Applications of Java to Physics Teaching (Part II)

1. Applications of Java to Physics Teaching (Part II) S S Tong Department of Physics CUHK

2. 6. Simple Animations • Animations • many frames displayed in a given order • Thread • part of the program running on its own • multi-tasking (actually multi-threading) • How to • create a thread? • display the frames? • terminate the thread under certain conditions?

3. calls the run() method • Implement the interface Runnable • Declare a Thread public class Spin extends Applet implements Runnable { Thread runner = null; ...... } • Load a series of image for (int i = 0; i < 16; i++) spin[i] = getImage(getCodeBase(), "images/Spin" + i + ".gif"); • Create and run a Thread runner = new Thread(this); runner.start();

4. Create a loop, call the paint method repeatedly public void run() { int i = 0; while (runner != null) { currentImage = spin[i]; repaint(); i++; if (i > 15) i = 0; pause(50); } } public void paint(Graphics screen) { if (currentImage != null) { screen.drawImage( currentImage, 100, 100, this); ...... } }

5. call paint(...)once init() images loaded ...getImage(...) Thread created and initialized ...new Thread(this) runner.start() run() paint(...) loop inside while (...) { ...... } draw images repaint() • Flow of the applet • Stop a thread runner.stop(); runner = null;

6. call the paint method directly • Reduce flickering • Override the update(...) method, so that the screen is not erased before repainting public void update(Graphics screen) { paint(screen); } • Create an off-screen image and a Graphics object • do all drawing on the off-screen image first • replace the on-screen image with the updated off-screen image ...... Graphics offScreen; Image offScreenImage; public void init() { offScreenImage = createImage(300,300); ......

7. associate the off-screen image and Graphics erasing off-screen drawing off-screen put the off-screen image on-screen • Reduce flickering (contiune) public void paint(Graphics screen) { j++; offScreen = offScreenImage.getGraphics(); if (currentImage != null) { offScreen.setColor(Color.white); offScreen.fillRect(0,0,300,300); offScreen.drawImage(currentImage, 100, 100, this); offScreen.setColor(Color.red); offScreen.drawString("Frame no " + j, 30, 30); screen.drawImage(offScreenImage, 0, 0, this); } }

8. 7 Building simple interface • Basic components Label TextField Button Checkbox Choice

9. items contents length • Adding components to an applet • Declare the object type Button theButton; Checkbox theCheckbox; Choice theChoice; TextField theField; • Assign labels and attributes to the components theButton = new Button("I am a button"); theCheckbox = new Checkbox("I am a checkbox"); theChoice = new Choice(); theChoice.add("I am item 1"); theChoice.add("I am item 2"); theChoice.add("I am item 3"); theField = new TextField("long field", 20); • Add the components to the applet add(...);

10. set background color set size draw something on the canvas • Canvas - an area for drawing • create a subclass of Canvas class MyCanvas extends Canvas { MyCanvas() { setBackground(Color.cyan); setSize(300,300); } public void paint(Graphics screen) { ...... } } • create an instance of this subclass in the applet canvas = new MyCanvas(); • repaint the canvas canvas.repaint();

11. North West Center East South • How to arrange the components? • Use Layout Manager • We discuss the BorderLayout() only public void init() { setLayout(new BorderLayout()); ......

12. add("Center", canvas); Panel p = new Panel(); p.add(theLabel); p.add(theButton); ...... add("South",p) • Adding components add("South", buttonA); add("Center", canvasB); • Using a panel

13. ActionListener for Button ItemListener for Choice and Checkbox Listeners are interfaces • Firing events from the components • Listeners: • Implementing the Listeners import java.awt.event.*; public class EventTest extends Applet implements ActionListener, ItemListener { ...... • Adding Listeners to the component button1.addActionListener(this); theChoice.addItemListener(this); theCheckbox.addItemListener(this);

14. identify which component triggers the event get the index of the selected item get the state (true/false) of the checkbox • Buttons trigger the actionPerformed method public void actionPerformed(ActionEvent evt) { if (evt.getSource() == button1) ...... if (evt.getSource() == button2) ...... canvas.repaint(); } • Choice menus and checkboxes trigger itemStateChanged method public void itemStateChanged(ItemEvent evt) { if (evt.getSource() == theChoice) { int itemNo = theChoice.getSelectedIndex(); ....... if (evt.getSource() == theCheckbox) { boolean boxState = theCheckbox.getState(); .......

15. EventCanvas is defined inside EventTest - an inner class • Updating the canvas import java.awt.event.*; ........ public class EventTest extends Applet implements ActionListener, ItemListener { String currentString = ""; ...... public void actionPerformed(ActionEvent evt) { ...... canvas.repaint(); } public void itemStateChanged(ItemEvent evt) { ...... canvas.repaint(); } class EventCanvas extends Canvas {....... screen.drawString(currentString, 50, 50); ...... } }

16. gives the text of field field.getText() field.setText("string") sets the text of field to "string" call to the superclass's constructor string  double double  string • Number I/O for a TextField • Converting numbers to strings and vice versa class MyTextField extends TextField { MyTextField(String s, int l) { super(s,l); } double getNumber() { return Double.valueOf( getText()).doubleValue(); } void setNumber(double num) { setText(String.valueOf(num)); } }

17. Example AnimateWaves.java • superposition of two waves • amplitude, wavelengths, periods read from text fields • mode of display - choice menu • start and stop - buttons • Exercise ProjectileEx.java • initial position and velocity read from text fields • start and reset - buttons • Example ShootHead.java • Modification of Projectile.java • Shooting a free-falling object

18. MouseListener mouse pressed, released, enter, exit mouse moved, dragged MouseMotionListener Listeners are interfaces • Listening to mouse events • Listeners: • Implementing the Listeners import java.awt.event.*; public class EventTest extends Applet implements MouseListener, MouseMotionListener { ...... • Adding Listeners to the component canvas.addMouseListener(this); canvas.addMouseMotionListener(this);

19. Let the canvas listens to mouse events • Interact with hot spots, drag and drop objects etc. • Mouse events trigger public void mousePressed(MouseEvent evt) {...} public void mouseClicked(MouseEvent evt) {...} public void mouseReleased(MouseEvent evt) {...} public void mouseEntered(MouseEvent evt) {...} public void mouseExited(MouseEvent evt) {...} • Mouse motion events trigger public void mouseMoved(MouseEvent evt) {...} public void mouseDragged(MouseEvent evt) {...} • Gets the mouse position inside this methods e.g., Point p = evt.getPoint();

20. Example AddVectors.java • addition and subtraction of two vectors • changing the text fields updates the screen • dragging the vectors updates the text fields

21. 8 References • L. Lemay and R. Cadenhead, Teach Yourself Java 2 in 21 days, SAMS • M. Campione and K. Walrath, The Java Tutorial (2nd Ed), Addison Wesley • http://java.sun.com/