java programming applets. topics write an html document to host an applet understand simple applets...

Java Programming

Applets

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

Applets

Four methods in every applet• public void init()

• public void start()

• public void stop()

• public void destroy()

Java can create these for you

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

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

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

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

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

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

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

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

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

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

Editable- The capacity for a field to accept keystrokes

setEditable() method- Used to change the editable status of a JTextField

Other JTextField Methods

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

Adding JButton Components to Swing Applets

setLabel() method• To change a JButton’s label

• readyJButton.setLabel(“Don’t press me again!”)

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

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

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

Event Driven Programming

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

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

Telling Your Swing Applet to Expect Events to Happen

addActionListener() method • To tell the applet to expect ActionEvents

• aButton.addActionListener(this);

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

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);

Understanding the Swing Applet Life Cycle

Override- When you write a method that has the same method header as an automatically provided method

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

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

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

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);