Sams Teach Yourself Java 2 in 24 Hours

By Rogers Cadenhead

• Table of Contents
  • Copyright
  • About the Author
  • About the Technical Editor
  • Acknowledgments
  • We Want to Hear from You!
  • Reader Services
  • Introduction
  • Hour 1. Becoming a Programmer
  • Hour 2. Writing Your First Program
    • What You Need to Write Programs
    • Creating the Saluton Program
    • Beginning the Program
    • Storing Information in a Variable
    • Saving the Finished Product
    • Compiling the Program into a Class File
    • Fixing Errors
    • Workshop: Running a Java Program
    • Summary
    • Q&A
    • Quiz
    • Activities
  • Hour 3. Vacationing in Java
  • Hour 4. Understanding How Java Programs Work
  • Part II: Learning the Basics of Programming
  • Hour 5. Storing and Changing Information in a Program
  • Hour 6. Using Strings to Communicate
  • Hour 7. Using Conditional Tests to Make Decisions
  • Hour 8. Repeating an Action with Loops
  • Part III: Working with Information in New Ways
  • Hour 9. Storing Information with Arrays
  • Hour 10. Creating Your First Object
  • Hour 11. Describing What Your Object Is Like
  • Hour 12. Making the Most of Existing Objects
  • Part IV: Programming a Graphical User Interface
  • Hour 13. Building a Simple User Interface
  • Hour 14. Laying Out a User Interface
  • Hour 15. Responding to User Input
  • Hour 16. Building a Complex User Interface
  • Part V: Creating Multimedia Programs
  • Hour 17. Creating Interactive Web Programs
  • Hour 18. Handling Errors in a Program
  • Hour 19. Creating a Threaded Program
  • Hour 20. Reading and Writing Files
  • Part VI: Creating Multimedia Programs
  • Hour 21. Using Fonts and Color
  • Hour 22. Playing Sound Files
  • Hour 23. Working with Graphics
  • Hour 24. Creating Animation
  • Part VII: Appendixes
  • Appendix A. Tackling New Features of Java 2 Version 1.4
  • Appendix B. Using the Java 2 Software Development Kit
  • Appendix C. Programming with the Java 2 Software Development Kit
  • Appendix D. Using Sun ONE Studio
  • Appendix E. Where to Go from Here: Java Resources
  • Appendix F. This Book's Web Site

Fixing Errors

If errors exist in your program when you compile it, a message is displayed that explains each error and the line on which it occurred. The following output illustrates an attempt to compile a program that has an error, and the error messages that is displayed as a result:

C:\J24Work>javac Saluton.java 
Saluton.java:4: cannot resolve symbol. 
symbol  : method print1n (java.lang.String) 
location: class java.io.PrintStream 
        System.out.print1n(greeting); 
                  ^ 
1 error 

C:\J24Work> 

Error messages displayed by the javac tool include the following information:

• The name of the Java program
• The number of the line where the error was found
• The type of error
• The line where the error was found

As you learned during the past hour, errors in programs are called bugs. Finding those errors and squashing them is called debugging. The following is another example of an error message you might see when compiling the Saluton program:

Saluton.java:4: cannot resolve symbol. 
symbol  : variable greting 
location: class Saluton 
        System.out.println(greting); 
                           ^ 

In this example, the 4 that follows the file name Saluton.java indicates that the error is on Line 4. This is where having a line-numbering word processor comes in handy—you can jump more easily to the Java statement that's associated with the error.

The actual error message, cannot resolve symbol in this case, can often be confusing to new programmers. In some cases, the message can be confusing to any programmer. When the error message doesn't make sense to you, take a look at the line where the error occurred.

System.out.print1n(greting);