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Autoboxing and Auto-Unboxing

In versions of Java prior to J2SE 5.0, if you wanted to insert a primitive value into a data structure that could store only Objects, you had to create a new object of the corresponding type-wrapper class, then insert this object in the collection. Similarly, if you wanted to retrieve an object of a type-wrapper class from a collection and manipulate its primitive value, you had to invoke a method on the object to obtain its corresponding primitive-type value. For example, suppose you want to add an int to an array that stores only references to Integer objects. Prior to J2SE 5.0, you would be required to "wrap" an int value in an Integer object before adding the integer to the array and to "unwrap" the int value from the Integer object to retrieve the value from the array, as in

 Integer[] integerArray = new Integer[ 5 ]; // create integerArray

 // assign Integer 10 to integerArray[ 0 ]
 integerArray[ 0 ] = new Integer( 10 );

 // get int value of Integer
 int value = integerArray[ 0 ].intValue();

Notice that the int primitive value 10 is used to initialize an Integer object. This achieves the desired result but requires extra code and is cumbersome. We then need to invoke method intValue of class Integer to obtain the int value in the Integer object.

J2SE 5.0 simplifies converting between primitive-type values and type-wrapper objects, requiring no additional code on the part of the programmer. J2SE 5.0 introduces two new conversionsthe boxing conversion and the unboxing conversion. A boxing conversion converts a value of a primitive type to an object of the corresponding type-wrapper class. An unboxing conversion converts an object of a type-wrapper class to a value of the corresponding primitive type. J2SE 5.0 allows these conversions to be performed automatically (called autoboxing and auto-unboxing). For example, the previous statements can be rewritten as

 Integer[] integerArray = new Integer[ 5 ]; // create integerArray
 integerArray[ 0 ] = 10; // assign Integer 10 to integerArray[ 0 ]
 int value = integerArray[ 0 ]; // get int value of Integer

In this case, autoboxing occurs when assigning an int value (10) to integerArray[ 0 ], because integerArray stores references to Integer objects, not int primitive values. Auto-unboxing occurs when assigning integerArray[ 0 ] to int variable value, because variable value stores an int value, not a reference to an Integer object. Autoboxing and auto-unboxing also occur in control statementsthe condition of a control statement can evaluate to a primitive boolean type or a Boolean reference type. Many of this chapter's examples use these conversions to store primitive values in and retrieve them from data structures that store only references to Objects.

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





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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