Rule
http://www.janeg.ca/scjp/gc.html
- the Java garbage collector consists of three basic activities: 
- monitors program objects to determine when they are no longer required 
- informs selected objects that they should release any non-memory resources 
- destroys objects and reclaims their memory resources 
- the gc operates as a seperate asynchronous background thread that tracks all program objects 
- an object ceases to be needed by a program when it is no longer reachable 
- an object is reachable if a reference to the object exists in any variables of any executing code 
- an object is subject to garbage collection when it can no longer be reached but it is not necessarily garbage collected immeadiately 
- there are no guarantees as to when the gc will reclaim an object or the order in which objects are reclaimed 
- there is no way to tell if and when an object will be collected, you can only tell when an object becomes eligible for garbage collection 
- you can request garbage collection by calling one of 

Runtime.getRuntime().gc() // no guarantee gc will run 

System.gc() // no guarantee gc will run   
- you can also request that the finalize() method be run for objects deemed eligible for collection but which have not yet had their finalization code run 

Runtime.runFinalization()

System.runFinalization()