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Graphical User Interfaces (Part I)

Graphical User Interfaces (Part I). Introduction to events. A Brief history. Event sources and listeners. The delegation model. Example 1: Handling key events. Example 2: Handling mouse events. Adapter classes. Coming Next: GUI Programming (Part II). Introduction to Events.

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Graphical User Interfaces (Part I)

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  1. Graphical User Interfaces (Part I) • Introduction to events. • A Brief history. • Event sources and listeners. • The delegation model. • Example 1: Handling key events. • Example 2: Handling mouse events. • Adapter classes. • Coming Next: GUI Programming (Part II). Gui Programming (Part I)

  2. Introduction to Events • GUI applications are event-driven. User interaction with the GUI results in events being generated to inform the application of user actions. Clicking a button, closing a window, or hitting a key results in an appropriate event being sent to the application • Events can be generated as a consequence of a person interacting with graphics components of a program. Examples: • Keyboard events (key up, key down) • Mouse events (mouse move, mouse drag, button up, button down). • Button events (user presses a button) • TextField events (user presses a return) • Menu events (user selects an item from a pull-down menu) • Events can also happen in cases where there is no direct user interaction. For example, an event may be generated when a timer expires, a counter exceeds a value, a software or hardware failure occurs, or an application is completed. Gui Programming (Part I)

  3. Introduction to Events (Cont’d) • When studying events and events handling, there are three main categories of classes to deal with: • Event classes. • Event source classes. • Event listener classes. • Event classes represent events and provide a consistent, easy-to-use means of encapsulating events. • Event sources are graphics components (like Buttons, Scollbar, Window) which generate and manage event listeners. For example, you receive key and mouse events from an applet (see Examples 1 and 2 in this lecture) • Listener classes are classes that implement one or more listener interfaces defined by the java.awt.event package. • Event class (e.g. MouseEvent). Contains information about the event (e.g. mouse location). • Event listener. Knows what to do when the event occurs. Implements listener interface (e.g. MouseListener). • Event source. Keeps a list of listeners and notifies them when the event occurs. Gui Programming (Part I)

  4. A Brief History • The original AWT (Abstract Windowing Toolkit)was suitable for Java applets but not for full-fledged application development. • AWT 1.1 (JDK 1.1) had better event handling but did not have enough GUI components and was too dependent on (nonportable) native code,. • In 1997 Netscape and Sun developed a set of GUI classes written entirely in Java. The Java Foundation Classes (JFC), including the Swing component set, were released with JDK 2.0. • A Swing program can have the same look and feel on a Mac, Windows, or Unix platform. Gui Programming (Part I)

  5. Event Classes & Source classes Gui Programming (Part I)

  6. Java Events Classes • At the root of the Java event class hierarchy is EventObject, which is in java.util (also see Horstmann Chapter 10). • The EventObject contains two methods: getSource() and toString() which return the source event and the string equivalent of the event respectively. • Event classes contain methods for getting information on the event, for example the MouseEvent class has methods getX() and getY() returning the coordinates of the mouse when the event occurs. • Here are some of the event classes which subclass EventObject: • ActionEvent : A component-defined action occurred. • ComponentEvent : A component moved, changed size, or changed visibility. • FocusEvent : A component has gained or lost the keyboard focus. • KeyEvent : A keystroke occurred in a Component. • MouseEvent : A mouse action occurred in a Component. • TextEvent : An object’s text changed. • WindowEvent : A window has changed its status. Gui Programming (Part I)

  7. Event Sources and Listeners • An object that generates an event is called a source. A source may generate more than one event. • When an event occurs (or is "fired"), it is received by one or more "listeners" which act on the event. • Note that an event source and the place where the event is handled can be separate. • Each event listener is an object of a class that implements a particular type of listener interface. • A listener class must implement the appropriate interface and all the event handling logic will go inside the listener class. • The most commonly handled events are those generated by the mouse, the keyboard, and various controls, such as a push button. Gui Programming (Part I)

  8. Event Sources and Listeners (Cont’d) • A source must register listeners in order for the listeners to receive notifications about a specific event. • The source object maintain la list of listener. example. keyboard event implements KeyListener interface Registration methods: are dependent on event type . Example KeyListener is addKeyListener . ActionEvent is addActionEvent • Each component type has the following pair of methods for registering and unregistering listeners • public void addTypeListener (TypeEvent t) throws java.util.TooManyListnersException • public void removeTypeListener (TypeEvent t) • Here, Type is the name of the event and t is a reference to the event listener. the TooManyListenersException is thrown in case of sources that allow only one listener to register. • Notifying many registered listeners for an event is called broadcasting the event. Event notification in sources that can have only one listener sources is called unicasting. Gui Programming (Part I)

  9. The Delegation Model mouseEvent keyEvent windowEvent Listener implements specific event handling method Events are sent from single source object 1Define a class that implements the appropriate listener interface your class provides the handler by implementing the methods that that are declared by the interface. Ex public class K implements KeyListener…MouseLisener…WindowListener. Implement keyPressed,keyReleased, keyTyped 2-Register an instance of the class with the component affected by the event Ex. void addxxxListener(xxListener object). Gui Programming (Part I)

  10. Example 1: Handling Key Events • In order to handle keyboard events, we must implement the KeyListener interface: void keyPressed(KeyEvent ke); void keyReleased(KeyEvent ke); void keyTyped(KeyEvent ke); • Note that when a key is pressed a KEY_PRESSED event is generated which results in calling the keyPressed() event handler. • When the key is released a KEY_RELEASED event is generated which results in calling the keyReleased() event handler. • If a character is generated by the keystroke, then a KEY_TYPED event is sent and the keyTyped() handler is invoked. • Thus, you will notice that, each time the user presses a key, at least two and often three events are generated. If you are interested in all these events then you must put the appropriate code in all these methods to achieve the behavior you desire. Gui Programming (Part I)

  11. Example 1: Handling Key Events (Cont’d) public void init(){ // pass the ref this to add itself as // a key listener addKeyListener(this); //The following method requests that //this component gets the input focus. //The component must be visible on the //screen for this request to be granted requestFocus(); } public void paint(Graphics g){ g.drawString(msg,startX,startY); } } import java.applet.Applet; import java.awt.*; import java.awt.event.*; public class KeyEventHandler extends Applet implements KeyListener{ private String msg = ""; private int startX = 10, startY = 10; public void keyPressed(KeyEvent ke){ showStatus("Key Down"); } public void keyReleased(KeyEvent ke){ showStatus("Key Up"); } public void keyTyped(KeyEvent ke){ msg += ke.getKeyChar(); repaint(); } Gui Programming (Part I)

  12. Example 2: Handling Mouse Events • In order to handle mouse events, we must implement the MouseListener interface which has these five methods: void mouseClicked(MouseEvent me); void mouseEntered(MouseEvent me); void mouseExited(MouseEvent me); void mousePressed(MouseEvent me); void mouseReleased(MouseEvent me); If mouse is clicked and released at the same point, mouseClicked() is invoked. When the mouse enters a component, the mouseEntered() method is called. When it leaves, mouseExited() is called. • The mousePressed() and mouseReleased() are invoked when the mouse is pressed and released respectively Gui Programming (Part I)

  13. Example 2: Handling Mouse Events (Cont’d) import java.applet.Applet; import java.awt.event.MouseEvent; import java.awt.event.MouseListener; public class MouseEventHandler extends Applet implements MouseListener{ public void mouseClicked(MouseEvent me) { System.out.println("Clicked at ("+me.getX()+", "+me.getY()+")"); } public void mouseEntered(MouseEvent me) { System.out.println("Entered at ("+me.getX()+", "+me.getY()+")"); } public void mouseExited(MouseEvent me) { System.out.println("Exited at ("+me.getX()+", "+me.getY()+")"); } public void mousePressed(MouseEvent me) { System.out.println("Pressed at ("+me.getX()+", "+me.getY()+")"); } public void mouseReleased(MouseEvent me) { System.out.println("Released at ("+me.getX()+", "+me.getY()+")"); } public void init() { addMouseListener(this); }} Gui Programming (Part I)

  14. Mouse Event Example Gui Programming (Part I)

  15. Introduction to Adapter Classes In order to make the button work, we need a WindowEvent listener. The obvious way to get such a listener is to write a listener class that implements the WindowListener interface. That turns out to be a bad idea., though. Implementing this interface requires writing seven methods(windowActivated()windowclosed(), windowIconified,windowOpened.., one for each type of window event. We’re interested in just one of these events, so we’d end up writing six methods for events that we don’t need. The java.awt.event package contains a class named WindowAdapter. This class implements the WindowListener interface, although the methods that it provides are all empty. All we have to do is extend the WindowAdapter class and override the WindowClosing method, which is the only one we’re interested in. WindowAdapter is an example of an adapter class . A class that we can extend instead of implementing an interface. Java provides matching adapter classes for most interfaces that have two or more methods. Gui Programming (Part I)

  16. Introduction to Adapter Classes (Cont’d) • When an interface contains only one method, there’s no need for an adapter class • Adapter classes save programmer’s time by providing an empty implementation of Listener’s methods • To use an Adapter you need to subclass it and override only the methods which you wish to use. • You can define an Adapter class as an inner, anonymous inner or as an external class. • Some adapter classes are: • java.awt.event.ComponentAdapter • java.awt.event.FocusAdapter • java.awt.event.KeyAdapter • java.awt.event.MouseAdapter • java.awt.event.MouseMotionAdapter • java.awt.event.WindowAdapter Gui Programming (Part I)

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