CGI

• Use HttpServletRequest request

GET

String param_value = request.getParameter("param_name");

• If the parameter was not sent, the value retrieved is null.
  

POST

String param_value = request.getParameter("param_name");

• If the parameter was not sent, the value retrieved is null.

Environment variables

String baseUrl   = request.getHeader("Referer");
String host      = request.getHeader("Host");
String userAgent = request.getHeader("User-Agent");

HTML forms

Text field

• If the field is empty, the value retrieved is an empty string.
  

Radio button

• If no item was selected, the value retrieved is null.
  

Check box

• If the field is unchecked, the value retrieved is null.
• If the field is checked, the value retrieved is "on".

Single-selection list

• If no item was selected, the value retrieved is null.
  

Multiple-selection list

• The value is retrieved as an array of strings.
• If no items were selected, the value retrieved is null.