NullPointerException on facebook login using servlet

Question

after solve class notfound exeption i have to find error when i login in face book using given example

http://code.google.com/p/facebook-java-api/wiki/Examples

java.lang.NullPointerException
    com.google.code.facebookapi.ExtensibleClient.<init>(ExtensibleClient.java:111)
    com.google.code.facebookapi.ExtensibleClient.<init>(ExtensibleClient.java:99)
    com.google.code.facebookapi.ExtensibleClient.<init>(ExtensibleClient.java:95)
    com.google.code.facebookapi.FacebookXmlRestClientBase.<init>(FacebookXmlRestClientBase.java:44)
    com.google.code.facebookapi.FacebookXmlRestClient.<init>(FacebookXmlRestClient.java:10)
    FaceBookCrawl.FacebookUserFilter.doFilter(FacebookUserFilter.java:107)
    FaceBookCrawl.FacebookUserFilter.doPost(FacebookUserFilter.java:181)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

Answer 1

It is open source; why are you not just examining the source code to answer your question? This is the latest version of ExtensibleClient. The line numbers are slightly different from the version you are using, but the only obvious source of NPE in a constructor is if you are passing null for secret.