Getting a visitor to supply input to a servlet

Previously, a servlet with a doGet method has been used to get HTML generated dynamically.
How can the visitor influence the execution of the bytecodes?
Often the author produces a WWW form for the visitor to provide input.
Here is a WWW page to obtain a Centigrade value:

0065: <HTML>
0066:    <BODY>
0067:       <FORM METHOD="POST" ACTION="http://localhost:8080/mytomcat/servlet/toFahrenheit">
0068:          Type in a Centigrade value
0069:          <INPUT TYPE="text" NAME="centigrade">
0070:          <BR>
0071:          <INPUT TYPE="submit" VALUE="Get Fahrenheit">
0072:       </FORM>
0073:    </BODY>
0074: </HTML>

Suppose this HTML is in a file called toFahrenheit.html.
When a browser visits this WWW page, the WWW form will appear.
The visitor types a value into the textbox and clicks on the button.
When the visitor does this, the action given in the ACTION attribute gets executed.
This means that the bytecodes of the toFahrenheit servlet will get executed.
Here is the code of a toFahrenheit servlet:

0075: import javax.servlet.http. HttpServlet;                 // toFahrenheit.java
0076: import javax.servlet.http. HttpServletRequest;
0077: import javax.servlet.http. HttpServletResponse;
0078: import java.io.            IOException;
0079: import java.io.            PrintWriter;
0080: import javax.servlet.      ServletException;
0081: public class toFahrenheit extends HttpServlet
0082: {
0083:    public void doPost(HttpServletRequest request,
0084:                       HttpServletResponse response)
0085:          throws IOException, ServletException
0086:    {      
0087:       String tCentigradeString = request.getParameter("centigrade");
0088:       double tCentigrade = Double.parseDouble(tCentigradeString);
0089:       double tFahrenheit = 32 + tCentigrade*9/5;
0090:       response.setContentType("text/html");
0091:       PrintWriter tResponsePrintWriter = response.getWriter();
0092:       StringBuffer tStringBuffer = new StringBuffer();
0093:       tStringBuffer.append("<html>\n" );
0094:       tStringBuffer.append("<head>\n" );
0095:       tStringBuffer.append("<title>Reply</title>\n" );
0096:       tStringBuffer.append("</head>\n" );
0097:       tStringBuffer.append("<body>\n" );
0098:       tStringBuffer.append("<p>\n" );
0099:       tStringBuffer.append("In Fahrenheit, this is " + tFahrenheit + "\n" );
0100:       tStringBuffer.append("</p>\n" );
0101:       tStringBuffer.append("</body>\n" );
0102:       tStringBuffer.append("</html>\n" );
0103:       tResponsePrintWriter.println(tStringBuffer);
0104:       tResponsePrintWriter.close();
0105:    }
0106: }

Because a WWW form is being used, an HTTP POST request will be generated.
For this reason, the above code overrides doPost rather than doGet.
Like doGet, the doPost method has two parameters.
It is the parameter called request that enables the values supplied on the WWW form to be accessed.
Because in the HTML we gave the textbox the name centigrade, ...
... the servlet can access the text typed into the centigrade textbox using:

0087:       String tCentigradeString = request.getParameter("centigrade");