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Programming with Java
1
Chapter 1
Introducing Java
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Programming with Java
2
Objectives
•
Recognize the strengths of the Java programming
language.
•
Understand the uses of Java for applets and applications.
•
Use the import statement to incorporate Java packages
into your code.
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Declare and add components to an interface.
•
Modify text using the Color and Font objects.
•
Write an applet and run it in a browser or applet viewer.
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Programming with Java
3
Java
•
•
•
•
You can write small programs called applets, which run in
a Web browser.
You can write stand-alone programs called applications,
which are independent of the browser.
Programs can be text based or have graphical user
interface (GUI).
Developed by Sun Microsystems in 1991.
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Programming with Java
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An Official Description
•
Java is a simple, object-oriented, robust, secure, portable,
high-performance, architecturally neutral, interpreted,
multithreaded, dynamic language.
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Programming with Java
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Object Oriented
•
Java is a true object oriented programming language (OOP).
•
In OOP, programmers create units called classes that contain
data and the instructions for handling data.
•
Well-designed classes are reusable components that can be
combined to create new applications.
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Programming with Java
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Object Oriented Continued
•
•
•
Applets running in a browser and GUI applications do not
follow a sequential logic.
Each user action can cause an event to occur, which can trigger
an event handler.
The clicking causes the event listener to notify your program,
and the program automatically jumps to a procedure you have
written to do the event.
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Programming with Java
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Traditional Languages are
Compiled
Source
Program
Compile
Object
Program
System Library
Routines
Link
Executable
Program
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Programming with Java
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Java Applets and Applications are
Compiled
Java Source
Program
System Library
Classes
(Packages)
Compile
Bytecodes
(Can transfer bytecodes to
another computer)
Java Virtual
Machine
interprets and
executes
bytecodes
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Programming with Java
9
Java Development Tools
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To run a Java program, you need the Java Runtime
Environment (JRE).
•
To develop or write Java applications and applets you need the
Java Development Kit (JDK) from Sun Microsystems.
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Programming with Java
10
The Java API
•
The Java application programming interface (API)
consists of classes created by Sun Microsystems and stored in
library files called packages.
•
We use classes from java.lang, java.awt, java.applet.
•
For the other classes see table on next slide.
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Programming with Java
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Partial List of Core Java API
Packages
Package
Purpose
java.applet
Create applets
java.awt
Graphical components using
abstract windows toolkit
java.beans
Create software components
java.io
Handle input and output of data
java.lang
Core functions of the language,
automatically included
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Programming with Java
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Partial List of Core Java API
Packages Continued
Package
Purpose
java.math
Handle math functions, very large
integers and decimal values
java.net
Networking
java.rmi
Remote objects
java.security
Manage certificates, signatures, and
other security
java.sql
Query databases
java.swing
GUI using Java Foundation Classes
java.text
Manipulate text including searches
java.util
Provide utilities such as dates
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Programming with Java
13
Source
code
Compiler
Java has Several
Levels of Security
ByteCode
(May transfer to
another computer)
Verifier
Class Loader
Interpreter
Security Manager
Program
Output
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Programming with Java
14
Classes and Objects
•
A class is a template or blueprint for an object, but is not the
object itself.
•
Let us take an example of a dog:
•
We can describe the characteristics of a certain breed of dog,
such as typical coloring and general size and whether the
breed barks, bites, or runs fast and descriptions are the
properties and methods of the class.
•
But an actual dog (an instance of the dog class) will have a
set of characteristics, such as a name and a size.
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This means you instantiate object of the dog class to create
an instance of the dog.
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Programming with Java
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Java Objects
•
In Java, all objects are inherited from the Object class from the
lang package (java.lang.Object).
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The Object class is the superclass of all other classes, which
are called subclasses.
•
See Fig 1.6 for the inheritance of the applet class, which
inherits from the Panel class, which inherits from the Container
class, which inherits from the Component, which inherits from
the Object.
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Programming with Java
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Inheritance of Java Objects
Object
Component
Container
Panel
Applet
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Programming with Java
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Inheritance
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An important concept of OOP is inheritance.
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The new class is based on an existing class.
•
In Java, we say that the new class extends or inherits from the
existing class.
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Programming with Java
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Encapsulation
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Another important concept of OOP is encapsulation.
•
Encapsulation refers to the combination of characteristics of a
class.
•
Encapsulation is often referred to as data hiding.
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The class can "expose" certain properties and methods and
keep others hidden.
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Programming with Java
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Punctuation
•
Java is very picky about punctuation.
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Every statement must be terminated by a semicolon (;).
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The braces ({}) enclose a block of statements.
•
The applet class contains a block of statements and the method
inside the class contains another block.
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Programming with Java
20
Case Sensitivity
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Java is case-sensitive, so it is critical that you observe the
capitalization.
•
Most errors on first programs are due to incorrect
capitalization.
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Programming with Java
21
Class Diagram
Class: HelloWorldApplet
Methods
init
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Variables and Components
lblMessage
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Programming with Java
22
Naming Rules
A Java name:
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must contain only letters, numbers, and underscores.
•
must begin with a letter or underscore.
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cannot have any embedded spaces.
•
is case sensitive.
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Cannot be one of Java's reserved words, such as
boolean, public, or import. (Note that all of the
Java reserved words are lowercase. If you use uppercase as
part of your names, you will never have a naming conflict.
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Programming with Java
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Naming Conventions
To create good Java names, you should:
•
Make all names meaningful. Do not use names such as a, b, c,
or x. Always create names that a person reading your code can
tell the purpose of. And do not abbreviate unless using a
standard abbreviation that has a clearly understood meaning.
•
Begin the name with a lowercase prefix that indicates the type
of component.
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Programming with Java
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Naming Conventions Continued
•
Begin the actual name (following the prefix) with an
uppercase letter. Use mixed upper- and lowercase for the
name, with a capital to begin each new word.
Examples
lblMessage
fntLargeText
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Programming with Java
25
Finding and Fixing Errors
•
Programming errors come in three varieties: compile
errors, run-time errors and logic errors.
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Programming with Java
26
Using HTML to Run the Applet in
a Browser
•
To run your applet in a browser, you need to create an
HTML file that calls the applet, and then open the HTML
file with the browser.
•
You can set the width and height to the value of your
choice, and then omit the resize method in your applet.
<HTML>
<BODY>
<Applet code = FontApplet.class width = 600 height =
200>
</Applet>
</BODY>
</HTML>
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Programming with Java
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Using HTML to Run the Applet in
a Browser Continued
•
The codes in HTML tags < > are not case sensitive, but the
name of the class file is case sensitive.
•
Your Java source code file has a .java extension and after the
compiler completes, it creates the .class file.
•
Follow the applet information with the closing tag </Applet>.
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Programming with Java
28
Life Cycle of an Applet
•
The execution of an applet in a browser follows a specific
cycle.
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Each applet always executes four methods: init,
start, stop, and destroy.
•
The only method you are writing currently is the init
method.
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Programming with Java
29
The Methods in the Life Cycle of
an Applet
Method
Purpose
init
Executes the first time an applet is loaded
start
Follows the init and reexecutes each time that the
page displays
stop
Executes when the browser leaves a Web page
containing the applet
destroy
Executes prior to shut down of the browser
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Programming with Java
30
Class Diagram
Class: FontsApplet
Methods
init
McGraw-Hill/Irwin
Variables and Components
lblName
lblPhone
fntName
fntPhone
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.