Re: Why is Java lying?

From:

dantarion@gmail.com

Newsgroups:

comp.lang.java.programmer

Date:

28 Nov 2006 12:20:05 -0800

Message-ID:

<1164745205.458855.324220@j72g2000cwa.googlegroups.com>

You have to cast from Object to String.

java does not know that that "Object" is a "String".
it does know that "String"s are "Object"s.

You have to cast whenever you go from general->specific

String v2 = (String) session.getValue(attr);

On Nov 28, 12:12 pm, Thomas Fritsch <i.dont.like.s...@invalid.com>
wrote:

laredotorn...@zipmail.com wrote:

I'm trying to compile a very simple JSP page on Tomcat 5.5, JDK 1.5

<%@ page import="java.util.*" %>
<%
   Object v = new String("b");
   session.setAttribute("a", v);

   Enumeration e = session.getAttributeNames();
   while (e.hasMoreElements()) {
       String attr = (String) e.nextElement();
       String v1 = session.getAttribute(attr);
       String v2 = session.getValue(attr);I suspect the JSP compiler complains about the line above, because

HttpSession.getValue is declared as
public Object getValue(String name)
but not as
public String getValue(String name)
See
<http://java.sun.com/j2ee/1.4/docs/api/javax/servlet/http/HttpSession....>.

       out.println("attr: " + attr + " v1:" + v1 + " v2:" + v2);
   } // while
%>

but I'm getting this compile error:

An error occurred at line: 2 in the jsp file: /session_vars.jsp
Generated servlet error:
Type mismatch: cannot convert from Object to String

An error occurred at line: 2 in the jsp file: /session_vars.jsp
Generated servlet error:
Type mismatch: cannot convert from Object to String

   org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:512)
   org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:377)
   org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
   org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
   javax.servlet.http.HttpServlet.service(HttpServlet.java:802)

First off, the line number is incorrect (line 2 is "<%") but more
importantly, I thought String extended Object.Don't ask me why the compiler says "line: 2", although it actually seems

to be line 10.

Any thoughts? - Dave--

Thomas