Once you understand the basics you may want to migrate from using the XMLHttpReuqest object directly to using one of the available libraries, e.g. Prototype. Those make the communication easier to deal with.
Originally posted by Suganya Rajarethinam: Now i want to develop refresh a page using Ajax with java(servlet).
I'm not at all sure what you mean by this. One of the whole points of Ajax is that you don't need to refresh the whole page -- you can go back to the server to fetch data without the need for full-page refresh.
And using a servlet as the server component is no problem -- Ajax doesn't care how the server side is implemented.