Chapter 5 Processing Input with Applets 2000 McGraw-Hl‖ Introduction to Object-Oriented Programming with Java-Wu Chapter 5-1
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 1 Chapter 5 Processing Input with Applets
Chapter 5 objectives After you have read and studied this chapter, you should be able to e Define an applet with multiple methods e Incorporate a simple event-handling routine to an applet to process input. e Construct input-processing applets using Label, TextField, and Button objects from the java. awt package e Convert string data to numerical data e Use the reserved word this in your programs e Run applets without using an applet viewer or browser. C 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5-2
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 2 Chapter 5 Objectives After you have read and studied this chapter, you should be able to Define an applet with multiple methods. Incorporate a simple event-handling routine to an applet to process input. Construct input-processing applets using Label, TextField, and Button objects from the java.awt package. Convert string data to numerical data. Use the reserved word this in your programs. Run applets without using an applet viewer or browser
Sample applet: GreetingApplet r The GreetingApplet applet accepts a name entered y the user and replies back with a personalized b greeting Applet viewer: Greeting Applet class回区 This is a text field Please enter your name Mireille where the user Nice to meet you, mireille enters his/her name The greeting is displayed when the user enters the name and presses the pple started ENTER key r We will develop this applet in two steps 1. Place GUi components 2. Add an action event handling method C 2000 McGraw-Hill troduction to Object-Oriented Programming with Java--Wu Chapter 5-3
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 3 Sample Applet: GreetingApplet The GreetingApplet applet accepts a name entered by the user and replies back with a personalized greeting. We will develop this applet in two steps: 1. Place GUI components 2. Add an action event handling method The greeting is displayed when the user enters the name and presses the ENTER key. This is a text field where the user enters his/her name
Template for an applet definition import java applet. *i Import import java. awt. *i Statements Ja vado Comment public class extends Applet Applet Name Declaration Method Include a sequence of methods C 2000 McGraw-Hill troduction to Object-Oriented Programming with Java--Wu Chapter 5-4
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 4 Template for an Applet Definition import java.applet.*; import java.awt.*; public class extends Applet { } javadoc Comment Applet Name Methods Include a sequence of methods Declaration Import Statements
GreetingApplet: Placing GUi objects public class GreetingApplet extends Applet private label prompt private label greeting; Declaration private TextField inputlinei public GreetingApplet( prompt new Label (Please enter your name")i greeting new Label( )i inputline new TextField( 15 )i add( prompt )i These statements add( greeting place Label and add( inputline )i TextField objects on this applet C 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5-5
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 5 GreetingApplet: Placing GUI Objects public class GreetingApplet extends Applet { private Label prompt; private Label greeting; private TextField inputLine; public GreetingApplet( ) { prompt = new Label(“Please enter your name”); greeting = new Label( ); inputLine = new TextField( 15 ); add( prompt ); add( greeting ); add( inputLine ); } } These statements place Label and TextField objects on this applet. Declaration
Greeting Applet: Handling Action Events r We will now add code to process the pressing of the ENTER key r Event sources generate or trigger events and Event listeners process the generated events r For this applet, the TextField object inputLine is the event source and GreetingApplet is the event listener. r TextField inputline generates an action event C 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5-6
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 6 GreetingApplet: Handling Action Events We will now add code to process the pressing of the ENTER key. Event sources generate or trigger events and Event listeners process the generated events. For this applet, the TextField object inputLine is the event source and GreetingApplet is the event listener. TextField inputLine generates an action event
GreetingApplet as an Action Event Handler r To set a Greeting Applet object be an action event handler, we must 1. Import the Java event-handling classes 2. Modify the class declaration to include the clause implements Action Listener. 3. Add the action Performed method to the class definition 4. Register the Greeting Applet object to the event source inputline as its action event listener. (An event source will call the registered listeners action Performed method when the event occurs.) C 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5-7
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 7 GreetingApplet as an Action Event Handler To set a GreetingApplet object be an action event handler, we must 1. Import the Java event-handling classes. 2. Modify the class declaration to include the clause implements ActionListener. 3. Add the actionPerformed method to the class definition. 4. Register the GreetingApplet object to the event source inputLine as its action event listener. (An event source will call the registered listeners’ actionPerformed method when the event occurs.)
Template for an Action Processing applet import java applet. *i Import import java. awt. *i Statements import java. event. * i avadoc Comment Applet Name public class extends Applet implements ActionListener Implement Clause Declaration Methods One of which is action Performed C 2000 McGraw-Hill troduction to Object-Oriented Programming with Java--Wu Chapter 5-8
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 8 Template for an Action Processing Applet import java.applet.*; import java.awt.*; import java.event.*; public class extends Applet implements ActionListener { } javadoc Comment Applet Name Methods One of which is actionPerformed Import Statements Declaration Implement Clause
Registering GreetingApplet r We register an GreetingApplet object as an action event Listener, of the event source inputLine by calling inputLine's addActionListener method. We do this in the constructor. public GreetingApplet //as before inputline. addActionListener( this )i r The reserved word this refers to the GreetingApplet object To refer to an object from within the object's method, we use the reserved word this C 2000 McGraw-Hill troduction to Object-Oriented Programming with Java--Wu Chapter 5-9
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 9 Registering GreetingApplet We register an GreetingApplet object as an action event listener of the event source inputLine by calling inputLine’s addActionListener method. We do this in the constructor: public GreetingApplet { //as before inputLine.addActionListener( this ); } The reserved word this refers to the GreetingApplet object. To refer to an object from within the object’s method, we use the reserved word this
The action Performed method public void actionPerformed( ActionEvent event greeting. setText(Nice to meet you,+ inputLine. getText()+.)i add( greeting )i dolayout()i C 2000 McGraw-Hill troduction to Object-Oriented Programming with Java--Wu Chapter 5-10
© 2000 McGraw-Hill Introduction to Object-Oriented Programming with Java--Wu Chapter 5 - 10 The actionPerformed Method public void actionPerformed( ActionEvent event ) { greeting.setText( “Nice to meet you, “ + inputLine.getText( ) + “.” ); add( greeting ); doLayout( ); }
