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
  • 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
    • Statements and Expressions
    • Assigning Variable Types
    • Naming Your Variables
    • Storing Information in Variables
    • Workshop: Using Expressions
    • All About Operators
    • Summary
    • Q&A
    • Quiz
    • Activities
  • 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

Storing Information in Variables

You can put a value into a variable at the same time that you create the variable in a Java program. You can also put a value in the variable at any time later in the program.

To set up a starting value for a variable upon its creation, use the equal sign (=). The following is an example of creating a floating-point variable called pi with the starting value of 3.14:

double pi = 3.14; 

All variables that store numbers can be set up in a similar fashion. If you're setting up a character or a string variable, quotation marks must be placed around the value as shown previously.

You can also set one variable equal to the value of another variable if they both are of the same type. Consider the following example:

int mileage = 300; 
int totalMileage = mileage; 

First, an integer variable called mileage is created with a starting value of 300. In the second line, an integer variable called totalMileage is created with the same value as mileage. Both variables will have the starting value of 300. In future hours, you will learn how to convert one variable's value to the type of another variable.

WarGame.java:7: Variable warships may not have been initialized. 
               warships = warships + 10; 
               ^ 
1 error 

Another naming convention in Java is to capitalize the names of variables that will not change in value. These variables are called constants. The following creates three constants:

int TOUCHDOWN = 7; 
int FIELDGOAL = 3; 
int PAT = 1; 

Because constants never change in value, you might wonder why one should ever be used—you can just use the value assigned to the constant instead. One of the advantages of using constants is that they can make a program easier to understand. For example, the variables Font.BOLD and Font.ITALIC are constants that hold integer values representing the style of the current font.

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