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Chapter 3 - Introduction to Java Applets

Outline 3.1 Introduction 3.2 Sample Applets from the Java 2 Software Development Kit 3.3 Simple Java Applet: Drawing a String 3.4 Drawing Strings and Lines 3.5 Adding Floating-Point Numbers 3.6 Java Applet Internet and World Wide Web Resources 3.7 (Optional Case Study) Thinking About Objects: Identifying the Classes in a Problem Statement

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3.1 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 Notes Mimic several features of Chapter 2 to reinforce them Focus on fundamental programming concepts first Explanations will come later

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3.2 Sample Applets from the Java 2 Software Development Kit

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/j2se/1.4.1/

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3.2 Sample Applets from the Java 2 Software Development Kit

Running applets In command prompt, change to demo subdirectory of applet cd c:\j2sdk1.4.1\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

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3.2 Sample Applets from the Java 2 Software Development Kit

You start as player "X"

Fig. 3.2 Sample execution of applet TicTacToe.

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3.2 Sample Applets from the Java 2 Software Development Kit

Fig. 3.4 Sample execution of applet DrawTest.

Drag the mouse pointer in the white area to draw.

Select the shape to draw by clicking the down arrow, then clicking Lines or Points. This GUI component is commonly known as a combo box, choice or drop-down list.

Select the drawing color by clicking the circle for the color you want. These GUI components are commonly known as radio buttons.

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3.2 Sample Applets from the Java 2 Software Development Kit

Demonstrates 2D drawing capabilities built into Java2

Try changing the options to see their effect on the demonstration.

Click a tab to select a two-dimensional graphics demo.

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3.3 Simple Java Applet: Drawing a String

Now, create applets of our own Take a while before we can write applets like in the demos Cover many of same techniques Upcoming program Create an applet to display "Welcome to Java Programming!" Show applet and HTML file, then discuss them line by line

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Java applet Program Output

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3.3 Simple Java Applet: Drawing a String

Comments Name of source code and description of applet Import predefined classes grouped into packages import declarations tell compiler where to locate classes used When you create applets, import the JApplet class (package javax.swing) import the Graphics class (package java.awt) to draw graphics Can draw lines, rectangles ovals, strings of characters import specifies directory structure

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3.3 Simple Java Applet: Drawing a String

Applets have at least one class declaration (like applications) Rarely create classes from scratch Use pieces of existing classes Inheritance - create new classes from old ones (ch. 9) Begins class declaration for class WelcomeApplet Keyword class then class name extends followed by class name Indicates class to extend (JApplet) JApplet : superclass (base class) WelcomeApplet : subclass (derived class) WelcomeApplet now has methods and data of JApplet

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3.3 Simple Java Applet: Drawing a String

Class JApplet defined for us Someone else defined "what it means to be an applet" Applets require over 200 methods! extends JApplet Inherit methods, do not have to declare them all Do not need to know every detail of class JApplet

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3.3 Simple Java Applet: Drawing a String

Class WelcomeApplet is a blueprint appletviewer or browser creates an object of class WelcomeApplet Keyword public required File can only have one public class public class name must be file name

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3.3 Simple Java Applet: Drawing a String

Our class inherits method paint from JApplet By default, paint has empty body 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

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3.3 Simple Java Applet: Drawing a String

Method paint Lines 11-19 are the declaration of 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

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3.3 Simple Java Applet: Drawing a String

Calls version of method paint from superclass JApplet Should be first statement in every applet’s paint method Body of paint Method drawString (of class Graphics) Called using Graphics object g and dot (.) 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)

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3.3 Simple Java Applet: Drawing a String

Running the applet Compile javac WelcomeApplet.java If no errors, bytecodes stored in WelcomeApplet.class Create an HTML file Loads the applet into appletviewer or a browser Ends in .htm or .html To execute an applet Create an HTML file indicating which applet the browser (or appletviewer) should load and execute

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3.3 Simple Java Applet: Drawing a String

Simple HTML file (WelcomeApplet.html) Usually in same directory as .class file Remember, .class file created after compilation HTML codes (tags) Usually come in pairs Begin with < and end with > Lines 1 and 4 - begin and end the HTML tags Line 2 - begins <applet> tag Specifies code to use for applet Specifies width and height of display area in pixels Line 3 - ends <applet> tag

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3.3 Simple Java Applet: Drawing a String

appletviewer only understands <applet> tags Ignores everything else Minimal browser Executing the applet appletviewer WelcomeApplet.html Perform in directory containing .class file

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3.3 Simple Java Applet: Drawing a String

Running the applet in a Web browser

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3.4 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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WelcomeApplet2

java 1. import 2. Class (extends JApplet) 3. paint 3.1 drawString 3.2 drawString on same x coordinate, but 15 pixels down

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1 // Fig. 3.9: WelcomeApplet2.java 2 // Displaying multiple strings in an applet. 3 4 // Java packages 5 import java.awt.Graphics; // import class Graphics 6 import javax.swing.JApplet; // import class JApplet 7 8 public class WelcomeApplet2 extends JApplet { 9 10 // draw text on applet’s background 11 public void paint( Graphics g ) 12 { 13 // call superclass version of method paint 14 super.paint( g ); 15 16 // draw two Strings at different locations 17 g.drawString( "Welcome to", 25, 25 ); 18 g.drawString( "Java Programming!", 25, 40 ); 19 20 } // end method paint 21 22 } // end class WelcomeApplet2

HTML file Program Output

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1 <html> 2 <applet code = "WelcomeApplet2.class" width = "300" height = "60"> 3 </applet> 4 </html>

WelcomeLines

java 2. Class (extends JApplet) 3. paint 3.1 drawLine 3.2 drawLine 3.3 drawString Program Output

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1 // Fig. 3.11: WelcomeLines.java 2 // Displaying text and lines 3 4 // Java packages 5 import java.awt.Graphics; // import class Graphics 6 import javax.swing.JApplet; // import class JApplet 7 8 public class WelcomeLines extends JApplet { 9 10 // draw lines and a string on applet’s background 11 public void paint( Graphics g ) 12 { 13 // call superclass version of method paint 14 super.paint( g ); 15 16 // draw horizontal line from (15, 10) to (210, 10) 17 g.drawLine( 15, 10, 210, 10 ); 18 19 // draw horizontal line from (15, 30) to (210, 30) 20 g.drawLine( 15, 30, 210, 30 ); 21 22 // draw String between lines at location (25, 25) 23 g.drawString( "Welcome to Java Programming!", 25, 25 ); 24 25 } // end method paint 26 27 } // end class WelcomeLines

HTML file

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1 <html> 2 <applet code = "WelcomeLines.class" width = "300" height = "40"> 3 </applet> 4 </html>

3.4 Drawing Strings and Lines

Method drawLine of class Graphics Takes as arguments Graphics object and line’s end points X and y coordinate of first endpoint X and y coordinate of second endpoint

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3.5 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 types double – double precision floating-point numbers float – single precision floating-point numbers Show program, then discuss

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AdditionApplet

java 1. import 2. Class (extends JApplet) 3. Fields 4. init 4.1 Declare variables 4.2 showInputDialog 4.3 parseDouble

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1 // Fig. 3.13: AdditionApplet.java 2 // Adding two floating-point numbers. 3 4 // Java packages 5 import java.awt.Graphics; // import class Graphics 6 import javax.swing.*; // import package javax.swing 7 8 public class AdditionApplet extends JApplet { 9 double sum; // sum of values entered by user 10 11 // initialize applet by obtaining values from user 12 public void init() 13 { 14 String firstNumber; // first string entered by user 15 String secondNumber; // second string entered by user 16 17 double number1; // first number to add 18 double number2; // second number to add 19 20 // obtain first number from user 21 firstNumber = JOptionPane.showInputDialog( 22 "Enter first floating-point value" ); 23 24 // obtain second number from user 25 secondNumber = JOptionPane.showInputDialog( 26 "Enter second floating-point value" ); 27 28 // convert numbers from type String to type double 29 number1 = Double.parseDouble( firstNumber ); 30 number2 = Double.parseDouble( secondNumber ); 31

5. Draw applet contents 5.1 Draw a rectangle 5.2 Draw the results HTML

file

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32 // add numbers 33 sum = number1 + number2; 34 35 } // end method init 36 37 // draw results in a rectangle on applet’s background 38 public void paint( Graphics g ) 39 { 40 // call superclass version of method paint 41 super.paint( g ); 42 43 // draw rectangle starting from (15, 10) that is 270 44 // pixels wide and 20 pixels tall 45 g.drawRect( 15, 10, 270, 20 ); 46 47 // draw results as a String at (25, 25) 48 g.drawString( "The sum is " + sum, 25, 25 ); 49 50 } // end method paint 51 52 } // end class AdditionApplet

1 <html> 2 <applet code = "AdditionApplet.class" width = "300" height = "65"> 3 </applet> 4 </html>

Program Output

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3.5 Adding Floating-Point Numbers

Lines 1-2: Comments Line 5: imports class Graphics import not needed if use full package and class name public void paint ( java.awt.Graphics g ) Line 8: specify entire javax.swing package * indicates all classes in javax.swing are available 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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3.5 Adding Floating-Point Numbers

Begin class declaration Extend JApplet, imported from package javax.swing Field declaration Each object of class gets own copy of the field Declared in body of class, but not inside methods Variables declared in methods are local variables Can only be used in body of method Fields can be used anywhere in class Have default value (0.0 in this case)

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3.5 Adding Floating-Point Numbers

Primitive type double Used to store floating point (decimal) numbers Method init Normally initializes fields and applet class Guaranteed to be first method called in applet First line must always appear as above Returns nothing (void), takes no arguments Begins body of method init

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3.5 Adding Floating-Point Numbers

Declare variables Two types of variables Reference variables (called references) Refer to objects (contain location in memory) Objects defined in a class definition Can contain multiple data and methods paint receives a reference called g to a Graphics object Reference used to call methods on the Graphics object Primitive types (called variables) Contain one piece of data

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3.5 Adding Floating-Point Numbers

Distinguishing references and variables If type is a class name, then reference String is a class firstNumber, secondNumber If type a primitive type, then variable double is a primitive type number1, number2

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3.5 Adding Floating-Point Numbers

Method JOptionPane.showInputDialog Prompts user for input with string Enter value in text field, click OK If not of correct type, error occurs In Chapter 15 learn how to deal with this Returns string user inputs Assignment statement to string Lines 25-26: As above, assigns input to secondNumber

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3.5 Adding Floating-Point Numbers

static method Double.parseDouble Converts String argument to a double Returns the double value Remember static method syntax ClassName.methodName( arguments ) Assignment statement sum an field, can use anywhere in class Not defined in init but still used

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3.5 Adding Floating-Point Numbers

Ends method init appletviewer (or browser) calls inherited method start start usually used with multithreading Advanced concept, in Chapter 16 We do not declare it, so empty declaration in JApplet used Next, method paint called Method drawRect( x1, y1, width, height ) Draw rectangle, upper left corner (x1, y1), specified width and height Line 45 draws rectangle starting at (15, 10) with a width of 270 pixels and a height of 20 pixels

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3.5 Adding Floating-Point Numbers

Sends drawString message (calls method) to Graphics object using reference g "The sum is" + sum - string concatenation sum converted to a string sum can be used, even though not defined in paint field, can be used anywhere in class Non-local variable

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3.6 Java Applet Internet and World Wide Web Resources

Many Java applet resources available java.sun.com/applets/ Many resources and free applets Has demo applets from J2SDK Sun site developer.java.sun.com/developer Tech support, discussion forums, training, articles, links, etc. Registration required www.jars.com Rates applets, top 1, 5 and 25 percent View best applets on web

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3.7 (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

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3.7 (Optional Case Study) Thinking About Objects: Identifying the

Classes in a Problem Statement

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

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3.7 (Optional Case Study) Thinking About Objects: Identifying the

Classes in a Problem Statement

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 ElevatorShaft, Elevator, Person, Floor, ElevatorDoor, FloorDoor, ElevatorButton, FloorButton, Bell, and Light

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3.7 (Optional Case Study) Thinking About Objects: Identifying the

Classes in a Problem Statement

Using UML to model elevator system Class diagrams models classes and relationships Model structure/building blocks of system Representing class Elevator using UML Top rectangle is class name Middle contains class’ attributes Bottom contains class’ operations

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3.7 (Optional Case Study) Thinking About Objects: Identifying the

Classes in a Problem Statement

Class associations using UML Elided diagram Class attributes and operations ignored Class relation among ElevatorShaft, Elevator and FloorButton Solid line is an association, or relationship between classes Numbers near lines express multiplicity values Indicate how many objects of class participate association

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3.7 (Optional Case Study) Thinking About Objects: Identifying the

Classes in a Problem Statement

Diagram shows two objects of class FloorButton participate in association with one object of ElevatorShaft FloorButton has two-to-one relationship with ElevatorShaft

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3.7 (Optional Case Study) Thinking About Objects: Identifying the

Classes in a Problem Statement

Associations can be named In diagram, “Requests” indicates association and arrow indicates direction of association One object of FloorButton requests one object of class Elevator Similar context with “Resets” and “Signals Arrival” Aggregation relationship Implies whole/part relationship Some object “has” some object Object attached with diamond is “owner” Object on other end is the “part” In diagram, elevator shaft “has an” elevator and two floor buttons

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3.7 (Optional Case Study) Thinking About Objects: Identifying the

Classes in a Problem Statement

Fig. 3.23 Class diagram for the elevator model.

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3.7 (Optional Case Study) Thinking About Objects: Identifying the

Classes in a Problem Statement

The complete class diagram for elevator model Several of many and aggregates 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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