1. Objects At Their Best— Java's Abstract Window Toolkit
GUI Aint Just Buttons
Other components:
Text fields
Windows
Associated with different events
Changing text
Closing a window
Different events, different Event classes
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

2. Objects At Their Best— Java's Abstract Window Toolkit
Event Summary
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

3. Objects At Their Best— Java's Abstract Window Toolkit
Event Classes
Each Event class is associated with
One or more component classes
event source
A listener interface
Implemented by event handler (listener) class
A registration method
allows listener to register with the event source
defined in component class (event source)
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

4. Java Event Naming Conventions
Event classes
EventTypeEvent
Example: TextEvent, ActionEvent
Listener interfaces
EventTypeListener
Example: TextListener, ActionListener
Listener registration method
addEventTypeListener or setEventTypeListener
Example: addTextListener, addActionListener
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

5. Objects At Their Best— Java's Abstract Window Toolkit
Listener Interfaces
Contain methods invoked by the event source
No strict naming conventions
Single method (usually)
Example: textValueChanged, actionPerformed
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

6. Listener Interfaces With Multiple Methods
Some listener interfaces have multiple methods
Example: MouseListener
mouseClicked, mouseEntered, mouseExited, mousePressed, mouseReleased
Often the event handler is interested in only one
mouseClicked
What a pain!
We want one method
Gotta implement five ???
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

7. Pain Reliever: Adapter Classes
Java supplies adapter classes for such interfaces.
class MouseAdapter implements MouseListener {
public void mouseClicked(MouseEvent e) { }
public void mouseEntered(MouseEvent e) { }
public void mouseExited(MouseEvent e) { }
public void mousePressed(MouseEvent e) { }
public void mouseReleased(MouseEvent e) { } }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

8. Using the Adapter Class
Event handler can now inherit from this class
just implement desired method (mouseClicked):
class MyMouseHandler extends MouseAdapter {
public void mouseClicked(MouseEvent e) {
// handle mouse click }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

9. The Model-View-Controller Paradigm
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

10. Objects At Their Best— Java's Abstract Window Toolkit
A Counter Applet
Two buttons
Value field
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

11. The CounterApplet Class
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class CounterApplet extends Applet
implements ActionListener {
public void init() {…}
public void actionPerformed(ActionEvent e) {…}
Button incButton, decButton; // Instance variables
TextField valField; }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

12. Objects At Their Best— Java's Abstract Window Toolkit
The init Method
public void init() {
incButton = new Button("+"); // Create Controls
decButton = new Button("-");
valField = new TextField();
valField.setText("0");
Panel p = new Panel(); // Lay them out
p.setLayout(new BorderLayout());
p.add(incButton, "North");
p.add(decButton, "South");
add(p);
add(valField);
// Register as listener
incButton.addActionListener(this);
decButton.addActionListener(this); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

13. The actionPerformed Method
public void actionPerformed(ActionEvent e) {
// Retrieve text field
int val = Integer.parseInt(valField.getText());
if (e.getSource() == incButton)
val++;
else
val--;
// Update text field
valField.setText(Integer.toString(val)); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

14. An Unexpected Benefit (Bug?)
Can directly modify the counter value through the TextField
We’re using the contents of the TextField as our counter value.
We’ll call it a feature
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

15. Objects At Their Best— Java's Abstract Window Toolkit
Concern I
CounterApplet is modeling too many things:
General behavior of an applet (inherited from Applet)
GUI of our particular application
Display of the two buttons and the text field
Role as listener to the two buttons
Behavior of an up/down counter
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

16. Objects At Their Best— Java's Abstract Window Toolkit
Concern II
The counter logic is all over the place
Initialization is in applet’s init method
Maintenance logic is in actionPerformed
No real counter value
"maintained" indirectly as a string in a text field
Makes it difficult to understand and maintain
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

17. Objects At Their Best— Java's Abstract Window Toolkit
Concern III
What if we want
a Scrollbar rather than a pair of Buttons
a Label rather than a TextField?
Requires changing the "counter" logic
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

18. Objects At Their Best— Java's Abstract Window Toolkit
Good Interface Design
Separate interface from application:
Interface: should not affect application logic
Application: should not know about interface.
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

19. Revising the Counter Applet
Have a Counter class
Encapsulate application logic of up/down counter
The CounterApplet class then uses the Counter class.
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

20. Objects At Their Best— Java's Abstract Window Toolkit
The Counter Class
public class Counter {
public Counter(int max) {this.max = max;}
public void reset() {val = 0;}
public int getVal() {return val;}
public void setVal(int val) {
if (val >= 0 && val <= max)
this.val = val; }
public void inc() {if (val < max) val++;}
public void dec() {if (val > 0) val--;}
private int max, val=0; // Instance variables
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

21. The CounterApplet Class
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class CounterApplet extends Applet implements ActionListener {
public void init() {…}
public void actionPerformed(ActionEvent e) {…}
Button incButton, decButton; // Instance variables
TextField valField;
Counter ctr; }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

22. Objects At Their Best— Java's Abstract Window Toolkit
The init Method
public void init() {
ctr = new Counter(10);
incButton = new Button("+");
decButton = new Button("-");
valField = new TextField();
valField.setText(Integer.toString(ctr.getVal()));
Panel p = new Panel();
p.setLayout(new BorderLayout());
p.add(incButton, "North");
p.add(decButton, "South");
add(p);
add(valField);
incButton.addActionListener(this);
decButton.addActionListener(this); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

23. The actionPerformed Method
public void actionPerformed(ActionEvent e) {
if (e.getSource() == incButton)
ctr.inc();
else
ctr.dec();
valField.setText(Integer.toString(ctr.getVal)); }
The event handler for the Button invokes the appropriate counter method and updates the TextField.
We no longer use the TextField for the counter value.
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

24. Objects At Their Best— Java's Abstract Window Toolkit
Better Applet, but...
actionPerformed still responsible for updating the textField.
What if we decide to change the TextField to a
Label?
A Scrollbar
A gauge?
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

25. Objects At Their Best— Java's Abstract Window Toolkit
One Possible Approach
We could isolate the display update logic into a helper method that can be called from anywhere.
However, the CounterApplet still has to know WHEN the Counter has changed.
We will try something a bit different.
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

26. Model-View-Controller (MVC)
Separates application, user input, and display logics.
Makes for an exceptionally clean design
Minimal coupling between interface and application
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

27. Objects At Their Best— Java's Abstract Window Toolkit
Model
Application
Interface-independent
Examples
Spreadsheet data
Counter class
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

28. Objects At Their Best— Java's Abstract Window Toolkit
View
Display
Responsible for maintaining an updated view of the model
Must be notified of changes to the model.
Examples
graph, matrix
TextField update
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

29. Objects At Their Best— Java's Abstract Window Toolkit
Controller
Input
Often initiates changes to the model.
Examples
keyboard, mouse
actionPerformed
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

30. Objects At Their Best— Java's Abstract Window Toolkit
Document-View
Often easier to combine view and controller.
Sometimes known as document-view.
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

31. Objects At Their Best— Java's Abstract Window Toolkit
Model as Observed
Must notify view when changes happen
Model must know:
Who is the view?
How to notify the view of the change?
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

32. Symmetric with Event Handling
View notification
Who is the view?
How to notify view?
Event Handling
Who is the listener?
How to notify listener?
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

33. Objects At Their Best— Java's Abstract Window Toolkit
View as Observer
Who is the view?
View registers with the model
How to notify view that change happened?
View implements an "observer" interface
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

34. The Symmetry Continues
View notification
View registers with model
View implements observer interface
Event Handling
Listener registers with event source
Listener implements listener interface
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

35. The Counter Applet Using MVC
class Counter— Model
class CounterApplet — View/Controller
interface CounterObserver — observer interface
developed for our applet— not predefined
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

36. CounterObserver Interface
Implemented by view to ‘listen’ for model changes
Single method with the Counter as a parameter.
When invoked allows view to be updated.
interface CounterObserver {
public void counterHasChanged(Counter ctr); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

37. Counter Class Overview
Same basic functionality, plus…
Maintains a CounterObserver as part of its state.
Provides observer registration method
registerAsObserver
Notifies observer of changes
Relies upon observer implementing CounterObserver
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

38. Objects At Their Best— Java's Abstract Window Toolkit
Counter Class
public class Counter {
public Counter(int max) {this.max = max;}
public void reset() {val = 0;}
public int getVal() {return val;}
// Manipulator methods-
// must notify observer the model has changed
public void setVal(int val) {…}
public void inc() {...}
public void dec() {...}
// Observer-related methods
public void registerAsObserver(CounterObserver observer){…}
private void notifyObserver() {...}
private int max, val=0;
CounterObserver observer; }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

39. Counter’s Manipulator Methods
public void setVal(int val) {
if (val >= 0 && val <= max) this.val = val;
notifyObserver(); }
public void inc() {
if (val < max) val++;
public void dec() {
if (val > 0) val--;
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

40. Counter’s Observer-Related Methods
public void registerAsObserver(CounterObserver observer) {
this.observer = observer;
notifyObserver(); // Allow initial display }
private void notifyObserver() {
observer.counterHasChanged(this);
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

41. CounterApplet Class Overview
Implements CounterObserver interface (view)
Registers with Counter during initialization (view)
Event-handlers invoke Counter methods but do not update view. (controller)
Updates TextField when counterHasChanged method is invoked. (view)
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

42. Objects At Their Best— Java's Abstract Window Toolkit
CounterApplet Class
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class CounterApplet extends Applet
implements ActionListener,
CounterObserver {
public void init() {…}
public void actionPerformed(ActionEvent e) {…}
public void counterHasChanged(Counter ctr) {…}
Button incButton, decButton;
TextField valField;
Counter ctr; }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

43. Objects At Their Best— Java's Abstract Window Toolkit
The init Method
public void init() {
incButton = new Button("+");
decButton = new Button("-");
valField = new TextField();
Panel p = new Panel();
p.setLayout(new BorderLayout());
p.add(incButton, "North");
p.add(decButton, "South");
add(p);
add(valField);
incButton.addActionListener(this);
decButton.addActionListener(this);
ctr = new Counter(10);
ctr.registerAsObserver(this); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

44. The Event-Handling Methods
public void actionPerformed(ActionEvent e) {
if (e.getSource() == incButton)
ctr.inc();
else
ctr.dec(); }
The event handler simply initiates model changes
NOT responsible for the display!
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

45. The counterHasChanged Method
public void counterHasChanged(Counter ctr) {
valField.setText(Integer.toString(ctrVal)); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

46. Objects At Their Best— Java's Abstract Window Toolkit
Now Let’s Have Some Fun!
We’ve shown you the basics of MVC
We’ve also claimed it results in a clean design
Model knows nothing of controller/view’s workings
All this leads to some very unexpected capabilities!
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

47. Objects At Their Best— Java's Abstract Window Toolkit
Counters...
Two pairs of synchronized counters:
East/West
North/South
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

48. Objects At Their Best— Java's Abstract Window Toolkit
… Logs, ...
A logging facility in synch with the counters
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

49. Objects At Their Best— Java's Abstract Window Toolkit
… and Alarms
An alarm panel keeping track of the counter values
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

50. Objects At Their Best— Java's Abstract Window Toolkit
Class Overview
Counter class: a model
same basic functionality, but ...
now allows multiple observers
CounterPanel class: a view and controller
creates our familiar counter GUI
observes the counter
similar structure to our previous CounterApplet class
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

51. Objects At Their Best— Java's Abstract Window Toolkit
Class Overview (II)
CounterLogger: a view
prints out counter changes to System.out
observes the counter
maintains counter name
CounterAlarmPanel: a view
observes multiple counters
indicates movement past error threshold
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

52. Objects At Their Best— Java's Abstract Window Toolkit
Class Overview (III)
CounterApplet
creates the various interface components
sits back and let’s them do all the work
CounterObserver interface
same as before
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

53. Objects At Their Best— Java's Abstract Window Toolkit
Counter Class
import java.util.*;
public class Counter {
public Counter(int max) {
this.max = max;
observers = new Vector(); }
public void reset() { /*Same as before*/ }
public int getVal() { /*Same as before*/ }
public void setVal(int val) { /*Same as before*/ }
public void inc() { /*Same as before*/ }
public void dec() { /*Same as before*/ }
public void registerAsObserver(CounterObserver observer){…}
private void notifyObservers() {…}
private int max, val=0;
Vector observers; // Allows multiple observers
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

54. The registerAsObserver Method
observers are added to the Vector
public void registerAsObserver(CounterObserver observer) {
observers.addElement(observer);
observer.counterHasChanged(this); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

55. The notifyObservers Method
Notification involves enumerating through the Vector and notifying each observer
private void notifyObservers() {
Enumeration enum = observers.elements();
while (enum.hasMoreElements()) {
CounterObserver observer =
(CounterObserver)enum.nextElement();
observer.counterHasChanged(this); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

56. Objects At Their Best— Java's Abstract Window Toolkit
CounterLogger Class
class CounterLogger implements CounterObserver {
public CounterLogger(Counter ctr, String counterName) {
this.counterName = counterName;
ctr.registerAsObserver(this); // Observes counter }
public void counterHasChanged(Counter ctr) {
System.out.println(counterName + " changed to " +
ctr.getVal());
private String counterName; // For id purposes
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

57. Objects At Their Best— Java's Abstract Window Toolkit
CounterPanel Class
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
public class CounterPanel extends Panel
implements ActionListener,
CounterObserver {
public CounterPanel(Counter ctr) {…}
public void actionPerformed(ActionEvent e) {…}
public void counterHasChanged(Counter ctr) {…}
Button incButton, decButton;
TextField valField;
Counter ctr; }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

58. CounterPanel Constructor
public CounterPanel(Counter ctr) {
incButton = new Button("+");
decButton = new Button("-");
valField = new TextField();
Panel p = new Panel();
p.setLayout(new BorderLayout());
p.add(incButton, "North");
p.add(decButton, "South");
add(p);
add(valField);
incButton.addActionListener(this);
decButton.addActionListener(this);
this.ctr = ctr;
ctr.registerAsObserver(this); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

59. The Event-Handler Method
identical to before
public void actionPerformed(ActionEvent e) {
if (e.getSource() == incButton)
ctr.inc();
else
ctr.dec(); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

60. The counterHasChanged Method
Same as before
public void counterHasChanged(Counter ctr) {
valField.setText(Integer.toString(ctrVal)); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

61. CounterAlarmPanel Class
import java.util.*;
import java.awt.*;
class CounterAlarmPanel extends Panel
implements CounterObserver {
public CounterAlarmPanel() {counters = new Vector();}
// add counter with error-threshold
void add(Counter ctr, int err) {…}
public void paint(Graphics g) {...}
public void counterHasChanged(Counter ctr) {…}
Vector counters; // counter/error-threshold pairs }
Counter/error-threshold pairs maintained in helper class, CounterAlarm
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

62. Objects At Their Best— Java's Abstract Window Toolkit
The add Method
Creates a CounterAlarm object from the counter/error-threshold
Adds the CounterAlarm to the Vector
Registers as an observer of the Counter
void add(Counter ctr, int err) {
counters.addElement(new CounterAlarm(ctr, err));
ctr.registerAsObserver(this); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

63. Objects At Their Best— Java's Abstract Window Toolkit
The paint Method
Enumerates through the CounterAlarm Vector
Paints a green circle if below the threshold; a red one otherwise
public void paint(Graphics g) {
Enumeration e = counters.elements();
int i = 0;
while (e.hasMoreElements()) {
CounterAlarm ctrAlarm =
(CounterAlarm)e.nextElement();
int ctrval = ctrAlarm.ctr.getVal();
if (ctrval >= ctrAlarm.err)
g.setColor(Color.red);
else
g.setColor(Color.green);
g.fillOval(50+i*30, 30, 20, 20);
g.setColor(Color.black);
g.drawOval(50+i*30, 30, 20, 20);
i++; }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

64. The counterHasChanged Method
Merely causes a repaint
public void counterHasChanged(Counter ctr) {
repaint(); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

65. Objects At Their Best— Java's Abstract Window Toolkit
CounterApplet Class
import java.applet.*;
import java.awt.*;
public class CounterApplet extends Applet {
public void init() {
Counter ctr1 = new Counter(10);
CounterPanel cp1 = new CounterPanel(ctr1);
CounterPanel cp2 = new CounterPanel(ctr1);
Counter ctr2 = new Counter(5);
CounterPanel cp3 = new CounterPanel(ctr2);
CounterPanel cp4 = new CounterPanel(ctr2);
CounterAlarmPanel cpa = new CounterAlarmPanel();
cpa.add(ctr1, 8); cpa.add(ctr2, 4);
setLayout(new BorderLayout());
add(cp1, "North"); add(cp2, "South");
add(cp3, "East"); add(cp4, "West");
add(cpa, "Center");
CounterLogger cl = new CounterLogger(ctr1, "Counter1");
CounterLogger c2 = new CounterLogger(ctr2, "Counter2"); }
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit

66. Objects At Their Best— Java's Abstract Window Toolkit
CounterAlarm Class
class CounterAlarm {
CounterAlarm(Counter ctr, int err) {
this.ctr = ctr;
this.err = err; }
Counter ctr;
int err;
April 4, 2019
Arnow and Weiss:
Objects At Their Best— Java's Abstract Window Toolkit