo o rLig h t : Lig h t
se c o n d Flo o r : Flo o r
: Ele v a t o rSha ft
f irstFlo o rDo o r : Flo o rDo o r
firstFlo o rBu t to n : Flo o rBu t to n
se c o n d Flo o rDo o r : Flo o rDo o r
se c o n d Flo o rBu t t o n : Flo o rBu t t o n
: Ele va t o rDo o r
: Ele v a t o r
: Be ll
: Ele v a t o rBu t t o n
Thinking About Objects
• Object diagrams
– Model objects (instances of classes) at a specific time in program
execution
– Snapshot of system structure while running
• Information about participation of objects at that time
– Links
• Relationships between objects represented as solid lines
• Object diagram when no people in building
– No objects of class Person exist in system at this point
– Objects written in form objectName:ClassName
• UML permits omission of object names instantiated only once
• If object name unknown, just include class name
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