Each uniquely named INPUT element in the FORM on the HTML page makes
an entry in the request object’s list of
parameters. These can be obtained as an enumeration, but more
commonly you request just one. Figure 18-2 shows a
simple form that asks you how many random numbers you want generated,
and makes up that many for you.
When I type the number 8 into the field and press the “Get Yours” button, I see the screen shot in Figure 18-3.
How does it work? The program obviously consists of both an HTML page and a Java servlet. The HTML page appears in Example 18-3; notice the FORM entry and the INPUT field.
Example 18-3. IntsServlet.htm
<HTML> <HEAD><TITLE>Random Numbers Page</TITLE></HEAD> <BODY BGCOLOR="white"> <H1>Random Numbers Page</H1> <P>This site will let you pick some random numbers for Lottery, lucky number or other purposes, all electronically.</P> <FORM METHOD=POST ACTION="/servlets/IntsServlet"> <H4>How Many Numbers Do You Want Today?</H4> <INPUT NAME=howmany SIZE=2> (default is 5) <BR> <INPUT TYPE="SUBMIT" VALUE="Get YOURS!"> </FORM> </BODY></HTML>
Example 18-4 shows the Java for the
servlet. Watch
for the use of
getParameter( )
.
Example 18-4. IntsServlet.java
import java.io.*;
import java.util.Random;
import javax.servlet.*;
import javax.servlet.http.*;
public class IntsServlet extends HttpServlet {
protected final int DEFAULT_NUMBER = 5;
/** Called when the form is filled in by the user. */
public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws IOException {
resp.setContentType("text/html");
PrintWriter out = resp.getWriter( );
// The usual HTML setup stuff.
out.println("<HTML>");
out.println("<HEAD>");
out.println("<BODY BGCOLOR="white">");
// HTML for this page
out.println("<TITLE>Your Personal Random Numbers</TITLE>");
out.println("<H1>Your Personal Random Numbers</H1>");
out.println("<P>Here are your personal random numbers,");
out.println("carefully selected by a");
out.println("<A HREF="http://java.sun.com">Java</A> program.");
out.println("<OL>");
// Figure out how many numbers to print.
int n = DEFAULT_NUMBER;
String num=req.getParameter("howmany");
if (num != null && num.length( ) != 0) {
try {
n = Integer.parseInt(num);
} catch (NumberFormatException e) {
out.println("<P>I didn't think much of ");
out.println(num);
out.println(" as a number.</P>");
}
}
// Now actually generate some random numbers.
Random r = new Random( );
for (int i=0; i<n; i++) {
out.print("<LI>");
out.println(r.nextInt(49)); // for Lotto 6/49
}
out.println("</OL>");
// Print a break and a back link.
out.println("<HR></HR>");
out.println("<A HREF="index.html">Back to main Page</A>");
out.println("</HTML>");
}
}