For Computational Finance Java Applets and the AWT Jeff Stephenson (slides adapted from Manu Kumar)

Java Applets? u Applets are Java programs which can run within your web browser u Applets vs. Applications –Applets t run in the web browser t subject to security restrictions t do not have a main() –Applications t full fledged Java programs t run from commandline t no security restrictions u So far we’ve built applications –for good reason too since we’ll need what we’ve learnt!

Java Applet Sandbox u Applets Vs. Application –Applet: runs in Web browser. –Application: runs in any Java VM. u Applet restrictions ( –no access to Client’s file System t cannot check for existence, read, write, or rename a file t cannot create or list the contents of a directory t cannot check a file’s type, timestamp or size. –Network connection ONLY to originating host t cannot connect to arbitrary servers / sites across the net t key issue when designing client-server applets

Java Architecture in light of Sandbox DB WWW Applet The Web server machine must act as the proxy for all calls to services on other machines An overly simplified description :

Java Displaying applets in the browser u To add a Java applet to your HTML page –use </APPLET tag t Full Syntax: t [ ] [Text for non-Java supporting browsers] u Browsers treatment of an applet tag –Java enabled browser will load applet –non-Java enabled browser will ignore t it will display Text for non-Java supporting browsers

Java JavaPhysical Example u Place compiled Applet classes in directory under web server u Include tag in HTML file – Your Browser is NOT Java Enabled! u Load up page in Java enabled browser!

Java Applet Class u Applets inherit from java.applet.Applet –provides basic structure for applet to run in a browser u What you must/should override –default constructor, init(), start(), stop(), destroy() u For a threaded applet –implement Runnable interface –override run() u For graphics –may override repaint(), update() and paint()

Java Applet Lifecycle u Browser loads HTML page –Find tag –Locates code using codebase –Downloads.class files to browser –Browser verifies.class files for security u Applet methods executed AUTOMATICALLY in this order: –init() –start() –leaving the page calls stop() –returning to the page calls start() again –exiting the browser or leaving the page permanently calls destroy()

Java HelloWorldApplet import java.awt.*; import java.applet.*; public class HelloWorld extends Applet { public void init() { resize (150, 25); } public void paint(Graphics g) { g.drawString("Hello World!", 50, 25); }

Java Applets in the Real World u To make applets more functional –May need a complicated interface –Must respond to events –Do something useful u Applet uses –Make information and functionality available via browser! –No download necessary –Works automatically –Browser becomes the “platform”

Java Handling Events u Sribble Applet u public boolean mouseDown(Event e, int x, int y) { lastX = x; lastY = y; return true; } u public boolean mouseDrag(Event e, int x, int y) { Graphics g = getGraphics(); g.drawLine(lastX, lastY, x, y); lastX = x; lastY = y; return true; }

Java Scribble Applet CodeWalk u Notice the two methods: –mouseDown –mouseDrag u These get call automatically when any mouse event occurs –all we need to do is override them and take the appropriate action u If we run the applet –we can draw anything we like with the mouse!

Java Making applet functional u Requires: –may more graphical components t buttons t labels t checkboxes t text fields etc. –handling events other than mouse events t click on button t check a checkbox t doubleclick in a list t type in a text box etc. u To do this we’ll need the AWT!!!

Java AWT u AWT –Abstract Windowing Toolkit –Awkward Windowing Toolkit ?!? u Java’s library for graphical elements –Makes EXTENSIVE use of inheritance and OO techniques! t That’s why you needed to learn them before we got to Applets –Hierarchical approach to developing “Components” t you’ll see when we look at the API –The AWT is platform independent t gives you the same components and “widgets” on all platforms! t Allows you to write code for the Java-platform as opposed to Windows or Mac or UNIX

Java AWT Classes u The two MAIN classes in the AWT are –Component –Container u Component –nearly all the “widgets” in the AWT inherit from Component t it inherits attributes and behavior from Component u Container –a placeholder –a Container can “contain” other Components –a Container is a Component t I.e. a Container can contain another Container

Java Layout Managers u AWT uses the concept of “Layout Managers” to define the user interface u Layout Managers –tell the AWT where to position a component or a container –tell the AWT what to do when we resize the application/applet! t Dynamic resizing t Layout Managers specify everything using “relationships” –There are lots of layout managers, we’ll look at a few… t GridLayout t BorderLayout

Java Handling Events u Two approaches to handling events –we saw one approach earlier t overriding specific event handler methods such as mouseDown, mouseDrag etc. –today we will see the better approach t using Listners

Java AWT Widgets u Frame u Panel u Canvas u Button u Checkbox u Choice u Label u List u ScrollBar u TextArea u TextField

Java AWT development process u Steps: –Create a Container t An applet extends Panel and is therefore already a Container –Set the layout manager –Instantiate the components –Add the components to the container using the layout manager –Call show() to actually draw the components on the screen u Tricks: –Remember the containers can be NESTED t VERY USEFUL TRICK! –Use multiple nested layouts for complex user interfaces

Java LayoutManagers u We’ll only look at two today –BorderLayout t Provide North, South, East, West and Center regions –GridLayout t Divides the container into equally spaced rows and columns u Lets see some working Demos and explanations!! –Off to the Java Tutorial from here… –

Java Nesting in AWT u Key Concept: –A Container is also a Component t in most cases we care about for this class u Example Problem –Say we wish to create the layout shown on the right –Characteristics: t Button Bar on the left t TextField at the bottom t TextArea in the rest –Note: t buttons are equally spaced

Java Nesting in AWT (2) u Process –Layout the design you want on paper or a whiteboard! –Analyze the design to see what sections you can identify t Buttons on the left can be one possible piece t TextArea and TextField are the other two pieces –Work from top to bottom t identify a suitable layout for the big pieces identified above –BorderLayout (surprise!) Buttons in the West TextField in the South TextArea in the North t But what about the “Buttons” –GridLayout for the buttons! Here is where we’ll use nesting

Java Nesting in AWT (3) u Sample Solution –(see ButtonPad.java): // set the top level layout setLayout(new BorderLayout(3,3)); // create the text area notesArea = new WrappedTextArea(10, 30); notesArea.setEditable(false); // add the text area add("Center", notesArea); // create the text field entryField = new TextField(40); // add the entry field add("South", entryField); // create a NEW PANEL for NESTING! Panel buttonPanel = new Panel(); //set the layout in this panel buttonPanel.setLayout(new GridLayout(0, 1, 5, 5)); //create and add the buttons button1 = new Button("Button 1"); button2 = new Button("Button 2"); button3 = new Button("Button 3"); button4 = new Button("Button 4"); button5 = new Button("Button 5"); buttonPanel.add(button1); buttonPanel.add(button2); buttonPanel.add(button3); buttonPanel.add(button4); buttonPanel.add(button5); //add the panel to the west of the main BorderLayout add("West", buttonPanel);

Java Handling Events u Two approaches to handling events –we saw one approach earlier t overriding specific event handler methods such as mouseDown, mouseDrag etc. –today we will see the better approach t using Listners

Java Handling Events u Types of events: –ACTION_EVENT, LIST_SELECT, LIST_DESELECT, WINDOW_DESTROY, WINDOW_ICONIFY, WINDOW_DEICONIFY, WINDOW_MOVED, MOUSE_DOWN, MOUSE_UP, MOUSE_DRAG, KEY_PRESS, KEY_ACTION, KEY_RELEASE, KEY_ACTION_RELEASE, GOT_FOCUS, LOST_FOCUS, MOUSE_ENTER, MOUSE_EXIT, MOUSE_MOVE u All can be handled with special listners –We’ll use a MouseListner as an example u OR you can override the methods corresponding to each event –what we saw in the Scribble applet

Java Handling Events (2) public class MouseEventDemo... implements MouseListener {...//where initialization occurs: //Register for mouse events on blankArea and applet (panel). blankArea.addMouseListener(this); addMouseListener(this);...

Java Handling Events (3) u Continued ….. public void mousePressed(MouseEvent e) { saySomething("Mouse pressed; # of clicks: " + e.getClickCount(), e); } public void mouseReleased(MouseEvent e) { saySomething("Mouse released; # of clicks: " + e.getClickCount(), e); } public void mouseEntered(MouseEvent e) { saySomething("Mouse entered", e); } …….. }