Double to String: String.valueOf(double)

String to double: Double.parseDouble(String s)

Say you have a bean called MyBeanwith with a field f1 ( a double)
In your jsp page:
<jsp:useBean id="bean_instance" scope="session" class="com.package.MyBean" />

Parsing an incoming parameter to a double:

<%
String double_str=request.getParameter("double_val");
double dbl = Double.parseDouble(double_str);
%>

<jsp:setProperty name="bean_instance" property="f1" value="<%=dbl%>" />

The other way round :

<%
String parsed_double=String.valueOf(dbl);
%>