JavaBeans

A JavaBean is an object instantiated from a Java class that obeys certain restrictions. These restrictions make it easier to use JavaBean objects. (See p. 1000 in the text.)

A JavaBean must have a zero-argument constructor. The best way to achieve this is not to declare any constructors at all. Then the only constructor will be the default zero-argument constructor.
A JavaBean must have no public instance variables.
Persistent values in JavaBeans are called Bean properties. They must be accessed through setxx() and getxx() (or for boolean values isxx()) methods.

In other words, a JavaBean is more or less a simple repository of variables and values that can be accessed by getxx() and setxx() (or isxx()).

To see a JavaBean in action:

Download the StringBean.jsp and StringBean.java files from this directory. (Right click and select Save Target As.) These are extensions of the same files from the book's www.corewebprogramming.com archive.
Store at least the StringBean.jsp file within the CoreWebProgramming webapp.
Compile StringBean.java and put the StringBean.class file in the cwp package in the CoreWebProgramming application, i.e., in %TOMCAT_HOME%/webapps/CoreWebProgramming/WEB-INF/classes/cwp.
Access StringBean.jsp within the CoreWebProgramming application, e.g., http://localhost:8080/CoreWebProgramming/cwp/StringBean.jsp

Things to notice:

Even though StringBean.java is a JavaBean, it has methods other than get() and set() that can be called directly. The reverseMessage() method is an example.
<jsp:getProperty name="stringBean" property="bgColor" /> is executed even though it is inside a quoted string as the value of the style attribute inside a <span ... > tag.
Try leaving out the final "/" at the end of a <jsp:getProperty ... /> and see what happens.
Bean properties may be associated with request parameters so that whenever a new value of a request parameter is received, it is automatically stored as the value of the associated Bean property. Type conversion is done automatically. (See pp. 1009 ... 1011 in the text.)
Use this convenience to pass the calculator's operation and argument directly from the client to the Bean. To illustrate automatic type conversion, let the operation be a char (use E to mean ME), and let the argument be an int (use -1 to mean "memory"). As far as the user is concerned, though, continue to show memory and ME on the screen in both the calculator and the log.