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![Page 1: Java Programming Applets. Topics Write an HTML document to host an applet Understand simple applets Use Labels with simple AWT applets Write a simple.](https://reader036.fdocuments.in/reader036/viewer/2022062518/56649f325503460f94c4d900/html5/thumbnails/1.jpg)
Java Programming
Applets
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Topics
Write an HTML document to host an applet
Understand simple applets
Use Labels with simple AWT applets
Write a simple Swing applet and use a JLabel
Add JTextField and JButton Components to Swing applets
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Learn about event-driven programming
Add output to a Swing applet
Understand the Swing applet life cycle
Create a more sophisticated interactive Swing applet
Use the setLocation() and setEnabled() methods
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To Write a Java Application:
Write the application in the Java programming language, and then save it with a .java file extensionCompile the application into bytecode using the javac command. The bytecode is stored in a file with a .class file extensionUse the java command to interpret and execute the .class file
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Writing an HTML Document to Host an Applet
Applets- Programs that are called from within another application• You run applets within
• a page on the Internet• an intranet• or a local computer from within another program
called Applet Viewer
• To view an applet, it must be called from within another document written in HTML
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Writing an HTML Document to Host an Applet
To create an applet: Write the applet in the Java programming language, and save it with a .java file extensionCompile the applet into bytecode using the javac commandWrite an HTML document that includes a statement to call your compiled Java classLoad the HTML document into a Web browser or run the AppletViewer program
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Writing an HTML Document to Host an Applet
Applets are popular because users can execute them using a Web browser • Web browser- A program that allows you to
display HTML documents on your computer screen
• Internet Explorer
• Netscape Navigator
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Writing an HTML Document to Host an Applet
Code to run an applet from within an HTML document• <applet>• </applet>
Applet tag attributes• CODE = is followed by the name of the compiled
applet you are calling• WIDTH = is followed by the width of the applet on the
screen• HEIGHT = is followed by the height of the applet on
the screen
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Applets
The WIDTH and HEIGHT attributes are measured in pixels• Pixels- Picture elements, or tiny dots that make
up the image on your video monitor
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Understanding Simple Applets
To write an applet you must also:
Include import statements to ensure that necessary classes are available
Learn to use some Windows components and applet methods
Learn to use the keyword extends
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Understanding Simple Applets
Component- A class that defines any object that you want to display
Container- A class that is used to define a component that can contain other components
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Understanding Simple Applets
Most AWT applets contain 2 import statements• import java.applet.*;
• import java.awt.*;
java.applet- Contains a class named Applet• Every applet you create is based on Applet
java.awt- The Abstract Windows Toolkit, or AWT
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Understanding Simple Applets
Most Swing applets contain 2 import statements• import javax.swing.*;
• import java.awt.*;
javax.swing- A package that contains classes that define GUI components (Swing components)
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Understanding Simple Applets
Swing classes- part of a more general set of GUI programming capabilities that are known as the Java Foundation Classes, or JFC
• JFC includes Swing component classes and selected classes from the java.awt package
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AWT and Swing Applets
AWT and Swing applets• Begin the same way as Java applications
• Must also include• extends Applet
• extends JApplet
• The extends keyword indicates the applet will build upon Applet and JApplet
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Applets
Four methods in every applet• public void init()
• public void start()
• public void stop()
• public void destroy()
Java can create these for you
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Using Labels with Simple AWT Applets
The java.awt package contains commonly used Windows components• Labels
• Menus
• Buttons
Label- Built-in class that holds text that you can display within an applet
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Using Labels with Simple AWT Applets
Label class contains fields that indicate font and alignment
You can assign some text to a label with the setText() method
Use the add() method to add a component to an applet window
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Writing a Simple Swing Applet and Using a JLabel
JLabel- Built-in class that holds text that you can display within an applet• The counterpart to the AWT Label
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Writing a Simple Swing Applet and Using a JLabel
Available constructors include:• JLabel() creates a JLabel instance with no image and an empty
string for the title• JLabel(Icon image) creates a JLabel instance with the specified
image• JLabel(Icon image, int horizontalAlignment) creates a JLabel
instance with the specified image and horizontal alignment• JLabel(String text) creates a JLabel instance with the specified
text• JLabel(String text, Icon icon, int horizontalAlignment) creates
a JLabel instance with the specified text, image, and horizontal alignment
• JLabel(String text, int horizontalAlignment) creates a JLabel instance with the specified text and horizontal alignment
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Writing a Simple Swing Applet and Using a JLabel
AWT components are added directly to the Applet
Swing components must use a content pane• The content pane is an object of the Container
class
• A container can be created using the getContentPane() method
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Changing a JLabel’s Font
Font object- Holds typeface and size information
setFont() method requires a Font object argument
To construct a Font object you need 3 arguments• Typeface
• Style
• Point size
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Changing a JLabel’s Font
To construct a Font object you need 3 arguments• Typeface
• String representing a font• Common fonts are Arial, Courier, and New Times Roman• Is only a request
• Style- applies an attribute to displayed text• Font.PLAIN• Font.BOLD• Font.ITALIC
• Point size• Integer that represents 1/72 of an inch• Printed text is usually 10- or 12 points
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Adding JTextField Components to Swing Applets
JTextField- Component into which a user can type a single line of text dataJText field can be constructed from• public JTextField() constructs a new JTextField• public JTextField(int numColumns) constructs a new
empty JTextField with a specified number of columns• public JTextField(String text) constructs a new
JTextField initialized with the specific text• public JTextField(String text, int columns) constructs a
new JTextField with the specified text and columns
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Other JTextField Methods
setText() method- Allows you to change the text in a JTextField that has already been created
getText() method- Allows you to retrieve the string of text in a JTextField
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Other JTextField Methods
Keyboard focus- When the user clicks within the JTextField, the JTextField has focus, which means the next entries from the keyboard will be at that location
requestFocus() method- To have the insertion point appear automatically within the TextField without requiring the user to click in it first
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Editable- The capacity for a field to accept keystrokes
setEditable() method- Used to change the editable status of a JTextField
Other JTextField Methods
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Adding JButton Components to Swing Applets
JButton- Creates a button
JButton can be constructed from• public JButton() constructs a button with no set text
• public JButton(Icon icon) creates a button with an icon of type Icon or ImageIcon
• public JButton(String text) creates a button with the specific text
• public JButton(String text, int columns) constructs a new JTextField with the specified text and columns
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Adding JButton Components to Swing Applets
setLabel() method• To change a JButton’s label
• readyJButton.setLabel(“Don’t press me again!”)
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Adding Multiple Components to a JApplet
To add multiple components in a container use a layout manager• To control component positioning
• Default behavior is to use a border layout
Border layouts
Flow layouts
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Adding Multiple Components to a JApplet
Border layouts• Created by the BorderLayout class
• Divide a container into 5 sections• North, South, East, West, and center
• Created with the BorderLayout() or BorderLayout(int, int) methods
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Adding Multiple Components to a JApplet
Flow Layouts• Places components in a row, and when a row is
filled, it automatically spills components onto the next row
• Default positioning of the row of components is centered in the container
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Event Driven Programming
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Learning about Event-Driven Programming
Event- Occurs when someone using your applet takes action on a componentProcedural- Programmers dictate the order in which events occurEvent-driven programs- The user can initiate any number of events in any orderSource- Component on which an event is generatedListener- Object that is interested in an event
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Preparing Your Swing Applet to Accept Event Messages
Prepare your applet to accept mouse events by: • importing the java.awt.event package• adding the phrase implements ActionListener to
the class header
ActionListener is an interfaceInterface- A set of specifications for methods that you can use with event objects
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Telling Your Swing Applet to Expect Events to Happen
addActionListener() method • To tell the applet to expect ActionEvents
• aButton.addActionListener(this);
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Telling Your Swing Applet How to Respond to Any Events That Happen
actionPerformed(ActionEvent e) method
• When a JApplet has registered as a listener with a JButton, and a user clicks the JButton the actionPerformed method executes
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Adding Output to a Swing Applet
You can add components to an applet using the add() method
You can also remove components from an applet using the remove() method• Remove(answer);
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Understanding the Swing Applet Life Cycle
Override- When you write a method that has the same method header as an automatically provided method
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Understanding the Swing Applet Life Cycle
start() method- Executes after the init() method• Executes every time the applet becomes active
after it has been inactive
stop() method- When a user leaves a web page• You do not usually write your own stop()
methods
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Understanding the Swing Applet Life Cycle
destroy() method- When the user closes the browser or AppletViewer• You do not usually write your own destroy()
methods
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Using the setLocation() and setEnabled() Methods
setLocation() method- Allows you to place a component at a specific location within the AppletViewer window• X-axis- Horizontal position in a window• X-coordinate- Value increases as you travel
from left to right across the window• Y-axis- Vertical position in the window• Y-coordinate- Value increases as you travel
from top to bottom in the window
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The setEnabled() Method
setEnabled() method- To make a component unavailable and, in turn, to make it available again
True if you want to enable a component
False if you want to disable a component
If (yLoc==280) pressButton.setEnabled(false);