Garbage Collection is a complex feature in Java Technology.
By Programme we cannot cause gc but we can request JVM using
gc() method but it does not gurantee. Actually when an
object in our programme become unreachable it is ready to
garbage collection..the garbage collection runs periodically
and then claim the memory it does occupy ( the unreachable
object)..and is how garbage collected is done.

Garbage Collection is carried out based on the reference 
type an object is currently in.
There are 4 types of references in java
1)strong reference
2)soft reference
3)weak reference 
4)phantom reference

the last one is the weakest of the weak.
JVM will try get all those of object that are fell in this 
category and try to collect it.
But it is not neccessary the object in phantom reference 
phase will always be garbage collected.