Checking uniqueness of data entry value

I have one registration form,in that one username field,
when user fill this text box and click on the "Check Availability" then my servlet have to check that user is exist or not.

and i want one boolean answer (true/false).

Have any other trick to fulfill my requirement?..

Please be sure to take the time to compose descriptive subjects for your posts; read this for more information.

A title such as "hrlp" is not helpful. What would happen if all posts had such a title?

Please go back and change your post to add a more meaningful subject by clicking the

button on your post.

With or without submitting the form?

Without Submitting form...
Only check availability of user?

Like When we create account in gmail,yahoo,hotmail and other.

Ajax.

P.S. Still waiting for that better subject!

Bear Bibeault wrote:P.S. Still waiting for that better subject!

Maybe you should hrlp them.

Sigh. Please take note of the subject I applied as a model for future posts.