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Event Handling Inner classes Event Handling and Inner Classes GUI Programming

Week Four. Event Handling Inner classes Event Handling and Inner Classes GUI Programming Threads and Web Servers. Event Handling. Used by the Abstract Window Toolkit (AWT) – for basic GUI programming Java 1.0

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Event Handling Inner classes Event Handling and Inner Classes GUI Programming

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  1. Week Four • Event Handling • Inner classes • Event Handling and Inner Classes • GUI Programming • Threads and Web Servers Carnegie Mellon University, Graduate School of Industrial Administration

  2. Event Handling • Used by the Abstract Window Toolkit (AWT) – for basic GUI • programming Java 1.0 • Used by Swing -- Better components than AWT Java 2 • Used by JavaBeans -- reusable software components, like • Visual Basic, that can be manipulated • in a builder tool Carnegie Mellon University, Graduate School of Industrial Administration

  3. Babysitting A baby in the home needs attention. Many events surrounding the baby are not easily ignored. Events can be of different types. Carnegie Mellon University, Graduate School of Industrial Administration

  4. One possible source of events Carnegie Mellon University, Graduate School of Industrial Administration

  5. Babysitting Before responding to an event, a babysitter typically takes note of the source and the type of event. Carnegie Mellon University, Graduate School of Industrial Administration

  6. Babysitting Handler Event type Event source The sitter needs to know both the event type and the event source. Carnegie Mellon University, Graduate School of Industrial Administration

  7. Event Handling The window manager software may generate hundreds of different events. Examples include mouse clicks, mouse movements, key strokes, and timer ticks. A programmer is typically interested in a small subset of all of the possible events that may occur. Carnegie Mellon University, Graduate School of Industrial Administration

  8. Events and Event Objects • Since events may be of different types but still exhibit some • shared traits (inheritance) Java represents the event classes • in a hierarchy. • The root class is called java.util.EventObject. The only common • feature shared by all events is a source object. So we find the • following method in the EventObject class : • public Object getSource(); Carnegie Mellon University, Graduate School of Industrial Administration

  9. Object Some classes and methods in the event hierarchy. EventObject AWTEvent ActionEvent ComponentEvent String getActionCommand() WindowEvent InputEvent boolean isAltDown() Window getWindow() MouseEvent KeyEvent int getX() char getKeyChar() Carnegie Mellon University, Graduate School of Industrial Administration

  10. Event handling usually involves three types of objects • Objects are used to hold and report on information about the event. • Objects are typically the source of events. • Objects, called listeners, are used to handle events. Carnegie Mellon University, Graduate School of Industrial Administration

  11. Example A mouse object An event object that describes, say, the x and y coordinate of where the mouse was clicked Event data and methods Event Source Listener The listener object has methods that are called for particular events Carnegie Mellon University, Graduate School of Industrial Administration

  12. Example A mouse object An event object that describes, say, the x and y coordinate of where the mouse was clicked Implements MouseListener A mouse object must be told who its listener is. The listener object has methods that are called for particular events The event object is sent to a listener method Carnegie Mellon University, Graduate School of Industrial Administration

  13. The Listener • There are different types of Listeners. • MouseListeners • WindowListeners • ScrollBarListeners • Etc.. • These Listeners are interfaces. Remember what an interface • provides? If class X implements an interface then class X promises • to provide (at least) the methods declared in the interface. Carnegie Mellon University, Graduate School of Industrial Administration

  14. Some of the Listener Hierarchy EventListener KeyListener ActionListener MouseListener abstract void actionPerformed(ActionEvent e); abstract void mouseClicked(MouseEvent e) Carnegie Mellon University, Graduate School of Industrial Administration

  15. A Listener Interface public interface MouseListener { void mouseClicked(MouseEvent e); void mouseEntered(MouseEvent e); void mouseExited(MouseEvent e); void mouseReleased(MouseEvent e); } //What does it mean if I claim that I implement this interface? Carnegie Mellon University, Graduate School of Industrial Administration

  16. An Example // MouseSpyApplet.java import java.applet.Applet; import java.awt.event.MouseEvent; import java.awt.event.MouseListener; Carnegie Mellon University, Graduate School of Industrial Administration

  17. class MouseSpy implements MouseListener { 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()); } 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()); } } I have to provide ALL of these methods!! Carnegie Mellon University, Graduate School of Industrial Administration

  18. public class MouseSpyApplet extends Applet { public MouseSpyApplet() { MouseSpy listener = new MouseSpy(); addMouseListener(listener); } } Carnegie Mellon University, Graduate School of Industrial Administration

  19. Java Applets need an HTML file. <html> <head> </body> </head> <B>Spying on the mouse</b><p> <applet code="MouseSpyApplet.class" width=400 height=200> </applet> </html> Carnegie Mellon University, Graduate School of Industrial Administration

  20. Another approach Suppose a friendly sole created this class: public class MouseAdapter implements MouseListener { void mouseClicked(MouseEvent e){} void mouseEntered(MouseEvent e){} void mouseExited(MouseEvent e){} void mouseReleased(MouseEvent e){} } Now, suppose I extend this class. What must I provide? Carnegie Mellon University, Graduate School of Industrial Administration

  21. Only those methods that I am interested in. The other methods, when called, do nothing. We’ll visit this issue again later. Carnegie Mellon University, Graduate School of Industrial Administration

  22. Inner Classes • Nested Top Level Classes (not inner) • Member Classes • Local Classes • Anonymous Classes Carnegie Mellon University, Graduate School of Industrial Administration

  23. Nested Top Level Class • Nested top-level classes are not inner classes. • Use as a convenient way to group related classes • Since the class must be static and has no 'this' pointer, it • has no access to the instance data of objects for its • enclosing class. • It behaves just like a 'normal' class or interface. Carnegie Mellon University, Graduate School of Industrial Administration

  24. //NestedTopLevelExample.java class Top { int i,j; static class SomeClass { // static makes it top-level nested int k; SomeClass() { System.out.println("Constructing SomeClass"); } void foo() { System.out.println("Hello"); } } Top() { System.out.println("Constructing a Top object"); } } Carnegie Mellon University, Graduate School of Industrial Administration

  25. public class NestedTopLevelExample { public static void main(String args[]) { Top myTop = new Top(); Top.SomeClass myObject = new Top.SomeClass(); myObject.foo(); } } Output Constructing a Top object Constructing SomeClass Hello Carnegie Mellon University, Graduate School of Industrial Administration

  26. Member Classes • Member classes (there is no such thing as a 'member‘ • interface) • This inner class (it's not a top-level class) has no static • keyword and can access the members of each object of • its outer class. • The class 'appears in every instance'. Carnegie Mellon University, Graduate School of Industrial Administration

  27. The parent class must declare an instance of an inner class, before it can invoke the inner class methods, assign to data fields (including private ones), and so on. • Unlike nested top-level classes, inner classes are not directly part of a package and are not visible outside the class in which they are nested. • Inner classes are often used for GUI event handlers. Carnegie Mellon University, Graduate School of Industrial Administration

  28. // MemberClassExample.java class Top { int i = 33; public class SomeClass { // access the outer object's state. private int k = i; SomeClass() { System.out.println("Constructing SomeClass"); } void foo() { System.out.println("Hello"); } } Top() { System.out.println("Constructing a Top object"); SomeClass sc = new SomeClass(); System.out.println(sc.k); } } Carnegie Mellon University, Graduate School of Industrial Administration

  29. public class MemberClassExample { public static void main(String args[]) { Top myObject = new Top(); } } // OUTPUT Constructing a Top object Constructing SomeClass 33 Carnegie Mellon University, Graduate School of Industrial Administration

  30. Local Classes • A Local class is an inner class. Typically, a local class • is declared within a method. It is not a member of an • enclosing class. It is visible only within the block. • These classes are used primarily as "adapter classes". • For example, a block of code that creates a Button object • could use a local class to define a simple implementation • of the ActionListener Interface. Then it could instantiate • this simple implementation and pass the resulting object • to the button's ActionListener method, thereby connecting • the button to the "callback" code that is executed when • the button is pressed. Carnegie Mellon University, Graduate School of Industrial Administration

  31. // Local Class example class Top { int i = 33; Top() { System.out.println("Constructing a Top object"); // define a class within a method class Wow { int t; Wow() { System.out.println("Building a Wow"); i = 8; t = 9; } } Wow h = new Wow(); System.out.println(" h.t == " + h.t); System.out.println(" i == " + i); } } Carnegie Mellon University, Graduate School of Industrial Administration

  32. public class LocalExample { public static void main(String args[]) { Top myObject = new Top(); } } // OUTPUT Constructing a Top object Building a Wow h.t == 9 i == 8 Carnegie Mellon University, Graduate School of Industrial Administration

  33. Anonymous Classes • An anonymous class is refinement of inner classes. • It allows you to combine the definition of the class • with the instance allocation. • Since it is instantiated in the same expression that defines • it, it can only be instantiated once. This is very similar to • local classes. • When writing a simple adapter class, the choice between • a named local class and an unnamed anonymous class • typically comes down to a matter of style and code clarity, • rather than any difference in functionality. • The new class can't have a constructor. Carnegie Mellon University, Graduate School of Industrial Administration

  34. // Anonymous.java interface SmallClass { public void foo(); } class Top { int i = 33; void someMethod(SmallClass s) { s.foo(); } void anotherMethod() { someMethod(new SmallClass() { public void foo() { System.out.println("Really fun"); } }); } Carnegie Mellon University, Graduate School of Industrial Administration

  35. Top() { System.out.println("Constructing a Top object"); someMethod(new SmallClass() { public void foo() { System.out.println("Strange but fun"); } }); } } Carnegie Mellon University, Graduate School of Industrial Administration

  36. public class Anonymous { public static void main(String args[]) { // We can't create interface objects // error: SmallClass s = new SmallClass(); Top myObject = new Top(); myObject.anotherMethod(); } } // OUTPUT Constructing a Top object Strange but fun Really fun Carnegie Mellon University, Graduate School of Industrial Administration

  37. Event Handling and Inner Classes register Source Object Listener Object Event object fire events After the listener object registers itself with the source object, the source object calls a method found in the listener object and passes an object that describes the event. Carnegie Mellon University, Graduate School of Industrial Administration

  38. Event Handling Suppose we have a Button object Button b = new Button(); We must determine what events a particular component generates. A Button component may generate an ActionEvent object. Button b ActionEvent Object Carnegie Mellon University, Graduate School of Industrial Administration

  39. Implementing a Listener • We need an object that will listen for the ActionEvent. • We implement the ActionListener class (this class listens • for ActionEvents from buttons, menus, etc.) and override • its actionPerformed method. class BabySitter implements ActionListener { public void actionPerformed(ActionEvent e) { // handle the event object e in some way } } Carnegie Mellon University, Graduate School of Industrial Administration

  40. Create a BabySitter object BabySitter sitter = new BabySitter(); sitter Carnegie Mellon University, Graduate School of Industrial Administration

  41. Registering The Listener • We want to listen for the ActionEvent object coming from the button. • So, we tell the button what object will listen for the ActionEvent object: • b.addActionListener(sitter) Carnegie Mellon University, Graduate School of Industrial Administration

  42. The button and its listener addActionListener Button Object Sitter Object The button calls the actionPerformed() method of the sitter object and passes an ActionEvent object as a parameter Carnegie Mellon University, Graduate School of Industrial Administration

  43. Once again but with a window Hit the X and the program exits Carnegie Mellon University, Graduate School of Industrial Administration

  44. Create a WindowCloseSitter Class import javax.swing.*; import java.awt.event.*; public class CloseDemo { public static void main(String[] args) { JFrame f = new JFrame("Example"); f.setSize(400,100); f.setVisible(true); WindowCloseSitter h = new WindowCloseSitter(); f.addWindowListener(h); } } Carnegie Mellon University, Graduate School of Industrial Administration

  45. But we have to implement ALL of the functions !! class WindowCloseSitter implements WindowListener { public void windowClosing(WindowEvent e) { System.exit(0); } public void windowClosed(WindowEvent e) { } public void windowOpened(WindowEvent e) { } public void windowIconified(WindowEvent e) { } public void windowDeiconified(WindowEvent e) { } public void windowActivated(WindowEvent e) { } public void windowDeactivated(WindowEvent e) { } } Carnegie Mellon University, Graduate School of Industrial Administration

  46. Java Provides Adapter Classes • ComponentAdapter • MouseMotionAdapter • WidowAdapter • ContainerAdapter • MouseAdapter • FocusAdapter • KeyAdapter Carnegie Mellon University, Graduate School of Industrial Administration

  47. The WindowAdapter class public abstract class WindowAdapter implements WindowListener { public void windowClosing(WindowEvent e) {} public void windowClosed(WindowEvent e) { } public void windowOpened(WindowEvent e) { } public void windowIconified(WindowEvent e) { } public void windowDeiconified(WindowEvent e) { } public void windowActivated(WindowEvent e) { } public void windowDeactivated(WindowEvent e) { } } A built in class -- we already have the empty bodies!! Carnegie Mellon University, Graduate School of Industrial Administration

  48. The Window again import javax.swing.*; import java.awt.event.*; public class CloseDemo2 extends WindowAdapter { public static void main(String[] args) { JFrame f = new JFrame("Example"); f.setSize(400,100); f.setVisible(true); f.addWindowListener(new CloseDemo2()); } public void windowClosing(WindowEvent e) { System.exit(0); } } Carnegie Mellon University, Graduate School of Industrial Administration

  49. Again with anonymous classes import javax.swing.*; import java.awt.event.*; public class CloseDemo3 { public static void main(String[] args) { JFrame f = new JFrame("Example"); f.setSize(400,100); f.setVisible(true); f.addWindowListener( new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } } ); } } Carnegie Mellon University, Graduate School of Industrial Administration

  50. GUI Programming with threads Java GUI programming and Java Threads GUI example taken from “Computing Concepts with Java 2” by Cay Horstmann Thread examples taken from “The Java Programming Language” By Arnold and Gosling and from Cay Horstmann’s “Core Java 2 Advanced” Carnegie Mellon University, Graduate School of Industrial Administration

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