Prentice Hall and Sun Microsystems. Personal use only; do not redistribute.

Chapter

he JavaBeans API provides a standard format for Java classes. Visual

manipulation tools and other programs can automatically discover

T

information about classes that follow this format and can then create

and manipulate the classes without the user having to explicitly write any code.

Full coverage of JavaBeans is beyond the scope of this book. If you want

details, pick up one of the many books on the subject or see the documen 

tation and tutorials at 

http://java.sun.com/beans/docs/

. For the pur 

poses of this chapter, all you need to know about beans are three simple

points:

1. A bean class must have a zero argument (empty) con 

structor. You can satisfy this requirement either by explicitly 

defining such a constructor or by omitting all constructors, which 

results in an empty constructor being created automatically. The 

empty constructor will be called when JSP elements create beans.

2. A bean class should have no public instance variables 

(fields). I hope you already follow this practice and use accessor 

methods instead of allowing direct access to the instance vari 

ables. Use of accessor methods lets you impose constraints on 

variable values (e.g., have the 

setSpeed

 method of your 

Car

class disallow negative speeds), allows you to change your inter 

nal data structures without changing the class interface (e.g., 

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