Developing JavaServer Pages - How to Use the Methods of the Request Object (Page 5 of 11 )
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
Method | Description |
| getParameter(String param) | Returns the value of the specified parameter as a string if it exists or null if it doesn't. Often, this is the value defined in the Value attribute of the control in the HTML page or JSP. |
| getParameterValues(String param) | Returns an array of String objects containing all of the values that the given request parameter has or null if the parameter doesn't have any values. |
| getParameterNames() | Returns an Enumeration object that contains the names of all the parameters contained in the request. If the request has no parameters, the method returns an empty Enumeration 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.
 Remember: this is from chapter four of Joel Murach's Java Servlets and JSP (Mike Murach & Associates, ISBN 1890774189, 2003). Grab a copy at your favorite book store today!
Buy this book now. |
Next: Retrieving Multiple Values >>
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