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Java Programming, 2E
Introductory Concepts
and Techniques
Chapter 2
Creating a Java
Application and Applet
Objectives
• Write a simple Java application
• Use TextPad
• Understand the different types and uses of
comments
• Use proper naming conventions for
classes and files
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Objectives
• Identify the parts of a class header and
method header
• Code output
• Use the println() method
• Compile a Java program
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Objectives
• Understand the common types of errors
• Run a Java Program
• Edit Java source code to insert escape
characters and a system date
• Print source code
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Objectives
• Differentiate between an application and
an applet
• Create an applet from Java source code
• Write code to display a graphic, text, color,
and the date in an applet
• Create an HTML host document
• Run a Java applet
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Introduction
• Users enter data and instructions into a
computer and receive feedback from the
computer through a user interface
• Programmers can create many types of user
interfaces in Java
• We will create a program with two types of user
interfaces
– Console application
• Command line interface
– Applet
• Graphical user interface displayed in a browser
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The Welcome to My Day Program
• This program will display a splash screen
– A splash screen is a screen that is displayed
before the main program starts
• The screen will contain a welcome
message, user’s name, and system date
– The console application will display text only
– The applet will contain text, color, and a
graphic
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Program Develpment
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Analysis and Design
• Verify that the requirements are specific
enough
• Design the user interface using a
storyboard
• Design the program logic using a flowchart
and event diagram
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Using TextPad
• TextPad has several window areas
– Coding window
– Selector window
– Clip Library window
• TextPad can display line numbers
– Helpful for finding compiler errors
• TextPad has color-coding capabilities
– Save a document before entering code to
enable Java related color-coding
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Coding the Program Comments as Documentation
• Purpose of comments
– Provides clear description when reviewing code
– Helps programmer think clearly when coding
• Placement of comments
– Use a comment header to identify a file and its
purpose
– Place a comment at the beginning of code for each
event and method
– Place comments near portions of code that need
clarification
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Coding the Program Comments as Documentation
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Coding the Program The Class Header
• Identify how the code can be accessed with an
access modifier
– public indicates that the code can be accessed by all
objects in the program and can be extended for a
subclass
• Specify a unique name for the class
– The class name at the beginning of the program must
match the file name exactly
– Java is case-sensitive
– By convention, uppercase letters are used for class
names and to distinguish words in class names
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Coding the Program The Class Header
• Use braces {} after the class header to enclose
the class body
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Coding the Program The Method Header
• The method header contains modifiers, return value,
method name, and parameters along with their data type
• Every stand-alone Java application must contain a
main() method, which is the starting point during
execution
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Coding the Program The Method Header
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Coding the Program The Method Header
• Modifiers set properties for a method
– public allows other programs to invoke this method
– static means this method is unique and can be
invoked with creating an instance
• Parameters are pieces of data received by the
method to help the method perform its operation
– Identifiers are used to name the variable sent to the
method
• Return type is the data type of the data returned
by the method
– If no data is returned, the keyword void is used
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Coding Output
• Call the System.out.println() method in the SDK
to display output to the monitor
– System is the class
– out is the object representing the output device
– println() is the method
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Coding Output
• When calling a method, arguments are placed in
parentheses
– String literals are placed in quotation marks
– Numeric literals and variables do not need quotation
marks
• Period delimiters separate the class, object, and
method
• Semicolons must be placed after every
statement except headers and braces
• Braces {} enclose the body of a method
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Testing the Solution
• Compile the source code
– javac.exe command
• In TextPad, use the Compile Java command
• At the command prompt, type javac filename.java
– A new bytecode file for each class is created with a
.class extension
• If the compiler detects errors, fix the errors and
compile again
• If the compilation was successful, run the
program
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Debugging the Solution
• System Errors
– System command is not set properly
– Software is installed incorrectly
– Location of stored files is not accessible
• Syntax Errors
– One or more violations of the syntax rules of Java
• Semantic Errors
– The code meaning is unrecognizable to the compiler
• Logic and Run-Time Errors
– Unexpected conditions during execution of a program
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Debugging the Solution
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Running the Application
• After compilation is successful, run the program
to test for logic and run-time errors
• Use the Run Java Application command in
TextPad
– TextPad automatically finds the class file with the
same name
• Use the java command from the command
prompt
– Syntax: java classname (no extension)
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Editing the Source Code
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Import Packages
• Use the import statement to access
classes in the SDK
– The java.lang package is automatically
imported
– Place the import statement before the class
header
– Use an asterisk (*) after the package name
and period delimiter to import all necessary
classes in the package
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Call a System Date Constructor
• Use the Date class in the java.util package to
access the system date
• Store the Date in an object variable
• Declare the object variable by calling the Date
constructor
– The constructor is a method denoted by the new
keyword followed by the object type and parentheses
– Declaration syntax:
objectType variableName = new objectType();
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Format Output Using Escape
Characters
• Use escape characters inside String arguments
to move the output of data
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Editing the source code - cont.
• Recompile and run the application
– The bytecode should be updated after any
changes to the source code
• Print a hard copy of the source code
– The final step of the program development
cycle is to document the solution
• Quit TextPad by clicking on the Close
button
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Moving to the Web
• Characteristics of an applet
– Applets run within a browser/viewer and are usually
delivered to the client machine via the Web
– Applets cannot use system resources or files on the
client machine
• Convert the application into an applet
– Import two packages
– Change the class name and extend the Applet class
– Include a paint method to draw text and display color
and a graphic
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Import Applet Packages
• Applet package (java.applet.*)
– Allows applets to inherit attributes and
methods
• AWT package (java.awt.*)
– Provides access to color, draw methods, and
GUI elements
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Change the Class Name and
Extend the Applet Class
• Change the class name and file name to
create a new applet file
• Edit the comment header in the applet file
• Add “extends Applet” in the class header
to inherit from the superclass, Applet
– Provides the init() method to load the applet in
the browser window
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The paint() Method
• Accepts a Graphics object as a parameter
• The Graphics object is commonly referred
to by the variable name g
– The variable g is created and initialized in the
init() method
– The variable g is a reference variable, or a
specific instance of an object
• The return type is void
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The drawString() Method
• Displays text in the applet window
• Accepts three arguments
– The String data
• If the data is not a String object, convert it to a
String object using the toString() method
– The horizontal and vertical coordinates of the
String
• The coordinates are measured in pixels
• Called by the Graphics object, g
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Draw an Image
• Declare an Image object
• Use the getImage() method to load the image
– The getImage() method calls the getDocumentBase() method to
pull the image from the current folder
• Use the drawImage() method to set the coordinates of
the image
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Set the Background Color
• Use the setBackground() method to change the
background color of the applet window
– The setBackground() method does not need to be
called from a reference variable
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Creating an HTML Host
Document
• A host program, such as a Web page executes
the applet
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Creating an HTML Host
Document
• The Web page contains HTML tags to
define a section or format
– A tag consists of a start tag, denoted by <>
and an end tag, denoted by </>
• The tag, <APPLET>…</APPLET>,
informs the browser of the applet
– The applet tag encloses the name of the
bytecode applet file and the width and height
of the applet window
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Running an Applet
• An applet is run by opening the HTML host
document
• In TextPad, use the Run Java Applet command
• At the command prompt, type appletviewer
followed by the name of the host document
• Use Applet Viewer to test the applet
– Ignores irrelevant HTML code
– Uses less memory than a browser
– Does not have to be Java-enabled
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Chapter Summary
• Use TextPad to write, compile, and run code
• Learn the basic form of an application and an
applet
• Insert comments as documentation
• Code Class and Method headers
• Create a console application
–
–
–
–
Use the println() method
Format output using escape characters
Import the java.util package
Call a Date constructor
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Chapter Summary
• Compile a Java program
• Debug a Java program
– Differentiate between types of errors
• Execute a Java program
• Edit the application to create an applet
– Import applet packages
– Extend the Applet class
– Use the paint(), drawString(), and getImage methods
• Create a HTML Host Document
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Java Programming, 2E
Introductory Concepts
and Techniques
Chapter 2 Complete