OOP Java1 Event Handling Overview Listeners, Adapters and Event Sources Inner classes Event Handling Details Applets and GUI Applications Event Handling

OOP Java2 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

OOP Java3 Babysitting A baby in the home needs attention. Many events surrounding the baby are not easily ignored. Events can be of different types.

OOP Java4 One possible source of events

OOP Java5 Babysitting Before responding to an event, a babysitter typically takes note of the source and the type of event.

OOP Java6 Babysitting The sitter needs to know both the event type and the event source. Event source Event type Handler

OOP Java7 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.

OOP Java8 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();

OOP Java9 EventObject AWTEvent ActionEventComponentEvent InputEvent WindowEvent MouseEvent KeyEvent Some classes and methods in the event hierarchy. Object String getActionCommand() int getX() char getKeyChar() boolean isAltDown()Window getWindow()

OOP Java10 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.

OOP Java11 Example A mouse object A mouse object An event object that describes, say, the x and y coordinate of where the mouse was clicked The listener object has methods that are called for particular events Event Source Event data and methods Listener

OOP Java12 Example A mouse object A mouse object An event object that describes, say, the x and y coordinate of where the mouse was clicked The listener object has methods that are called for particular events Implements MouseListener The event object is sent to a listener method A mouse object must be told who its listener is.

OOP Java13 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.

OOP Java14 Some of the Listener Hierarchy EventListener ActionListener abstract void actionPerformed(ActionEvent e); MouseListener abstract void mouseClicked(MouseEvent e) KeyListener

OOP Java15 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?

OOP Java16 An Example // MouseSpyApplet.java import java.applet.Applet; import java.awt.event.MouseEvent; import java.awt.event.MouseListener;

OOP Java17 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!!

OOP Java18 public class MouseSpyApplet extends Applet { public MouseSpyApplet() { MouseSpy listener = new MouseSpy(); addMouseListener(listener); }

OOP Java19 Spying on the mouse Java Applets need an HTML file.

OOP Java20 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 mousePressed(MouseEvent event) {} void mouseReleased(MouseEvent e){} } Now, suppose I extend this class. What must I provide?

OOP Java21 Only those methods that I am interested in. The other methods, when called, do nothing. We’ll visit this issue again later.

OOP Java22 Inner Classes Nested Top Level Classes (not inner) Member Classes Local Classes Anonymous Classes

OOP Java23 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.

OOP Java24 //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"); }

OOP Java25 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

OOP Java26 Member Classes (1) 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'.

OOP Java27 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. Member Classes (2)

OOP Java28 // 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); }

OOP Java29 public class MemberClassExample { public static void main(String args[]) { Top myObject = new Top(); } // OUTPUT Constructing a Top object Constructing SomeClass 33

OOP Java30 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 a 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.

OOP Java31 // 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); }

OOP Java32 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

OOP Java33 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. Anonymous Classes

OOP Java34 // 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"); } }); }

OOP Java35 Top() { System.out.println("Constructing a Top object"); someMethod(new SmallClass() { public void foo() { System.out.println("Strange but fun"); } }); }

OOP Java36 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

OOP Java37 Event Handling – Details register Source ObjectListener 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. Event object

OOP Java38 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 ActionEvent Object

OOP Java39 Implementing a Listener class BabySitter implements ActionListener { public void actionPerformed(ActionEvent e) { // handle the event object e in some way } 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.

OOP Java40 Create a BabySitter object BabySitter sitter = new BabySitter(); sitter

OOP Java41 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)

OOP Java42 The button and its listener Button Object Sitter Object The button calls the actionPerformed() method of the sitter object and passes an ActionEvent object as a parameter addActionListener

OOP Java43 Hit the X and the program exits Once again but with a window

OOP Java44 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); }

OOP Java45 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) { } } But we have to implement ALL of the functions !!

OOP Java46 Java Provides Adapter Classes ComponentAdapter MouseMotionAdapter WidowAdapter ContainerAdapter MouseAdapter FocusAdapter KeyAdapter

OOP Java47 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!!

OOP Java48 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); }

OOP Java49 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); } ); } }