Introduction to Computing
Using Java
Exception Handling, Array & GUI
1
Copyright by Michael P.F. Fung
Exception (例外/情況/鑊)
• When running a program, there may be
unexpected conditions or errors. E.g.
– Network outage causing read error.
– I/O Error (disk damage, disk full, etc.)
• It’s an art to handle the exceptions
gracefully and correctly.
X
– We won’t expect a blue screen during the
course of running our programs!
2
Copyright by Michael P.F. Fung
Exceptionally Ugly
3
Copyright by Michael P.F. Fung
How to Deal With Exceptions
• Each exception is given a meaningful name, indicating
the nature of it.
• We may define our own exceptions with our given
names.
• Java has already defined some commonly used ones.
– IOException, ArithmeticException, EOFException, etc.
• In any method, we may:
– Raise exceptions (throw)
– Detect exceptions (try) and Handle exceptions (catch)
– Ignore exceptions (declare throws clause and propagate)
• Ignore means the method would suicide.
• The suicided method would become a killer (exception propagation).
4
Copyright by Michael P.F. Fung
IOException Objects
• Used when something gone wrong during
general Input/Output operations.
• We may create (new) an IOException
object from the class java.io.IOException.
import java.io.*;
...
... new IOException();
5
Copyright by Michael P.F. Fung
Throwing IOException
• If we found/felt something wrong with I/O,
we may create and throw an IOException
object to the message sender.
double tan(double angle)
{
// read a 4-figure table from file
if (something_wrong_during_input)
throw new IOException();
else
...
return answer;
}
… void main(…)
{
double value = tan(30);
}
Normal return
6
Copyright by Michael P.F. Fung
Throwing IOException
• If we found/felt something wrong with I/O,
we may create and throw an IOException
object to the message sender.
double tan(double angle)
{
// read a 4-figure table from file
if (something_wrong_during_input)
throw new IOException();
else
...
return answer;
}
… void main(…)
{
double value = tan(30);
}
Abnormal return
“throwing exception”
7
Copyright by Michael P.F. Fung
Throwing IOException
• If we found/felt something wrong with I/O,
we may create and throw an IOException
object to the message sender.
double tan(double angle)
{
// read a 4-figure table from file
if (something_wrong_during_input)
throw new IOException();
else
...
return answer;
}
… void main(…)
{
double value = tan(30);
}
Rest skipped, method terminated
8
Copyright by Michael P.F. Fung
Still Something Wrong:
Exception Declaration
• If we compile the above “program”, we will get a
compilation error.
• We have to tell message senders: be ready to
receive (possible) IOException’s: declaration.
double tan(double angle) throws IOException
{
// read a 4-figure table from file
if (something_wrong_during_input)
throw new IOException();
else
...
return answer;
}
9
Copyright by Michael P.F. Fung
Detecting Exceptions
• In the previous example, main() will be
puzzled on receiving the IOException object
from tan().
• What should we do in the message sender?
• We have to define a try block in the message
sender to detect exceptions.
… void main(…)
{
double value = tan(30);
}
10
Copyright by Michael P.F. Fung
Try Block
• From the signature of tan(), we know
that sending this message is risky.
double tan(double angle) throws IOException
{
…
}
… void main(…)
{
double value;
try {
value = tan(30);
value = value * 3.14159;
}
}
11
Copyright by Michael P.F. Fung
Try Block
• In case of receiving any exceptions from any
statements in the try block, the rest (subsequent)
statements in the try block will be skipped.
double tan(double angle) throws IOException
{
… throw new IOException();
}
… void main(…)
{
double value;
try {
value = tan(30);
value = value * 3.14159; // wow wow, skipped
}
}
12
Copyright by Michael P.F. Fung
Limited Liability
• Skipping the remaining statements in the try
block means saving the method from dying!
Something
is missing
here…
See next
slide...
… void main(…)
{
double value;
try {
value = tan(30);
value = value * 3.14159; // wow wow, skipped
}
double result = value + 19 * 97 - 2002;
}
• It avoids skipping the rest statements in the
whole method body! i.e. rescuing the method.
13
Copyright by Michael P.F. Fung
Twins: Detection and Handling
• In fact, try and catch are twins.
• Try is responsible for detection.
• Catch is responsible for identification and
handling.
14
Copyright by Michael P.F. Fung
Handling Exceptions
double tan(double angle) throws IOException
{
if (read_4_figure_table_error)
throw new IOException();
…
}
… void main(…)
{
double value;
try {
value = tan(30);
value = value * 3.14159; // wow wow, skipped
…
}
catch (IOException io_exception_object_ref) {
System.out.println(“Input Output Exception received!”);
value = 3.14159;
}
// the program continues normally hereafter
}
15
Copyright by Michael P.F. Fung
Handling Exceptions
• We catch certain type of exception by:
catch (ExceptionType an_object_reference) {
statements to remedy the condition;
}
// resume normal execution hereafter
• an_object_reference lets us access the
fields/methods of the exception object.
• Thus further information may be passed from the
exception thrower to the exception handler
through the exception object.
16
Copyright by Michael P.F. Fung
Catching Several Kinds of
Possible Exceptions
double tan(double angle) throws IOException, ArithmeticException
{
if (read_4_figure_table_error)
throw new IOException();
if (angle == 90)
throw new ArithmeticException();
}
… void main(…)
{
double value;
try {
value = tan(90);
// tan 90 = infinity!
value = value * 3.14159; // wow wow, skipped
}
catch (IOException io_exception_object_ref) {
System.out.println(“Input Output Exception received!”);
value = 3.14159;
}
catch (ArithmeticException arithmetic_exception_object_ref) {
System.out.println(“Arithmetic Exception received!”);
value = 0.0;
}
// the program continues normally hereafter
}
17
Copyright by Michael P.F. Fung
Exception Propagation
• Unless encountering a try-catch block which
handles the received exception, the exception
will cause termination of the current method.
• In such case, the method re-throws (propagates)
the same exception object to its message
sender.
• The involved method should therefore declare
that it may throw that exception in its method
signature.
18
Copyright by Michael P.F. Fung
Exception Propagation
double tan(double angle) throws IOException, ArithmeticException
{
if (read_4_figure_table_error)
throw new IOException();
if (angle == 90)
throw new ArithmeticException();
}
… void main(…) throws ArithmeticException
{
double value;
try {
value = tan(90);
// tan 90 = infinity!
value = value * 3.14159; // wow wow, skipped
}
catch (IOException io_exception_object_ref) {
System.out.println(“Input Output Exception received!”);
System.out.println(“We have handled it gracefully!”);
value = 3.14159;
}
// ArithmeticException is not caught
// this method terminates and propagates the arithmetic exception object
// on receiving ArithmeticException
}
19
Copyright by Michael P.F. Fung
Top-Level Boss
• The Java Virtual Machine is the first message
sender which sends a message to main().
• It is thus the ultimate receiver of any un-handled
exceptions.
• The exception propagation stops either on:
– Reaching a try-catch block which handles the
exception.
– Or reaching the JVM and causing program
termination.
20
Copyright by Michael P.F. Fung
Array
• Popular and important data structure
– use an index to access variables of the same
name (identifier) and type:
i[0] = 5;
i[654] = -378;
• How to do that?
int[] i = new int[1000];
– OR( C++ style )
int i[] = new int[1000];
21
Copyright by Michael P.F. Fung
Declaration and Array Creation
int[] i = new int[1000];
is equivalent to
int[] i;
i = new int[1000];
// i is nothing yet
// i keeps something
22
Copyright by Michael P.F. Fung
Another Form of Array Creation
• By enumerating its initial values:
char[] vowels = {'a', 'e', 'i', 'o', 'u'};
• Then vowels is an array of 5 char variables with
vowels[0]
vowels[1]
vowels[2]
vowels[3]
vowels[4]





'a'
'e'
'i'
'o'
'u'
There are 5 char
variables!
• Array index must be an integer: [0 to length – 1].
23
Copyright by Michael P.F. Fung
Syntax of Creating Arrays
type[] array_name = new type[length];
type[] array_name = {value1, value2, ...};
e.g.
double[]
String[]
String
char[]
GPA
= new double[50];
countryCode = new String[175];
address[]
= new String[30];
vowels = {‘a’, ‘e’, ‘i’, ‘o’, ‘u’};
• type may be primitive type, class type or
even an other array type.
24
Copyright by Michael P.F. Fung
Properties of Array
• A bounded (fixed length) and indexed collection of
elements of the same type.
• Array length is fixed at the time of creation.
• Element access is done using an index [ ].
– vowels[0] to vowels[vowels.length – 1]
– vowels[-8]  ArrayIndexOutOfBoundsException
• To get the length (size) of an array:
– vowels.length
– NOT vowel.length() [confused with String]
25
Copyright by Michael P.F. Fung
Example: The Parameter in main()
class TestArgs {
public static void main(String[] args) {
System.out.println("There are " + args.length + " arguments:");
int i;
for (i = 0; i < args.length; i++)
System.out.println( args[i] );
}
}
C:\LetSee\>
There are 0
C:\LetSee\>
There are 2
Apple
Orange
java TestArgs
arguments:
java TestArgs Apple Orange
arguments:
26
Copyright by Michael P.F. Fung
Example: The Parameter in main()
• A Java application program can receive some parameters
at start-up.
• These start-up parameters are called command-line
arguments.
• The main() method is the receiver of such arguments.
• Such arguments are stored in a String array created by
the JVM.
• JVM sends a message to main() with such an array.
27
Copyright by Michael P.F. Fung
Array of Object References
int[] i;
// a null integer array reference
i = new int[100];// create a new integer array
i[5] = 87;
// let i refer to the array
// initially, i[0] = … = i[99] = 0
Octopus[] deck;
// a null Octopus array reference
deck = new Octopus[10];
// initially, deck[0] = … = null
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
• Creating a new array  Creating members
28
Copyright by Michael P.F. Fung
Array Itself is Also a Reference
deck[ ]
(reference)
?
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
29
Copyright by Michael P.F. Fung
Array Itself is Also a Reference
deck[ ]
(reference)
deck[2]
(reference)
deck[1]
(reference)
deck[0]
(reference)
?
?
?
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
30
Copyright by Michael P.F. Fung
Array Itself is Also a Reference
deck[ ]
(reference)
deck[2]
(reference)
deck[1]
(reference)
deck[0]
(reference)
??
Octopus
(class)
Octopus
(object)
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
31
Copyright by Michael P.F. Fung
Array Itself is Also a Reference
deck[ ]
(reference)
deck[2]
(reference)
deck[1]
(reference)
deck[0]
(reference)
?
Octopus
(class)
Octopus
(object)
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
32
Copyright by Michael P.F. Fung
Array Itself is Also a Reference
Octopus
(object)
deck[ ]
(reference)
Octopus
(class)
Octopus
(object)
deck[2]
(reference)
deck[1]
(reference)
deck[0]
(reference)
Class type
array
Octopus[] deck;
deck = new Octopus[3];
deck[0] = new Octopus();
deck[1] = deck[0];
deck[2] = new Octopus();
33
Copyright by Michael P.F. Fung
Array Itself is Also a Reference
i[ ]
(reference)
Primitive type
array

Class type
array
i[2]
(int)
9
i[1]
(int)
7
i[0]
(int)
7
int[] i;
i = new int[3];
i[0] = 7;
i[1] = i[0];
i[2] = 9;
34
Copyright by Michael P.F. Fung
Assignment of the Whole Array
i[ ]
(reference)
j[ ]
(reference)
They refer to the same array!
i[2]
(int)
9
i[1]
(int)
7
i[0]
(int)
7
int[] i = new int[3];
int[] j;
j = i; // object reference copying
j[2] = 9; // i[2] = 9
i[1] = 7; // j[2] = 7
j[0] = 7; // i[0] = 7
35
Copyright by Michael P.F. Fung
Assignment of the Whole Array
i[ ]
(reference)
j[ ]
(reference)
j[4]
j[3]
j[2]
(int)
j[1]
(int)
j[0]
0(int)
0(int)
0(int)
00
OR, Create another one!
i[2]
(int)
9
i[1]
(int)
7
i[0]
(int)
7
int[] i = new int[3];
int[] j;
j = new int[5];
36
Copyright by Michael P.F. Fung
Assignment of the Whole Array
i[ ]
(reference)
j[ ]
(reference)
j[4]
j[3]
j[2]
(int)
j[1]
(int)
j[0]
0(int)
0(int)
0(int)
00
Remember to Keep It Well!
i[2]
(int)
9
i[1]
(int)
7
i[0]
(int)
7
int[] i = new int[3];
int[] j;
j = new int[5];
j = i;
37
Copyright by Michael P.F. Fung
New Concept: Primitive Type
Array Argument Passing
class Student {
public static void studyHard(double[] newGPAs)
{
newGPAs[0] = 4.0;
newGPAs[1] = 4.0;
}
Start here 
public static void main(String[] args)
{
double[] GPAs = new double[2];
GPAs[0] = 1.0;
GPAs[1] = 1.5;
Student.studyHard(GPAs);
System.out.println(GPAs[0]);
System.out.println(GPAs[1]);
}
newGPAs
(Reference)
GPAs[1]
1.5
4.0
GPAs[0]
1.0
4.0
GPAs
(Reference)
}
Copy array reference
to formal parameter when sending message.
Change
toMichael
the formal
Copyright by
P.F. Fung parameter DOES NOT affect actual parameter!
38
New Concept: Object Type
Array Argument Passing
Start here
class CUHK {
public static void fire(Employee[] victims)
{
for (int i = 0; i < victims.length; i++)
victims[i].salary = 0;
}
 public static void main(String[] args)
{
Employee[] TAs = new Employee[3];
TAs[0] = new Employee(1000);
TAs[1] = new Employee(2000);
TAs[2] = new Employee(5000);
CUHK.fire(TAs);
}
}
TAs
(reference)
class Employee {
public int salary;
public Employee(int initialSalary)
{
salary = initialSalary;
}
}
victims
(reference)
Employee
(object)
salary
1000
TAs[0]
(reference)
Employee
(object)
TAs[1]
(reference)
Employee
(object)
salary
2000
salary
5000
TAs[2]
(reference)
39
Copyright by Michael P.F. Fung
Table (2-Level Array)
What’s the type?
double
// There are 176 students, 8 assignments
// record their marks in double
double[][] mark = new double[176][8];
mark[6][0]
= 99.34;
// mark: 7th student, Asg1
mark[175][6] = 89.12;
// mark: last student, Asg7
double[]
double[] singleStudent;
singleStudent = mark[175]; // refer to the
singleStudent[6] = 45.67; // marks of the last one
System.out.println(mark[175][6]); // would print 45.67
• Elements of an array could be arrays.
• Array reference of array references.
40
Copyright by Michael P.F. Fung
Table Illustrated
mark[ ][ ]
(reference)
Array of Array
of double
mark[2] mark[1] mark[0]
(reference) (reference) (reference)
mark[2][3]
(double) 9.45
mark[1][3]
(double) 8.48
mark[0][3]
(double) 9.11
mark[2][2]
(double) 2.49
mark[1][2]
(double) 3.40
mark[0][2]
(double) 1.42
mark[2][1]
(double) 3.43
mark[1][1]
(double) 6.13
mark[0][1]
(double) 5.43
mark[2][0]
(double) 1.75
mark[1][0]
(double) 1.15
mark[0][0]
(double) 0.35
41
Copyright by Michael P.F. Fung
Duplicating an int Array
i[ ]
(reference)
Copy the elements one-by-one
i[2]
(int)
9
i[1]
(int)
7
i[0]
(int)
7
int[] i = {7, 7, 9};
int[] j;
j = new int[i.length];
for (int count = 0;
count < i.length;
count++)
j[count] = i[count];
42
Copyright by Michael P.F. Fung
Duplicating an int Array
i[ ]
(reference)
j[ ]
(reference)
?
Copy the elements one-by-one
i[2]
(int)
9
i[1]
(int)
7
i[0]
(int)
7
int[] i = {7, 7, 9};
int[] j;
j = new int[i.length];
for (int count = 0;
count < i.length;
count++)
j[count] = i[count];
43
Copyright by Michael P.F. Fung
Duplicating an int Array
i[ ]
(reference)
Copy the elements one-by-one
i[2]
(int)
9
j[ ]
(reference)
j[2]
(int)
0
j[1]
(int)
0
j[0]
(int)
0
i[1]
(int)
7
i[0]
(int)
7
int[] i = {7, 7, 9};
int[] j;
j = new int[i.length];
for (int count = 0;
count < i.length;
count++)
j[count] = i[count];
44
Copyright by Michael P.F. Fung
Duplicating an int Array
i[ ]
(reference)
Copy the elements one-by-one
i[2]
(int)
9
j[ ]
(reference)
j[2]
(int)
9
j[1]
(int)
7
j[0]
(int)
7
i[1]
(int)
7
i[0]
(int)
7
int[] i = {7, 7, 9};
int[] j;
j = new int[i.length];
for (int count = 0;
count < i.length;
count++)
j[count] = i[count];
45
Copyright by Michael P.F. Fung
Duplicating an Object Array
deck[ ]
(reference)
deck[2]
(reference)
deck[1]
(reference)
deck[0]
(reference)
Octopus
(object)
Octopus
(class)
Octopus
(object)
Octopus[] deck;
...
Octopus[] newDeck;
newDeck = new Octopus[deck.length];
for (int count = 0;
count < deck.length;
count++)
newDeck[count] = deck[count];
46
Copyright by Michael P.F. Fung
Duplicating an Object Array
newDeck[ ]
(reference)
?
deck[ ]
(reference)
deck[2]
(reference)
deck[1]
(reference)
deck[0]
(reference)
Octopus
(object)
Octopus
(class)
Octopus
(object)
Octopus[] deck;
...
Octopus[] newDeck;
newDeck = new Octopus[deck.length];
for (int count = 0;
count < deck.length;
count++)
newDeck[count] = deck[count];
47
Copyright by Michael P.F. Fung
Duplicating an Object Array
newDeck[ ]
(reference)
newDeck[2]
(reference)
newDeck[1]
(reference)
newDeck[0]
(reference)
deck[ ]
(reference)
deck[2]
(reference)
deck[1]
(reference)
deck[0]
(reference)
Octopus
(object)
Octopus
(class)
Octopus
(object)
Octopus[] deck;
...
Octopus[] newDeck;
newDeck = new Octopus[deck.length];
for (int count = 0;
count < deck.length;
count++)
newDeck[count] = deck[count];
48
Copyright by Michael P.F. Fung
Duplicating an Object Array
newDeck[ ]
(reference)
newDeck[2]
(reference)
newDeck[1]
(reference)
newDeck[0]
(reference)
deck[ ]
(reference)
deck[2]
(reference)
deck[1]
(reference)
deck[0]
(reference)
Octopus
(object)
Octopus
(object)
Octopus
(class)
Only the object
references are
copied!
Octopus[] deck;
...
Octopus[] newDeck;
newDeck = new Octopus[deck.length];
for (int count = 0;
count < deck.length;
count++)
newDeck[count] = deck[count];
49
Copyright by Michael P.F. Fung
7-Minute Break
50
Copyright by Michael P.F. Fung
Graphics User Interface(GUI)
• Graphical User Interface
• Basic Elements
• Event Driven Model
• Trigger and Callback
51
Copyright by Michael P.F. Fung
Command-line User Interface
52
Copyright by Michael P.F. Fung
Graphical User Interface
53
Copyright by Michael P.F. Fung
GUI – Computer Display
• Use of graphical representations
– Windows
– Icons
– Buttons
–…
• To convey the underlying concepts
– An icon represents a file
– A button represents certain function
54
Copyright by Michael P.F. Fung
GUI – User Input
• Use of various interactive input devices
–
–
–
–
Keyboard
Mouse
Touch screen
…
• To gather command and control from user
– A mouse click opens a file
– A mouse drag moves a window
– Pressing <Enter> means “OK”
55
Copyright by Michael P.F. Fung
GUI and OOP
•
•
•
•
A window is an object.
A button is an object.
We create windows and buttons from classes.
Such objects
–
–
–
–
store state of the component (field/property);
perform certain function (method);
generates events (event objects);
respond to user actions (callback method);
56
Copyright by Michael P.F. Fung
1) Store State
• Basic properties
– Colour
– Size
– Visibility
• Dynamic states
– On/off state of a button
– Value of a text field
57
Copyright by Michael P.F. Fung
2) Perform Action
Paint the window
Draw circles
Display text
58
Copyright by Michael P.F. Fung
3) Generate Event
Detect if you
dragged the
scrollbar
Detect if you
clicked a button
Detect if you
dragged the
mouse pointer
59
Copyright by Michael P.F. Fung
4) Handle Event
• On clicking the buttons, moving the mouse,
dragging the mouse, … over a component,
events are generated.
• On receiving an event, the corresponding
callback method of a listener is invoked.
• The method may update the screen,
update the state, perform some function,
etc.
60
Copyright by Michael P.F. Fung
How to Do it Using Java?
• One of the provided packages java.awt
(Abstract Windowing Toolkit) is readily
used.
• There are plenty of component classes
well-defined in the package.
– Frame: basically a window
– Button: a push button with a label
– TextField…
61
Copyright by Michael P.F. Fung
Simple Example
import java.awt.*;
/* This is not the usual way we call up GUI
* Normally, we should subclass (extends) some
* GUI components in order to modify the behaviour
*/
class SimpleGUI {
public static void main(String[] args) {
Frame myWindow = new Frame();
myWindow.setTitle("Simple GUI");
myWindow.setSize(200, 100);
myWindow.setVisible(true);
}
}
62
Copyright by Michael P.F. Fung
Components List - Classes
63
Copyright by Michael P.F. Fung
How to Use the Components?
•
•
•
•
Read the API Manual and Books!
Check for the components you need
Create (new) the components
Set their properties
– Basic properties and initial state
– Relationship between the components
– Action to be performed on event happening
64
Copyright by Michael P.F. Fung
Component - Basic Properties
• Colour
: setBackground(Color)
setForeground(Color)
• Size
: setSize(int, int)
• Position : setLocation(int, int)
• State
: setEnabled(boolean)
• Font
: setFont(Font)
• Visibility : setVisible(boolean)
• …
65
Copyright by Michael P.F. Fung
Component - Relationship
• Sibling/sub-ordinary relationship
Button1 and Button2 are
contained/embedded in the
Frame
window.add(button1);
Button1 is on the left of
Button2 on the same row
• Relative position between the components
66
Copyright by Michael P.F. Fung
Component - Event Generation
• Events are automatically generated during
interaction with the user.
• Events are normally happened in
conjunction with certain component(s).
• If the involved component(s) do not listen
to (pay attention to) that event, normally
nothing would happen.
67
Copyright by Michael P.F. Fung
Component - Event Listening
• To listen to an event, we need a listener:
Frame myWindow = new Frame();
/* Adapter is a kind of Listener */
WindowAdapter adapter = new WindowAdapter();
myWindow.addWindowListener(adapter);
• add*Listener() are methods of components.
• We register a listener to listen to certain kind of
events, say MouseEvent.
– e.g. MouseListener may be MouseAdapter objects.
68
Copyright by Michael P.F. Fung
Event Handling
Button1

User action
Generates
MouseEvent
MouseListener
MouseAdapter
Event Handler may:
- Update the appearance of the button
- Modify the state of the button
- Perform programmer-defined action
such as “Say Hello”
mouseClicked
mouseEntered
Event
mousePressed
Dispatching
…
69
Copyright by Michael P.F. Fung
Listener Example
import java.awt.*;
import java.awt.event.*;
class ListenGUI {
public static void main(String[] args) {
Frame myWindow = new Frame();
myWindow.setTitle("Simple GUI");
myWindow.setSize(200, 100);
myWindow.setVisible(true);
myWindow.addWindowListener(new MyWindowAdapter());
}
}
File ListenGUI.java
class MyWindowAdapter extends WindowAdapter {
public void windowClosing(WindowEvent e) {
System.out.println(“Terminating the program!”);
System.exit(1);
// a message to ask the System to exit
// ... OR you may open other windows!!!
}
}
70
Copyright by Michael P.F. Fung
Handler (Callback) Method
• An adapter in fact normally listens to a set
of related events.
• For each kind of event, a corresponding
handler method is defined.
• On receiving a MouseEvent which
indicates mouse button pressed, the
mousePressed() method of the
MouseAdapter object will be invoked.
71
Copyright by Michael P.F. Fung
Customizing the Handling
• Without customizing (overriding) the
default handler methods provided in the
adapter, nothing would happen.
• That’s why we extends the
WindowAdapter, MouseAdapter… classes.
• By re-defining (overriding) their methods,
we can achieve desired behaviour in event
handling.
72
Copyright by Michael P.F. Fung
Examples
•
•
•
•
•
NormalMouseAdapter
NormalWindowAdapter
NormalGUI
ListenGUI
ListenGUI2
73
Copyright by Michael P.F. Fung
What Kinds of Events?
• Examples:
– WindowEvent needs WindowListener.
WindowAdapter is a kind of WindowListener.
– MouseEvent needs MouseListener.
MouseAdapter is a kind of MouseListener.
• The events themselves are generated by some
components under user interaction.
74
Copyright by Michael P.F. Fung
After-Life of main()
• When and how does a GUI program end?
• main() is the first method to be invoked.
• Normally, after main() finishes, the
program ends.
• But this is not the case for GUI programs...
• Why? The underlying event dispatching
loop of the system takes over the control.
75
Copyright by Michael P.F. Fung
Advanced Topics
• Using Swing/ AWT with NetBeans
– Setting properties and layout
– Creating call-back methods
• Inner Classes
76
Copyright by Michael P.F. Fung