ServletRequest.getParameterMap() returns Map and ServletRequest.getParameter() returns String?

Question

Can someone explain to me why ServletRequest.getParameterMap() returns type

Map 

ServletRequest

Answer 1

In the case with multi-value controls (checkbox, multi-select, etc), the request.getParameterValues(..) is used to fetch the values.

Answer 2

If you have a multi-value control like a multi-selectable list or a set of buttons mapped to the same name multiple selections will map to an array.

Answer 3

It returns all parameter values for controls with the same name.

For example:

<input type="checkbox" name="cars" value="audi" /> Audi
<input type="checkbox" name="cars" value="ford" /> Ford
<input type="checkbox" name="cars" value="opel" /> Opel

or

<select name="cars" multiple>
    <option value="audi">Audi</option>
    <option value="ford">Ford</option>
    <option value="opel">Opel</option>
</select>

Any checked/selected values will come in as:

String[] cars = request.getParameterValues("cars");

It's also useful for multiple selections in tables:

<table>
    <tr>
        <th>Delete?</th>
        <th>Foo</th>
    </tr>
    <c:forEach items="${list}" var="item">
        <tr>
            <td><input type="checkbox" name="delete" value="${item.id}"></td>
            <td>${item.foo}</td>
        </tr>
    </c:forEach>
</table>

in combination with

itemDAO.delete(request.getParameterValues("delete"));

Answer 4

http://foo.com/bar?biff=banana&biff=pear&biff=grape

"biff" now maps to {"banana","pear","grape"}

Answer 5

The real function to get all parameter values is

   request.getParameterValues();

getParameter() is just a shortcut to get first one.