By using request.getParameter() we can retrive the values that are submitted by an HTML form or that was included in a query string in URL. If we are using servlet code to set an attribute, we have to use the request.setAttribute("xxx","zzz") to set it and request.getAttribute("xxx") to retrieve it. The getAttribute() method always returns an Object and the getParameter() method always returns a String.
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I think your question should be what is the difference between a parameter and an attribute. Once you understand them clearly, you will understand the ways to manipulate them. Look at HFSJ for the 5th chapter. Your question is answered there more clearly!