Session 1: Classes and Applets

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1 Session 1: Classes and Applets

2 Literature Sprechen Sie Java, ISBN , dpunkt deutsch Java für Studenten, ISBN , PearsonStudium deutsch Java in a Nutshell, ISBN: , O'Reilly english Java Examples in a Nutshell, ISBN , O'Reilly english 3

deutsch Java in a Nutshell, ISBN: 0-59600-283-1, O'Reilly

3 Selbsttest: Klassen Wozu dienen Klassen? Vor-/ Nachteile von Datenabstraktion Was ist beim Aufruf einer Methode immer automatisch dabei, wenn auch versteckt? Was passiert, wenn ein Objekt mit dem Aufruf Point p = new Point(); angelegt wird? Import und Export Was kann importiert und exportiert werden? Wie wird es importiert bzw. exportiert? Welches Problem könnte man bei statischen Initialisierungsblöcken erzeugen? 4

auch versteckt? Was passiert, wenn ein Objekt mit dem Aufruf Point p = new Point(); angelegt wird?

4 Programmieraufgabe: Klassen Implementieren Sie eine Klasse Time zur Speicherung von Uhrzeiten. Jedes Time-Objekt soll eine Zeit in Form von Stunden und Minuten speichern. Es soll folgende Methoden geben: Sinnvoller Konstruktor Addieren einer Zeit zu einer Anderen (z.b.: 5h 42min + 3h 27min = 9h 9min) Berechnung der Differenz zwischen zwei Zeiten in Minuten. (z.b.: 5h 20min 3h 10min = 130min) Umrechnung einer Zeit in Minuten (z.b.: 3h 20min = 200min) 5

5 Applets Motivation? We introduce Applets first, in order to enhance the visual input and output capabilities of our sample programs! Applets can run within a Web-Browser or inside the appletviewer AppletContainer.html: <html> <head> <title>title of the web page</title> </head> <body> <applet code= AppletClassName.class width="100" height="100"></applet> </body> </html> Start with Appletviewer: 6

sample programs! Applets can run within a Web-Browser or inside the appletviewer AppletContainer.

6 Applets: GUI Container An applet is derived from java.awt.container i.e. it is a container for assembling GUI-widgets: setlayout(layoutmanager mgr) add(component guicomp) remove(component guicomp) removeall() 7

7 Applets: AWT Example import java.applet.*; import java.awt.*; public class HelloWorldApplet extends Applet { public void init() { setlayout(new BorderLayout()); add(new Label("Hello World!")); 8

8 Applets: AWT The AWT package offers following GUI-widgets: Button Canvas Checkbox Choice Label List Panel Scrollbar TextArea TextField LayoutManagers (can be combined to create new layout styles) BorderLayout FlowLayout GridLayout GridBagLayout 9

TextArea TextField LayoutManagers (can be combined to create

9 Applets: AWT Events and Listeners To react on GUI-events, it is necessary to register as an EventListener EventListener has to implement an EventListenerInterface Example: Listen on button clicks Create Widget: Button b = new Button( Click me! ); Register listener with Button: b.addactionlistener(actionlistenerobject); Class of actionlistenerobject implements the ActionListener interface: XYZClass implements ActionListener { public void actionperformed(actionevent eve) { // react on event 10

10 Applets: Painting Two dimensional painting of points, lines, circles, images,... Painting on Applet surface with method paint(graphics g) import java.applet.*; import java.awt.*; public class PaintApplet extends Applet { public void paint (Graphics g) { g.drawline(100,100,200,200); 11

.. Painting on Applet surface with method paint(graphics g) import java.

11 Applets: Lifecycle import java.applet.*; import java.awt.*; public class Lifecycle extends Applet { // Called when the applet has to paint its surface (Resize,...) public void paint(graphics g) { System.out.println("paint"); // Called when the applet initializes its resources public void init() { System.out.println("init"); // Here the work happens public void start() { System.out.println("start"); // Called when the applet stops (user surfs to another page,...) public void stop() { System.out.println("stop"); // Called when the applet has to free its resources (Browser closes,...) public void destroy() { System.out.println("destroy"); 12

out.println("start"); // Called when the applet stops (user surfs to another page,...) public void stop() { System.out.println("stop"); // Called when the applet has to free its resources (Browser closes,.

12 Applets: Sandbox Applets are code that comes from the net and then runs locally at the client side. Client side code could harm the client computer (format c:, send to everybody,...) To avoid harmful operations, Applets run under the control of a SecurityManager ("in a Sandbox") From within the Sandbox the following is not allowed: Read, write or change the local harddisk Open network connections to hosts (except to the origin host) Load any libraries from the client side... 13

..) To avoid harmful operations, Applets run under the control of a SecurityManager ("in a Sandbox") From within the

13 Applets: 2D-Drawing Demo During development you only need to start the appletviewer once! "Reload" updates the displayed applet class. 14

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