CSTP WS00CS423 (cotter)1 Java Applets Objective: Learn how to develop Java programs that interact with users through a Web browser

CSTP WS00CS423 (cotter) 2 Java Applet Lifecycle Methods init ( ) –Called when applet is loaded onto user’s machine. Prep work or one-time-only work done at this time. start ( ) –Called when applet becomes visible (page called up). Called every time applet becomes visible. stop ( ) –Called when applet becomes hidden (page loses focus). destroy ( ) –Guaranteed to be called when browser shuts down.

CSTP WS00CS423 (cotter) 3 Applets vs. Applications Extend Applet instead of Frame Use of init( ) vs. constructor No use of main( ) Default layout managers –application: BorderLayout –applet: FlowLayout Security

CSTP WS00CS423 (cotter) 4 Applet Presentation Uses applet Tag in HTML MyNewApplet Here’s My first Applet

CSTP WS00CS423 (cotter) 5 Applet Inheritance (extension) Object - “cosmic base class” Component - button, listBox Container - dialog box Panel - “sub-window”, group of buttons, etc. Applet - small application, web enabled MyApplet - specific instance of an applet

CSTP WS00CS423 (cotter) 6 MyTextApplet import java.applet.*; import java.awt.*; import java.awt.event.*; public class MyTextApplet extends Applet { public void init() { setBackground(Color.gray); Panel p = new Panel(); ButtonAction aButtonAction = new ButtonAction(); p.setLayout(new FlowLayout()); tickButton = new Button("Tick"); tickButton.addActionListener(aButtonAction); p.add(tickButton);

CSTP WS00CS423 (cotter) 7 MyTextApplet setButton = new Button("Set time"); setButton.addActionListener(aButtonAction); p.add(setButton); hourField = new TextField("12", 3); p.add(hourField); minuteField = new TextField("00", 3); p.add(minuteField); timeField = new TextField("", 12); p.add(timeField); add(p); }

CSTP WS00CS423 (cotter) 8 MyTextApplet Class ButtonAction implements ActionListener { public void actionPerformed(ActionEvent evt) { String buttoncommand = evt.getActionCommand(); if (buttoncommand.equals("Tick")) { int minutes = Integer.parseInt(minuteField.getText()); minutes += 1; String min = String.valueOf(minutes); minuteField.setText(min); }

CSTP WS00CS423 (cotter) 9 MyTextApplet else if (buttoncommand.equals("Set time")) { int hours = Integer.parseInt(hourField.getText()); int minutes = Integer.parseInt(minuteField.getText()); String tim = hourField.getText() + ":" + minuteField.getText(); timeField.setText(tim); } } //end of ButtonAction private TextField hourField; private TextField minuteField; private TextField timeField; private Button tickButton; private Button setButton; }

CSTP WS00CS423 (cotter) 10 HTML file for MyTextApplet My Text Applet Demo Here's my Demo Applet

CSTP WS00CS423 (cotter) 11 MyTextApplet Output

CSTP WS00CS423 (cotter) 12 Parameter Passing to Applets import java.applet.*; import java.awt.*; public class FontApplet extends Applet { public void paint (Graphics g) { String fontName = getParameter ("font"); int fontSize = Integer.parseInt (getParameter("size")); Font f = new Font(fontName, Font.BOLD, fontSize); g.setFont (f); g.drawString("Welcome to CS423", 25, 50); }

CSTP WS00CS423 (cotter) 13 HTML file for FontApplet FontApplet Here’s a demo of parameter passing to an Applet

CSTP WS00CS423 (cotter) 14 FontApplet.htm

CSTP WS00CS423 (cotter) Using Applet Parameters in VJ++

CSTP WS00CS423 (cotter) 16 Java Applet Lifecycle Methods (again...) init ( ) –Called when applet is loaded onto user’s machine. Prep work or one-time-only work done at this time. start ( ) –Called when applet becomes visible (page called up). Called every time applet becomes visible. stop ( ) –Called when applet becomes hidden (page loses focus). destroy ( ) –Guaranteed to be called when browser shuts down.

CSTP WS00CS423 (cotter) 17 LifecycleApplet.java import java.applet.Applet; import java.awt.Graphics; public class LifecycleApplet extends Applet { public LifecycleApplet() { System.out.println("Constructor running..."); } public void init() { System.out.println("This is init."); } public void start() { System.out.println("Applet started."); } public void paint(Graphics theGraphics) { theGraphics.drawString("Hello, World!", 0, 50); System.out.println("Applet just painted."); } public void stop() { System.out.println("Applet stopped."); } public void destroy() { System.out.println("Applet destroyed."); } }

CSTP WS00CS423 (cotter) 18 LifecycleApplet.java

CSTP WS00CS423 (cotter) 19 LifecycleApplet.java

CSTP WS00CS423 (cotter) 20 Summary Java Applet managed through applet lifecycle Applet can interact with browser through HTML code Uses Event programming model to interact with user (using buttons, textfields, etc.)