Well thank you sir but i still have a problem, the code needs a certain package because I can't compile and run it coz the main class which is Collision2D_e, extends this right ..org.opensourcephysics.ejs.AbstractModel and so with the other java programs so i don what to do it just can't run without this org package

EJS will generate all the java class files and pack it into jar for you.
I  do not understand why you need to compile it again.
If you really need to do it, you will find osp.jar,ejs.jar... under ejs/bin

Please write down what is your final goal? May be there is some other easy way to do it (Not the way you were doing right now.)