Init() Vs Constructor

Constructor is for initialization and so y they introduced init()?

init method is the life cycle method of servlet and constructor by default it exist if it is a java class.
If you look at the API for the following method init(ServletConfig config) it says
"Called by the servlet container to indicate to a servlet that the servlet is being placed into service. "

Through constructor container cannot place servlet for the service.

init is more specific to the container.

This gets asked quite often here.

You can either look at our entry in the FAQ:
http://faq.javaranch.com/view?ServletsFaq

Or search this forum with keywords "constructor" and "init" to find the question asked and answered many times.