1) what is the difference between doPost() and doGet()
2) what is difference between sendRedirect() and forward()
3) can we call destroy() in servlet class
4) advantages and disadvantages of Hidden Form fields,URLRewritting,Cookies
5) can we write a constructor for a servlet class
Is this your homework or are you really serious?
I'm pretty much stunned by the questions and I got an answer for you RTFM.
You haven't asked 1 usefull question that can't be found in ANY book, tutorial, document, related website or what so ever.
Please atleast read a book or do some tutorial, ask specific questions that you have problems _AFTER_ searching for answers yourself, then I (and probably others) will try and help you.