I just wanted to know that Do we have any API using which it is possible to
build our own GIS tool...

Or something like, a tool which can be easily interfaced with java to work as a
GIS system..

I have got this Geotools APIs and i am going through it right now... but i would
like to know more about this tool and also about any other alternate tool that
we can use

please help me out in this regard....