1. Chapter 5 Processing Input with Applets
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Introduction to Object-Oriented Programming with Java--Wu
Intro to OOP w/Java--Wu

2. Introduction to Object-Oriented Programming with Java--Wu
Chapter 5
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.
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Introduction to Object-Oriented Programming with Java--Wu
Intro to OOP w/Java--Wu

3. Sample Applet: GreetingApplet
The GreetingApplet applet accepts a name entered by the user and replies back with a personalized greeting.
This is a text field where the user enters his/her name.
The greeting is displayed when the user enters the name and presses the ENTER key.
We will develop this applet in two steps:
Place GUI components
Add an action event handling method
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Introduction to Object-Oriented Programming with Java--Wu

4. Template for an Applet Definition
import java.applet.*;
import java.awt.*;
public class extends Applet { }
Import Statements
javadoc
Comment
Applet Name
Declaration
Methods
Include a sequence of methods
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Introduction to Object-Oriented Programming with Java--Wu

5. GreetingApplet: Placing GUI Objects
Chapter 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 ); }
Declaration
These statements place Label and TextField objects on this applet.
To write an input processing applet, you need to know how to define a class with multiple methods. To understand how an applet processes input data, you must learn what events are and how events are handled in Java programs.
Actions such as moving a mouse, selecting a menu item, entering data, and pressing the ENTER key are all events. When the user makes one of these actions, we say an event is generated. We program applets so that a designated method will be executed in response to the generated events. A more detailed discussion on event handling is given in Chapter 13.
The standard applet classes we discuss in this chapter are from the java.awt and java.applet packages. For the discussion on the Swing version of applets, please visit Dr. Caffeine’s website. Please notice that an understanding of this chapter is required before studying the Swing version.Discussion on the Swing version of applets can be found at
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Introduction to Object-Oriented Programming with Java--Wu
Intro to OOP w/Java--Wu

6. GreetingApplet: Handling Action Events
Chapter 5
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.
In addition to action events, there are mouse events, mouse movement events, item changed events, and so forth.
For each different type of event, we have a corresponding event listener. For example, we have action event listeners for action events, mouse motion listeners for mouse movement events, and so forth.
Many GUI objects generate some types of events, but not all of them do. For instance, Label objects do not generate events, and therefore, are not event sources.
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Introduction to Object-Oriented Programming with Java--Wu
Intro to OOP w/Java--Wu

7. GreetingApplet as an Action Event Handler
To set a GreetingApplet object be an action event handler, we must
Import the Java event-handling classes.
Modify the class declaration to include the clause implements ActionListener.
Add the actionPerformed method to the class definition.
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.)
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Introduction to Object-Oriented Programming with Java--Wu

8. Template for an Action Processing Applet
import java.applet.*;
import java.awt.*;
import java.event.*;
public class extends Applet
implements ActionListener { }
Import Statements
javadoc
Comment
Applet Name
Implement Clause
Declaration
Methods
One of which is actionPerformed
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Introduction to Object-Oriented Programming with Java--Wu

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.
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Introduction to Object-Oriented Programming with Java--Wu

10. The actionPerformed Method
Chapter 5
The actionPerformed Method
public void actionPerformed( ActionEvent event ) {
greeting.setText( “Nice to meet you, “ +
inputLine.getText( ) + “.” );
add( greeting );
doLayout( ); }
Inside the actionPerformed method, we include the code to process the ENTER key event. Inside the method body, we
1. Fetch the name entered in inputLine by calling its getText method.
2. Concatenate the name returned by the getText method with the message Nice to meet you and the period as
"Nice to meet you, " + inputLine.getText() + "."
3. Assign the resulting string from Step 2 to the Label object greeting by calling its setText method.
4. Call the add method to place greeting and call the doLayout method.
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Introduction to Object-Oriented Programming with Java--Wu
Intro to OOP w/Java--Wu

11. GreetingApplet Object Diagram
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Introduction to Object-Oriented Programming with Java--Wu

12. Absolute Positioning of GUI Objects
Placement of GUI objects an applet is control by the layout manager. Default layout manager for an applet is FlowLayout.
It is possible to set the layout manager to null
setLayout( null );
and place GUI objects by specifying their position and size as
<object>. setBounds ( x, y, width, height ) ;
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Introduction to Object-Oriented Programming with Java--Wu

13. Introduction to Object-Oriented Programming with Java--Wu
The setBounds Method
inputLine.setBounds( 65, 55, 150, 25 );
150 25 55 65
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Introduction to Object-Oriented Programming with Java--Wu

14. Introduction to Object-Oriented Programming with Java--Wu
The Button Class
A type of button called pushbutton is one of most ubiquitous GUI objects used today’s window-based programs.
We use the Button class from java.awt to create pushbuttons.
Button helloButton;
helloButton = new Button(“Hello”);
A button will generate an action event when it is clicked. Register an action event listener to a button to process the action events.
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Introduction to Object-Oriented Programming with Java--Wu

15. Converting Text to a Numerical Value
A TextField object we to input data from the user will accept data as text value.
To input numerical data, we must convert the text value to a numerical value.
There are Double, Float, Integer, and others that provide conversion routines.
There is also the Convert class in the javabook package that provides a consistent way of converting values. The Convert class uses the Double, Float, Integer, and other standard classes internally to provide conversion services. Please consult its reference and source code.
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Introduction to Object-Oriented Programming with Java--Wu

16. Converting Text to a Double Value
To convert “ ” to a double value, we write
Doulbe doubleObj = new Double(“ ”);
double number = doubleObj.doubleValue();
Alternatively, we can write
double number;
number = Double.parseDouble(“ ”);
Conversion of a text value to other numerical data types follows the same pattern as above.
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Introduction to Object-Oriented Programming with Java--Wu

17. Sample Program: Body Mass Index (BMI)
Problem Statement
Write an applet that displays a BMI of a person given his or her weight in kilograms and height in meters.
Major Tasks
Get two input values
Convert the input text values to double and compute the BMI
Display the result
Key Formula (given)
BMI = w / h2
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Introduction to Object-Oriented Programming with Java--Wu

18. Introduction to Object-Oriented Programming with Java--Wu
BMIApplet – Design
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Introduction to Object-Oriented Programming with Java--Wu

19. Loan Calculator – Object Diagram
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Introduction to Object-Oriented Programming with Java--Wu

20. Loan Calculator – Development Steps
Start with a skeleton BMIApplet class. The skeleton class will include data member declarations and a partially defined constructor that creates and places GUI objects.
Add code to the constructor to create and place all GUI objects on the applet.
Define the actionPerformed method to process the button click action event. We will include a dummy output statement to verify the button click event is properly detected.
Add code to compute and display the BMI and finalize the code.
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Introduction to Object-Oriented Programming with Java--Wu