Chapter 2 - Part 2

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

2. Chapter Objectives • Code output • Use the println() method • Compile a Java program • 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

3. 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

4. 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

5. 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

6. Testing the Solution • Compile the source code • Clean and Build • If the compiler detects errors, fix the errors and compile again • If the compilation was successful, run the program

7. 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

8. Debugging the Solution

9. Running the Application • After compilation is successful, run the program to test for logic and run-time errors • Use the Run command • Use the java command from the command prompt • Syntax: java classname (no extension)

10. Editing the Source Code

11. Java Code Packages • The Java Software Development Toolkit (SDK) provides libraries of code which can be used in programs. • System.out.println() is a method provided in the SDK.

12. 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

14. 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();

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

16. Editing the Source Code - cont. • Recompile and run the application • The bytecode should be updated after any changes to the source code

17. 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

18. 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

20. Creating a Java Applet Class • Right click on the Welcome package and select New • If “Applet” is not listed click on “Other” and choose “Applet” and click Next • Set the Class Name to “WelcomeApplet” and the package to “welcome” • Click Finish

21. Adding the Java SDK packages • Add: import java.util.Date; import java.awt.*;

22. 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 a reference variable, or a specific instance of an object • The return type is void

23. 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

24. 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

25. 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

26. Creating an HTML Host Document • A host program, such as a Web page executes the applet • If you run the file, NetBeans will build a HTML file for you in the “build” folder. • Copy the image.gif file into the folder and open the HMTL

27. Final Result…

28. Rest of Today • Create the Java Applet for the Welcome Program • Add your name to Applet • Pick a different color • Show me the result when your done