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Ade Azurat, Advanced Programming, 2004
Review: Slide 1
Review
Why Java ?
Application and Applet
User Interface
Exception
Object-Oriented Programming
Ade Azurat, Advanced Programming, 2004
Review: Slide 2
What is Java
Java:
A simple, object-oriented, network-savvy,
interpreted, robust, secure, architecture neutral,
portable, high-performance, multithreaded,
dynamic language. (Sun Microsystems, Inc)
Ade Azurat, Advanced Programming, 2004
Review: Slide 3
What is Java
Java consists of three parts:
the Java programming language
the Java Virtual Machine
the Java platform
The Java programming language is the language
in which Java applications (including
applets,servlets, and JavaBeans) are written.
When a Java program is compiled, it is converted
to byte codes that are the portable machine
language of a CPU architecture known as the
Java Virtual Machine (or JVM).
Ade Azurat, Advanced Programming, 2004
Review: Slide 4
What is Java
The JVM can be implemented directly in hardware
(a Java chip), but it is usually implemented in the
form of a software program (an interpreter) that
interprets and executes byte codes.
The Java platform is distinct from both the Java
language and Java VM. The Java platform is the
predefined set of Java classes that exist on every
Java installation. These classes are available for
use by all Java programs.
The Java platform is also referred to as the Java
runtime environment or the core Java APIs.
The Java platform can be extended with optional
standard extensions.
Ade Azurat, Advanced Programming, 2004
Review: Slide 5
Why Java
The Java language provides a powerful addition to
the tools that programmers have at their disposal.
Java makes programming easier because it is
object-oriented and has automatic garbage
collection.
In addition, because compiled Java code is
architecture-neutral, Java applications are ideal
for a diverse environment like the Internet.
JavaBeans, support for component based software
engineering.
Ade Azurat, Advanced Programming, 2004
Review: Slide 6
Application
class LessonTwoD {
String text;
//Constructor
LessonTwoD(){
text = "I'm a Simple Program";
}
//Accessor method
String getText(){
return text;
}
public static void main(String[] args){
LessonTwoD progInst = new LessonTwoD();
String retrievedText = progInst.getText();
System.out.println(retrievedText);
}
}
Ade Azurat, Advanced Programming, 2004
Review: Slide 7
Application Structure and Elements
Ade Azurat, Advanced Programming, 2004
Review: Slide 8
Constructors
Classes have a special method called a constructor
that is called when a class instance is created.
The class constructor always has the same name
as the class and no return type.
If you do not write your own constructor, the
compiler adds an empty constructor, which calls
the no-arguments constructor of its parent class.
The empty constructor is called the default
constructor.
The default constructor initializes all noninitialized fields and variables to zero.
Ade Azurat, Advanced Programming, 2004
Review: Slide 9
Applet
Ade Azurat, Advanced Programming, 2004
Review: Slide 10
Applet (code)
import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Color;
public class SimpleApplet extends Applet{
String text = "I'm a simple applet";
public void init() {
text = "I'm a simple applet";
setBackground(Color.cyan);
}
Ade Azurat, Advanced Programming, 2004
Review: Slide 11
Applet (code)
public void start() {
System.out.println("starting...");
}
public void stop() {
System.out.println("stopping...");
}
public void destroy() {
System.out.println("preparing to unload...");
}
Ade Azurat, Advanced Programming, 2004
Review: Slide 12
Applet (code)
public void paint(Graphics g){
System.out.println("Paint");
g.setColor(Color.blue);
g.drawRect(0, 0,
getSize().width -1,
getSize().height -1);
g.setColor(Color.red);
g.drawString(text, 15, 25);
}
}
Ade Azurat, Advanced Programming, 2004
Review: Slide 13
Run the Applet
To see the applet in action, you need an HTML file
with the Applet tag as follows:
<HTML>
<BODY>
<APPLET CODE=SimpleApplet.class WIDTH=200
HEIGHT=100>
</APPLET>
</BODY>
</HTML>
Ade Azurat, Advanced Programming, 2004
Review: Slide 14
Applet Structure and Elements
The Java API Applet class provides what you need
to design the appearance and manage the behavior
of an applet.
This class provides a graphical user interface
(GUI) component called a Panel and a number of
methods.
The applet's appearance is created by drawing
onto the Panel or by attaching other GUI
components such as push buttons, scrollbars, or
text areas to the Panel.
To create an applet, you extend (or subclass) the
Applet class and implement the appearance and
behavior you want.
Ade Azurat, Advanced Programming, 2004
Review: Slide 15
Extending a Class
Most classes of any complexity extend other
classes.
To extend another class means to write a new
class that can use the fields and methods defined
in the class being extended.
The class being extended is the parent class, and
the class doing the extending is the child class.
Another way to say this is the child class inherits
the fields and methods of its parent or chain of
parents.
Child classes either call or override inherited
methods. This is called single inheritance.
Ade Azurat, Advanced Programming, 2004
Applet : Extending a Class
The SimpleApplet
class extends Applet
class, which extends
the Panel class, which
extends the
Container class. The
Container class
extends Object,
which is the parent of
all Java API classes.
Review: Slide 16
Ade Azurat, Advanced Programming, 2004
Review: Slide 17
SimpleApplet
The Applet class provides the init, start, stop,
destroy, and paint methods you saw in the example applet.
The SimpleApplet class overrides these methods to do
what the SimpleApplet class needs them to do. The Applet
class provides no functionality for these methods.
However, the Applet class does provide functionality for the
setBackground method,which is called in the init method.
The call to setBackground is an example of calling a
method inherited from a parent class in contrast to
overriding a method inherited from a parent class.
Ade Azurat, Advanced Programming, 2004
Review: Slide 18
Why ?
You might wonder why the Java language provides
methods without implementations.
?
It is to provide conventions for everyone to use for
consistency across Java APIs. If everyone wrote their
own method to start an applet, for example, but gave it
a different name such as begin or go, the applet code
would not be interoperable with other programs and
browsers, or portable across multiple platforms. For
example, Netscape and Internet Explorer know how to
look for the init and start methods.
Ade Azurat, Advanced Programming, 2004
Review: Slide 19
Graphical User Interface (Swing)
In contrast to the applet where the user interface
is attached to a panel object nested in a top-level
browser, the Project Swing application in this lesson
attaches its user interface to a panel object nested
in a top-level frame object. A frame object is a toplevel window that provides a title, banner, and
methods to manage the appearance and behavior of
the window.
Ade Azurat, Advanced Programming, 2004
Review: Slide 20
Layout Manager
Border Layout
CardLayout
FlowLayout
GridBagLayout
…
//Create panel
panel = new JPanel();
//Specify layout manager and background color
panel.setLayout(new BorderLayout(1,1));
panel.setBackground(Color.white);
//Add label and button to panel
getContentPane().add(panel);
panel.add(BorderLayout.CENTER, text);
panel.add(BorderLayout.SOUTH, button);
Ade Azurat, Advanced Programming, 2004
Review: Slide 21
Action
In your code, you implement the
actionPerformed method to take the
appropriate action based on which button is
clicked.
button = new JButton("Click Me");
//Add button as an event listener
button.addActionListener(this);
Ade Azurat, Advanced Programming, 2004
Review: Slide 22
Listener
The component classes have the appropriate add
methods to add action listeners to them. In the
code the JButton class has an addActionListener
method. The parameter passed to
addActionListener is this, which means the
SwingUI action listener is added to the button so
button-generated actions are passed to the
actionPerformed method in the SwingUI object.
Ade Azurat, Advanced Programming, 2004
Review: Slide 23
Event Handling
The actionPerformed method is passed an event
object that represents the action event that
occurred. Next, it uses an if statement to find out
which component had the event, and takes action
according to its findings.
public void actionPerformed(ActionEvent event){
Object source = event.getSource();
if (_clickMeMode) {
text.setText("Button Clicked");
button.setText("Click Again");
_clickMeMode = false;
} else {
text.setText("I'm a Simple Program");
button.setText("Click Me");
_clickMeMode = true;
}
}
Ade Azurat, Advanced Programming, 2004
Review: Slide 24
Exception
An exception is
a class that descends from
either
java.lang.Exception or
java.lang.RuntimeExcep
tion that defines mild
error conditions your
program might encounter.
Rather than letting the
program terminate, you can
write code to handle
exceptions and continue
program execution.
Ade Azurat, Advanced Programming, 2004
Review: Slide 25
Exception (Terminology)
Exception is a signal that indicates that some
sort of exceptional condition (such as an error)
has occurred
To throw an exception is to signal an
exceptional condition.
To catch an exception is to handle it – to take
whatever actions are necessary to recover
from it.
Ade Azurat, Advanced Programming, 2004
Review: Slide 26
public static int c(int i) throws MyException, MyOtherException {
switch (i) {
case 0: // processing resumes at point 1 above
throw new MyException("input too low");
case 1: // processing resumes at point 1 above
throw new MySubException("input still too low");
case 99:// processing resumes at point 2 above
throw new MyOtherException("input too high");
default:
return i*i;
}
}
Ade Azurat, Advanced Programming, 2004
Review: Slide 27
Exception Handling
try
The try clause simply establishes a block of code
that is to have its exceptions and abnormal exists
(through break, continue, return, or exception
propagation)
catch
When an exception occurs, the first catch clause
that has an argument of the appropriate type is
invoked. The type of the argument must match the
type of the exception object, or it must be a
superclass of the exception.
Ade Azurat, Advanced Programming, 2004
Review: Slide 28
Exception Handling
Finally
The finally clause is generally used to clean up (close files,
release resources, etc) after the try clause. The code in a
finally block is guaranteed to be executed, regardless of
how the code in the try block completes.
Exception propagation
Exceptions propagate up to through the lexical block
structure of a java method, and then up to the method call
stack.
If an exception is never caught, it propagates all the way to
the main() method from which the program started, and
cause the Java Interpreter to print an error message and a
stack trace and exit.
Ade Azurat, Advanced Programming, 2004
Review: Slide 29
Exception
public static void b(int i) throws MyException {
int result;
try {
System.out.print("i = " + i);
result = c(i);
System.out.print(" c(i) = " + result);
}
catch (MyOtherException e) {
// Point 2
// Handle MyOtherException exceptions:
System.out.println("MyOtherException: " + e.getMessage());
System.out.println("Handled at point 2");
}
finally {
// Terminate the output we printed above with a newline.
System.out.print("\n");
}
}
Ade Azurat, Advanced Programming, 2004
Review: Slide 30
Defining your own Exception
// Here we define some exception types of our own.
// Exception classes generally have constructors but no data or
// other methods. All these do is to call their superclass constructors.
class MyException extends Exception {
public MyException() { super(); }
public MyException(String s) { super(s); }
}
class MyOtherException extends Exception {
public MyOtherException() { super(); }
public MyOtherException(String s) { super(s); }
}
Ade Azurat, Advanced Programming, 2004
Review: Slide 31
Object Oriented Programming
Classes
A class is a structure that defines the data and
the methods to work on that data. When you write
programs in the Java language, all program data is
wrapped in a class, whether it is a class you write
or a class you use from the Java platform API
libraries.
Objects
An instance is an executable copy of a class.
Another name for instance is object. There can be
any number of objects of a given class in memory
at any one time.
Ade Azurat, Advanced Programming, 2004
Review: Slide 32
Example
class ExampleProgram {
public static void main(String[] args){
String text = new String("I'm a simple Program ");
System.out.println(text);
String text2 = text.concat(
"that uses classes and objects");
System.out.println(text2);
}
}
The output looks like this:
I'm a simple Program
I'm a simple Program that uses classes and objects
Ade Azurat, Advanced Programming, 2004
Review: Slide 33
Ade Azurat, Advanced Programming, 2004
Review: Slide 34
(cont.)
Inheritance
Inheritance defines relationships among classes in
an object-oriented language. In the Java
programming language, all classes descend from
java.lang.Object and implement its methods.
Ade Azurat, Advanced Programming, 2004
Review: Slide 35
Ade Azurat, Advanced Programming, 2004
Review: Slide 36
(cont.)
Polymorphism
Another way objects work together is to define
methods that take other objects as parameters.
You get even more cooperation and efficiency
when the objects are united by a common
superclass.
All classes in the Java programming language have
an inheritance relationship.
Ade Azurat, Advanced Programming, 2004
Review: Slide 37
(cont.)
String custID = "munchkin";
Integer creditCard = new Integer(25);
Set s = new HashSet();
s.add(custID);
s.add(creditCard);
Ade Azurat, Advanced Programming, 2004
Review: Slide 38
(cont.)
Fields and Methods
Fields and methods can be declared private, protected,
public, or package. If no access level is specified, the field
or method access level is package by default.
private: A private field or method is accessible only to the
class in which it is defined. protected: A protected field or
method is accessible to the class itself, its subclasses, and
classes in the same package.
public: A public field or method is accessible to any class of
any parentage in any package. In Part 2, Lesson 6:
Internationalization server data accessed by client programs
is made public.
package: A package field or method is accessible to other
classes in the same package.
Ade Azurat, Advanced Programming, 2004
Review: Slide 39
Next…
Multithreading
