I was wondering whether java will be possible to make online games like maplestory.i want to make a nice game like maplestory no matter how hard or how long i will take,but now i will need to find a good programming language to make something like maplestory or better.so can someone if possible tell me whether java will be able to make such a game? 
Absolutely, but if you are asking this question you are in for some difficult times (regardless of programming language)! Good luck 
Well Haha im kinda new,but what does slick2d helps me with? :o
Its a small java games library that will give you fast hardware accelerated graphics, sound and input. Its ideal for 2d games and has many useful tools to help you along the way. Currently its the best java 2d games library and should hep you get started quickly.
Well then,i visited the site and realised that the games they show does not look like it is suitable to play online.looks more like flash games,so i was wondering is it really possible to add your own graphics,sounds,effects,etc. And allow people to play online like maplestory?
Im confused… ???
Well slick2d would just be a for front end stuff, and yes you have to add all your own graphics and sound effects.
For online play you’d have to write your own networking code for a server and clients (possibly even use a networking library to make it easier).
Writing MMO style games is not an easy task and not recommended for the lone developer especially if your new to games coding.
Ok just a few questions,what is a networking library? And may i know how does the server side and cilent side works?is slick 2d made for the server side or cilent?
A networking library, is a package to handle normal networking code.
Slick2D is to handle the client side graphics.
I would recommend first learning the language, part of the learning process is getting a basic understanding of Java networking (get a Java programming book).
And how do i start learning the basics first?just by reading?
Start by lowering your exceptions to something much smaller and manageable. A simple game like space invaders or similar should be your initial goal. You should start with basic java tutorials to get familiar with the language first before moving to starting on a game.
Only once you’ve written a few small games and are confident with your skills, you’ll know whether your ready to attempt such a large task.
However most newbies generally learn the hard way by jumping in to a really large project, working really hard on it and then giving up after a while when they realise that the project is out of their league.
Thanks alot for your help!i will try