Primitive Types vs. Reference Types

Data types in Java are divided into two categoriesprimitive types and reference types (sometimes called nonprimitive types). The primitive types are boolean, byte, char, short, int, long, float and double. All nonprimitive types are reference types, so classes, which specify the types of objects, are reference types.

A primitive-type variable can store exactly one value of its declared type at a time. For example, an int variable can store one whole number (such as 7) at a time. When another value is assigned to that variable, its initial value is replaced. Primitive-type instance variables are initialized by defaultvariables of types byte, char, short, int, long, float and double are initialized to 0, and variables of type boolean are initialized to false. Programmers can specify their own initial values for primitive-type variables. Recall that local variables are not initialized by default.

Programs use variables of reference types (normally called references) to store the locations of objects in the computer's memory. Such variables are said to refer to objects in the program. Objects that are referenced may each contain many instance variables and methods. Line 14 of Fig. 3.8 creates an object of class GradeBook, and the variable myGradeBook contains a reference to that GradeBook object. Reference type instance variables are initialized by default to the value nulla reserved word that represents a "reference to nothing." This is why the first call to getCourseName in Fig. 3.8 returned nullthe value of courseName had not been set, so the default initial value null was returned. The complete list of reserved words and keywords is listed in Appendix C, Keywords and Reserved Words.

A reference to an object is required to invoke (i.e., call) the object's methods. In the application of Fig. 3.8, the statements in method main use the variable myGradeBook to send messages to the GradeBook object. These messages are calls to methods (like setCourseName and getCourseName) that enable the program to interact with the GradeBook objects. For example, the statement (in line 23)


 

myGradeBook.setCourseName( theName ); // set the course name

uses myGradeBook to send the setCourseName message to the GradeBook object. The message includes the argument that setCourseName requires to perform its task. The GradeBook object uses this information to set the courseName instance variable. Note that primitivetype variables do not refer to objects, so such variables cannot be used to invoke methods.

Software Engineering Observation 3.4

A variable's declared type (e.g., int, double or GradeBook) indicates whether the variable is of a primitive or a reference type. If a variable's type is not one of the eight primitive types, then it is a reference type. For example, Account account1 indicates that account1 is a reference to an Account object).


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

Arrays

Classes and Objects: A Deeper Look

Object-Oriented Programming: Inheritance

Object-Oriented Programming: Polymorphism

GUI Components: Part 1

Graphics and Java 2D™

Exception Handling

Files and Streams

Recursion

Searching and Sorting

Data Structures

Generics

Collections

Introduction to Java Applets

Multimedia: Applets and Applications

GUI Components: Part 2

Multithreading

Networking

Accessing Databases with JDBC

Servlets

JavaServer Pages (JSP)

Formatted Output

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

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Java(c) How to Program
Java How to Program (6th Edition) (How to Program (Deitel))
ISBN: 0131483986
EAN: 2147483647
Year: 2003
Pages: 615
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