garbage collection happens when jvm finds that any methods or variables declared but does not have any action and kept dummy then garbage is invoked..
only jvm decides when to collect garbage we can not instruct jvm..
the main thing is garbage collection is also a destructor in java.. it frees memory..

garbage collector(gc) is a Daemon thread that runs on the sake of jvm. gc is called, not only wen jvm required memory but also any random time assigned by java manager thread. nobody knows wen gc will be called. u can also call gc method( System.gc()) for the request of garbage collection but remember, this is just a explicit request but not the order.