System development with Java

1. System development with Java Instructors: Rina Zviel-Girshin Lecture 10 Rina Zviel-Girshin @ARC

2. Overview • Applet API • GUI Rina Zviel-Girshin @ARC

3. What applet can do? • Load data files specified relative to the URL of the applet or the page in which it is running. • Display short status strings. • Play sounds. • Find other applets running in the same page. Rina Zviel-Girshin @ARC

4. Finding and Loading Data Files • Whenever an applet needs to load some data from a file it uses either the code base (getCodeBase()) or the document base to form the complete URL (getDocumentBase()). • getCodeBase() method returns a URL that specifies the directory from which the applet's classes were loaded • getDocumentBase() method returns the directory of the HTML page that contains the applet. Rina Zviel-Girshin @ARC

5. Image • Java supports two types of images: .gif and .jpeg • Images must first be loaded and then displayed. • Images are loaded to the applet using getImage() method. • getImage() method have two syntax forms: • Image my =getImage(url); • Image my = getImage(url, relativePath); • Images are drawn using • drawImage( image, x, y, ImageObserver) where the last argument can be this. Rina Zviel-Girshin @ARC

6. Example import java.awt.*; import java.applet.*; public class DrawImage extends Applet { Image image; //Image instance public void init() { //image is in subdirectory of the applet code image = getImage(getCodeBase(), "images/cat.gif"); } public void paint(Graphics g) { // drawing the image in the center of the applet g.drawImage(image, (this.getSize().width-image.getWidth(this))/2, (this.getSize().height-image.getHeight(this))/2, this); } } // end of class Rina Zviel-Girshin @ARC

7. Status Strings • Status string is a string that appears on the status line at the bottom of the applet viewer window. • Applets display status lines with the showStatus() method. • Syntax: showStatus(“String message”); • You should never put crucial information in the status line because status line can be overwritten by other applets or by the browser. Rina Zviel-Girshin @ARC

8. Example import java.applet.*; import java.awt.*; public class PaintStatus extends Applet { int count = 0; public void paint(Graphics g) { g.drawString("hello",this.getSize().height/2, this.getSize().width/2-10); count += 1; showStatus("paint() called " + count + " time" + ((count > 1) ? "s":"")); } public void update(Graphics g) { paint(g); } } Rina Zviel-Girshin @ARC

9. Sounds • To play a sound you use play() method of the Applet class. • Syntax: • play(url) • play(url, relativePath) • Main disadvantage: plays exactly once. Rina Zviel-Girshin @ARC

10. Example import java.awt.*; import java.applet.*;  public class PlayAudio extends Applet { public void init() { play(getCodeBase(), "images/train.au"); } public void paint(Graphics g) { g.drawString("This is a simple audio applet", 20, 20); } } Rina Zviel-Girshin @ARC

11. Inter-applet Communication • Applets can find other applets and send messages to them- inter-applet communication. • Many browsers require that: • the applets originate from the same server. • the applets originate from the same directory on the server (the same code base). • The Java API requires that the applets be running on the same html page. • An applet can find another applet either • by looking it up by name getApplet method • or by finding all the applets on the page getApplets method. Rina Zviel-Girshin @ARC

12. Finding by name • getApplet method finds applet by its name. • If method is successful it returns to the caller an Applet objects and then the caller can invoke methods on the object. • getApplet(String name) - returns a reference to the applet called name used in the Name attribute in the applet tag. • The name attribute should be added to applet tag. • <applet name=“rina”..> Rina Zviel-Girshin @ARC

13. Finding by name • We will show some examples later. Rina Zviel-Girshin @ARC

14. What your applet has access to? • getParameter(String)-if there are parameters as part of the applet's invocation • appletContext • a hook into the browser • showDocument(URL) / showDocument(URL, target) • showStatus(String) • getApplet() /getApplets() • HTML <applet codebase=url code=“Name.class" width=600 height=100> <param name=“name" value=“value"> </applet> Rina Zviel-Girshin @ARC

15. GUI • Components are graphical user interface (GUI) widgets like • labels, • menus, • windows, • buttons, • text fields, • applets, and more. • In Java all components are subclasses of java.awt.Component. Rina Zviel-Girshin @ARC

16. Label • The simplest component is java.awt. • A Label object is a component for placing text in a container. • A label displays a single line of read-only text. • Syntax: Label lname=new Label(“string”); • The text can be changed by the application, but a user cannot edit it directly. Rina Zviel-Girshin @ARC

17. Label • After creation Label should be added to the container (Panel, Applet,..). • Syntax: this.add(lname); • Stages of adding a component: • Declare the component • Initialize the component • Add the component to the layout. Rina Zviel-Girshin @ARC

18. Example import java.applet.*; import java.awt.*; public class LabelExample extends Applet { public void init() { Label mylabel = new Label("This is a Label"); this.add(mylabel); } } Rina Zviel-Girshin @ARC

19. Result Rina Zviel-Girshin @ARC

20. Components paint() • There is no paint() method in the previous applet. • But label gets drawn on the screen anyhow. • How does this happen? • Components know how to paint themselves. • When a container like an applet is repainted it calls the paint() method for all the components it contains. • java.awt.Label has its own paint() method which knows how to paint() itself. Rina Zviel-Girshin @ARC

21. Format • You can format your components. • You can set font, foreground color, background color, name… • Resizes this component so that it has new width and height. • Moves component to a new location. • Syntax: (setAttribute) • setSize() • setFont() • setLocation() Rina Zviel-Girshin @ARC

22. Example import java.awt.*; import java.applet.*; public class LabelExample1 extends Applet { public void init() { Label mylabel = new Label("Formatted label!"); mylabel.setForeground(Color.blue); mylabel.setBackground(Color.red); this.add(mylabel); } }//end class Rina Zviel-Girshin @ARC

23. Buttons • Button creation: Button bname=new Button(“String”); • After creation button should be added to the applet. • Button creation and addition can be done in one line: add(new Button(“String")); Rina Zviel-Girshin @ARC

24. Example import java.applet.*; import java.awt.*; public class ButtonExample extends Applet { public void init () { this.add(new Button("Button with no actions")); } } Rina Zviel-Girshin @ARC

25. Result Rina Zviel-Girshin @ARC

26. Actions • In last example button did not do anything when it was pressed. • But buttons do things when we press them. • So we need to specify what to do when button is pressed. • When the mouse is clicked on a Button, the Button fires an ActionEvent. • To be ready to respond to this event we must register an ActionListener with the Button. Rina Zviel-Girshin @ARC

27. Action Listener • Syntax: Button b= new Button(“Button with action"); add(b); b.addActionListener(myActionListener); // assign the button a listener • where myActionListener is a reference to an object which implements the java.awt.event.ActionListener interface. • This interface has a single method: actionPerformed() Rina Zviel-Girshin @ARC

28. Action Listener • Since ActionListener is an interface and not a class, it can be implemented in our applet. • Applet signature will be: public class Name extends Applet implements ActionListener { … } • Method actionPerformed() will be implemented inside the applet. Rina Zviel-Girshin @ARC

29. Example import java.awt.*; import java.applet.*; import java.awt.event.*; public class ButtonAL extends Applet implements ActionListener { public void init () { Button b = new Button("Change applet background"); this.add(b); b.addActionListener(this); } public void actionPerformed(ActionEvent e) { this.setBackground(Color.green); } } Rina Zviel-Girshin @ARC

30. Mouse Listener • What about the mouse? • MouseListener interface listen to the mouse events (press, release, click, enter, and exit) on a component. • The listener object added to the component using addMouseListener method. • When a mouse event occurs, the relevant method in the listener object is invoked, and the MouseEvent is passed to it. Rina Zviel-Girshin @ARC

31. Example import java.awt.*; import java.applet.*; import java.awt.event.*; public class MouseClick extends Applet implements MouseListener { int xpos, ypos; public void init() { addMouseListener(this); } public void paint(Graphics g) { g.drawString("("+xpos+","+ypos+")",xpos,ypos); } Rina Zviel-Girshin @ARC

32. Example //methods of MouseListener interface public void mouseClicked (MouseEvent me) { xpos = me.getX(); ypos = me.getY(); repaint(); } //empty methods public void mousePressed (MouseEvent me) {} public void mouseReleased (MouseEvent me) {} public void mouseEntered (MouseEvent me) {} public void mouseExited (MouseEvent me) {} } Rina Zviel-Girshin @ARC

33. MouseMotionListener • The listener interface for receiving mouse motion events on a component: MouseMotionListener. • A mouse motion event is generated when the mouse is moved or dragged. • MouseMotionListener interface has 2 methods: • mouseDragged • mouseMoved Rina Zviel-Girshin @ARC

34. Example import java.awt.*; import java.applet.*; import java.awt.event.*; public class MouseMotion extends Applet implements MouseMotionListener { int xpos,ypos; public void init() { addMouseMotionListener(this); } public void paint(Graphics g) { g.drawString("("+xpos+","+ypos+")",xpos,ypos); } Rina Zviel-Girshin @ARC

35. Example //MouseMotionListener methods public void mouseMoved(MouseEvent me) { xpos = me.getX(); ypos = me.getY(); repaint(); } public void mouseDragged(MouseEvent me){} } Rina Zviel-Girshin @ARC

36. Any Questions? Rina Zviel-Girshin @ARC