no no u can never force the jvm for garbage collection. System.gc() method only sends the request to jvm for the garbage collection. its all depend on jvm wheather it has to run the garbage collector or not.
simply call the system garbage collector System.gc(), for more information see the gc() documentation....
http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/System.html#gc()
