json object in JSP-Collection of common programming errors

It’s just telling that the mentioned class is missing in the webapp’s runtime classpath. You need to download and drop the JAR file holding the classes of net.sf.json package in /WEB-INF/lib folder (don’t forget to do the same for its dependencies as well, if necessary).

Unrelated to the concrete problem: writing Java code in a JSP file is a poor practice. Reading Roseindia.net tutorials is also a very bad idea as they only shows bad practices.