1. Creating a Java Application and Applet
Chapter 2
Creating a Java Application and Applet

2. Chapter Objectives Write a simple Java application Use TextPad
Understand the different types and uses of comments
Use proper naming conventions for classes and files

3. Chapter Objectives
Identify the parts of a class header and method header
Code output
Use the println() method
Compile a Java program

4. Chapter 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

5. Chapter 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

6. 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

7. 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

9. Program Development

10. 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

14. 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

15. 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

16. Coding the Program - Comments as Documentation

17. 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

18. Coding the Program - The Class Header
Use braces { } after the class header to enclose the class body

19. 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

20. Coding the Program - The Method Header

21. 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 without creating a subclass or instance
Return value is the data type of the data returned by the method
If no data is returned, the keyword void is used
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

22. 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 default display
println() is the method

23. 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

24. 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

25. 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

26. Debugging the Solution

27. 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)

28. Editing the Source Code

29. 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

31. 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();

32. Format Output Using Escape Characters
Use escape characters inside String arguments to move the output of data

33. 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

34. 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

35. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. Creating an HTML Host Document
A host program, such as a Web page executes the applet

43. 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

44. 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

45. Chapter Summary Write a simple Java application Use TextPad
Understand the different types and uses of comments
Use proper naming conventions for classes and files

46. Chapter Summary Identify the parts of a class header and method header
Code output
Use the println() method
Compile a Java program

47. Chapter Summary 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

48. Chapter Summary 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

49. Chapter 2 Complete