Variable names such as number1, number2 and sum actually correspond to locations in the computer's memory. Every variable has a name, a type, a size and a value.
In the addition program of Fig. 2.7, when the statement (line 18)
number1 = input.nextInt(); // read first number from user
executes, the number typed by the user is placed into a memory location to which the name number1 has been assigned by the compiler. Suppose that the user enters 45. The computer places that integer value into location number1, as shown in Fig. 2.8. Whenever a value is placed in a memory location, the value replaces the previous value in that location. The previous value is lost.
Figure 2.8. Memory location showing the name and value of variable number1.
When the statement (line 21)
number2 = input.nextInt(); // read second number from user
executes, suppose that the user enters 72. The computer places that integer value into location number2. The memory now appears as shown in Fig. 2.9.
Figure 2.9. Memory locations after storing values for number1 and number2.
After the program of Fig. 2.7 obtains values for number1 and number2, it adds the values and places the sum into variable sum. The statement (line 23)
sum = number1 + number2; // add numbers
performs the addition, then replaces sum's previous value. After sum has been calculated, memory appears as shown in Fig. 2.10. Note that the values of number1 and number2 appear exactly as they did before they were used in the calculation of sum. These values were used, but not destroyed, as the computer performed the calculation. Thus, when a valueis read from a memory location, the process is nondestructive.
Figure 2.10. Memory locations after calculating and storing the sum of number1 and number2.
Introduction to Computers, the Internet and the World Wide Web
Introduction to Java Applications
Introduction to Classes and Objects
Control Statements: Part I
Control Statements: Part 2
Methods: A Deeper Look
Classes and Objects: A Deeper Look
Object-Oriented Programming: Inheritance
Object-Oriented Programming: Polymorphism
GUI Components: Part 1
Graphics and Java 2D™
Files and Streams
Searching and Sorting
Introduction to Java Applets
Multimedia: Applets and Applications
GUI Components: Part 2
Accessing Databases with JDBC
JavaServer Pages (JSP)
Strings, Characters and Regular Expressions
Appendix A. Operator Precedence Chart
Appendix B. ASCII Character Set
Appendix C. Keywords and Reserved Words
Appendix D. Primitive Types
Appendix E. (On CD) Number Systems
Appendix F. (On CD) Unicode®
Appendix G. Using the Java API Documentation
Appendix H. (On CD) Creating Documentation with javadoc
Appendix I. (On CD) Bit Manipulation
Appendix J. (On CD) ATM Case Study Code
Appendix K. (On CD) Labeled break and continue Statements
Appendix L. (On CD) UML 2: Additional Diagram Types
Appendix M. (On CD) Design Patterns
Appendix N. Using the Debugger
Inside Back Cover