Now that you have had some experience writing computer programs, you might be
feeling one of the strongest emotions of the programmer: compiler angst. Even though
it takes no more than 15 seconds to compile most programs, that time can seem interminable
when you're debugging a program. Write, Save, Compile, Aargh--an error! Write, Save,
Compile, Aargh! Write, Save, Compile, Aargh!... As this vicious cycle repeats itself,
it's easy to become world-weary as a program is compiled and recompiled.
One of the driving forces behind parameter use in Java applets is the fear and
loathing of compilation. Parameters enable you to change elements of an applet without
editing or recompiling anything. They also make the program more useful.
The following topics will be covered during this hour:
Sending parameters to Java applets
Receiving parameters in an applet program
Checking for nonexistent parameters
Converting parameters from one type to another
Writing a program that uses parameters
Sending Parameters
from a Web Page
Parameters are stored as part of the Web page that contains an applet. They are
created using the HTML tag <PARAM> and its two attributes: NAME
and VALUE. You can have more than one <PARAM> tag with an
applet, but all of them must be between the opening <APPLET> tag and
the closing </APPLET> tag. The following is an <APPLET>
tag that includes several parameters:
This example could be used to send news headlines to an applet that scrolls them
across the screen. Because news changes all the time, the only way to create a program
of this kind is with parameters. No other solution would work; just imagine how long
it would take to recompile a Java program every time a Dallas Cowboy ran afoul of
the law.
You use the NAME attribute to give the parameter a name. This attribute
is comparable to giving a variable a name. The VALUE attribute gives the
named parameter a value.
Receiving Parameters
in the Applet
You have to do something in your Java program to retrieve the parameters on the
Web page or they will be ignored. The getParameter() method of the Applet
class retrieves a parameter from a <PARAM> tag on a Web page. The
parameter name, which is specified with the NAME attribute on the page,
is used as an argument to getParameter(). The following is an example of
getParameter() in action:
String display1 = getParameter("Headline1");
The getParameter() method returns all parameters as strings, so you have
to convert them to other types as needed. If you want to use a parameter as an integer,
you could use statements such as the following:
int speed;
String speedParam = getParameter("SPEED");
if (speedParam != null)
speed = Integer.parseInt(speedParam);
This example sets the speed variable by using the speedParam
string. You have to test for null strings before setting speed
because the parseInt() method cannot work with a null string. When
you try to retrieve a parameter with getParameter() that was not included
on a Web page with the <PARAM> tag, it will be sent as null,
which is the value of an empty string.
Workshop: Handling
Parameters in an Applet
This hour's workshop project has little practical value, except perhaps as a taunting
device. The ShowWeight applet takes a person's weight and displays it under
several different units. The applet takes two parameters: a weight in pounds and
the name of the person who weighs that amount. The weight is used to figure out the
person's weight in ounces, kilograms, and metric tons, and all of these are displayed.
Create a new file with your word processor and give it the name ShowWeight.java.
Enter Listing 15.1 into the file. Then save and compile the file.
The init() method is where the two parameters are loaded into the applet.
Because they come from the Web page as strings, they must be converted into the form
you need: a floating-point number for the lbs variable and a string for
name. Converting a string to a floating-point number requires two steps:
converting the string to a Float object and then converting that object
to a variable of the type float.
As you learned with strings, objects and variables are treated differently in
Java programs, and there are different things you can do with them. The reason there
is a Float object and a float variable type is so you can use a
floating-point number as either an object or a variable. The Float object
class also has useful methods such as valueOf() and floatValue()
that you can use to convert floating-point numbers into different types of variables.
Lines 20-22 are used to convert the lbs variable into different units
of measure. Each of these statements has (float) in front of the conversion
equation. This is used to convert the result of the equation into a floating-point
number. You can use this structure to convert variables of one type to another. Put
a variable type in parentheses in front of an equation that produces a result, and
it will convert the result to that type.
The paint() method of the applet uses the drawString() method
of the Graphics class to display a line of text on-screen. The paint()
method has three arguments: the text to display and the x and y positions where the
text should be shown.
Before you can test the ShowWeight applet, you need to create a Web page
that contains the applet. Open up a new file on your word processor and name it ShowWeight.asp.
Enter Listing 15.2 and save it when you're done.
Use the appletviewer tool to see the ShowWeight applet. This demonstration
uses Konishiki as its example because the American-born sumo wrestling champion weighs
in at more than 605 pounds, making him the largest of the bikini-wearing behemoths.
You can substitute anyone whose weight is either exemplary or well-known. Figure
15.1 shows an example of output from the applet. As you can see, Konishiki's workout
regimen doesn't include a lot of fat-free SnackWell's Devil's Food Cakes.
To make the applet display a different name along with a different value for the
"weight" parameter, all you have to change is the ShowWeight.asp
file. The applet itself will continue to work correctly.
Summary
If you visit a site such as Gamelan on the World Wide Web (http://www.gamelan.com),
you'll see links to numerous applets that have been made available for public use,
including some that offer the source file of the program. These applets often use
parameters, especially for animation or scrolling text programs. During this hour
you covered all aspects of parameter use in your applets. Parameters can greatly
improve the usefulness and performance of applets.
Even if you're the paragon of programming patience and you can wait out the slowest
compiler, you'll be using parameters in your Java applets. Like arguments for Java
applications, parameters are a useful way to make one program perform in a variety
of ways.
Q&A
Q Why can't parameters be used with Java applications?
A Parameters must be stored in some sort of document, such as a Web page. With
a Java application that runs from the command line, there's nowhere to put them.
However, you can specify arguments on the command line when you run the program,
and these are brought in as strings also. Handling arguments in applications has
many similarities to handling parameters in applets.
Q Does the name of a parameter on a Web page have to be capitalized exactly as it
is shown as an argument to the getParameter() method?
A Like all HTML tags and attributes, the NAME and VALUE attributes
of the <PARAM> tag do not have to be capitalized a specific way to
work. The text "Speed" and "speed" and "SPEED"
all refer to the same parameter.
Quiz
Test your knowledge of parameters with the following questions.
Questions
1. What's the name of the HTML tag that is used to send a parameter to
a Java program?
(a) <APPLET> (b) <VARIABLE> (c) <PARAM>
2. When the getParameter() method is used to load a parameter value
into a program, what type of information is loaded?
(a) a String variable (b) a different type depending on the parameter (c) an array of characters
3. If you try to load a parameter that is not included on the Web page that
contains the applet, what will getParameter() do?
(a) crash the program with an error (b) return the empty string null as a value (c) nothing
Answers
1. c. <PARAM> is the tag to send a parameter, and <APPLET>
is the tag to load the applet itself.
2. a. All parameters are received by an applet as String variables.
They have to be converted to another type of variable or object if you want to use
them differently.
3. b. The getParameter() method must return some kind of value. The
null value is used to signify the lack of a parameter that matches the argument
of getParameter().
Activities
If you're not overstuffed from the weighty subject of parameters, the following
activities are suggested:
Create an applet version of the Repeat application from Chapter 8, "Repeating
an Action with Loops," that takes the line to display over and over again as
a parameter.
Add a parameter to the ShowWeight applet that specifies the ideal weight
of the person. Display how many weeks of dieting it would take them to reach it,
given five pounds of weight loss per week.
