How to Use the Methods of the Request Object - Java

In the last figure, you learned how to use the getParameter method to return the value that the user entered into a textbox. Now, Figure 5 summarizes that method and illustrates it in a new context. This figure also summarizes and illustrates two more methods of the implicit request object.

Figure 5: How to use the methods of the request object

Three methods of the request object

More Scriptlets

A scriptlet that determines if a checkbox is checked

<%
    String rockCheckBox = request.getParameter("Rock");
    // returns the value or "on" if checked, null otherwise.
    if (rockCheckBox != null){
%>
        You checked Rock music!
<% 
    }
%>

A scriptlet that reads and displays multiple values from a list box

<%
    String[] selectedCountries = request.getParameterValues("country");
    // returns the values of items selected in list box.
    for (int i = 0; i < selectedCountries.length; i++){
%>
       <%= selectedCountries[i] %> <br>
<%
    }
%>

A scriptlet that reads and displays all request parameters and values

<%
    Enumeration parameterNames = request.getParameterNames();
    while (parameterNames.hasMoreElements()){
        String parameterName = (String) parameterNames.nextElement();
        String parameterValue = request.getParameter(parameterName);
%>
        <%= parameterName %> has value <%= parameterValue %>. <br>
<%
     }
%>

Description

You can use the getParameter method to return the value of the selected radio button in a group or the selected item in a combo box. You can also use it to return the value of a selected check box or independent radio button, but that value is null if it isn't selected.

If an independent radio button or a checkbox doesn't have a Value attribute, this method returns "on" if the control is selected or null if it isn't.

In most cases, the getParameter method returns the value of the parameter. For a textbox, that's usually the value entered by the user. But for a group of radio buttons or a combo box, that's the value of the button or item selected by the user.

For checkboxes or independent radio buttons that have a Value attribute, the getParameter method returns that value if the checkbox or button is selected and a null value if it isn't. For checkboxes or independent radio buttons that don't have a Value attribute, though, the getParameter method returns an "on" value if the checkbox or button is selected and a null value if it isn't. This is illustrated by the first example in this figure.