Computer Science (CSCI) Department College of Engineering, Computer Science, & Technology (ECST)

JETT Workshops Object-Oriented Programming Fall 2004

(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann.
Visit www.wiley.com/college/horstmann for information on the current edition of this excellent text.)

• Interfaces vs. Classes

  • all methods in an interface are abstract--no implementation
  • all methods in an interface are automatically public
  • an interface doesn't have instance variables
    
    

• Event Classes
  

  • an event source: generates events and manages listeners
    • example: applet
      
  • a listener: notified when event happens
    • example: MouseListener
      
  • an event class: describes event
    • example: MouseEvent
      
      

• The MouseListener Interface

  public interface MouseListener { void mousePressed(MouseEvent event); //Called when a mouse button has been pressed on a component   void mouseReleased(MouseEvent event);   //Called when a mouse button has been released on a component   void mouseClicked(MouseEvent event); //Called when the mouse has been clicked on a component   void mouseEntered(MouseEvent event); //Called when the mouse enters a component void mouseExited(MouseEvent event); //Called when the mouse exits a component }

  
  
  • in order to receive mouse events from the event source
    • define a class that implements the MouseListener interface
    • create an object of that class
    • pass the reference to the object to the event source (Applet in this case)
    • the event source will use that reference to call the appropriate methods that your class defines
      
      
  • when a class implements an interface it is agreeing to provide the methods specified by the interface
    
    

    public class MouseSpy implements MouseListener { public void mousePressed(MouseEvent event) { System.out.println("Mouse pressed. x = " + event.getX() + " y = " + event.getY()); } public void mouseReleased(MouseEvent event) { System.out.println("Mouse released. x = " + event.getX() + " y = " + event.getY()); } public void mouseClicked(MouseEvent event) { System.out.println("Mouse clicked. x = " + event.getX() + " y = " + event.getY()); } public void mouseEntered(MouseEvent event) { System.out.println("Mouse entered. x = " + event.getX() + " y = " + event.getY()); } public void mouseExited(MouseEvent event) { System.out.println("Mouse exited. x = " + event.getX() + " y = " + event.getY()); } }

    
    
  • the event source can just store the reference as a MouseListener reference - it doesn't need to know anything else about it

• MouseSpy.java
• MouseSpyApplet.java
  
  

• Mouse Listener that Moves a Rectangle

  • typically an event listener will need to modify the private data of the class that wants the events
  • defining the listener class as an inner class gives it access to the outer class instance variables
  • here the listener class, MousePressedListener is defined as an inner class
  • the outer class is MouseApplet
    
    

    public MouseApplet() { ... //add mouse press listener class MousePressListener implements MouseListener { public void mousePressed(MouseEvent event) { int x = event.getX(); int y = event.getY(); box.setLocation(x,y); repaint(); } //do-nothing methods public void mouseReleased(MouseEvent event){} public void mouseClicked(MouseEvent event){} public void mouseEntered(MouseEvent event){} public void mouseExited(MouseEvent event){} } MouseListener listener = new MousePressListener(); addMouseListener(listener); ... }

    
    
    • this listener does the following
      • gets the mouse position from the event object
      • call setLocation on box, an instance variable of type Rectangle in the outer class
      • invokes repaint method on the outer class
      • repaint triggers paint method
        • you should always call repaint to trigger painting
        • don't call paint directly
    • notice the type of the variable listener is MouseListener - this is passed to the addMouseListener method
      
      
  • MouseApplet.java
    
    

• An Applet with a Control Panel

• control panel components
• uses Swing package javax.swing
• Swing class names start with J

  JTextField xField = new JTextField(width); JLabel xLabel = new JLabel("x = "); Icon buttonIcon = new ImageIcon("hand.gif"); JButton moveButton = new JButton("Move", buttonIcon);

  
  
• button listener
• a new interface - ActionListener
• the listener moves a rectangle when the button is clicked
• it reads xField, yField variables from enclosing scope

  public ButtonApplet() { ... class MoveButtonListener implements ActionListener { public void actionPerformed(ActionEvent event) { int x =Integer.parseInt(xField.getText()); int y =Integer.parseInt(yField.getText()); box.setLocation(x,y); repaint(); } }; ActionListener listener = new MoveButtonListener(); moveButton.addActionListener(listener); ... }

• placing components in a panel
• JPanel is a container for components
• you add components to the panel

  JPanel panel = new JPanel(); panel.add(xLabel); panel.add(xField); . . . // place the panel inside the frame JFrame frame = new JFrame(); frame.setContentPane(panel); // size frame to just contain the components and show frame.pack(); frame.show();

  
  
• ButtonApplet.java
• hand.gif - save this in the same directory
  
  

• Multiple Buttons with Similar Behavior

  • just for fun - now check out this one...
  • ButtonApplet.java