Creating a Java Application and Applet

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Chapter 2

• Creating a Java Application and Applet

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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, users
  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 ltgt and
  an end tag, denoted by lt/gt
• The tag, ltAPPLETgtlt/APPLETgt, 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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Chapter 2 Complete