Garbage Collection is an automated process in Java unlike C 
and C++. It executes to wipe out those objects which no 
longer are used in a program or stay idle for a very long 
time. This to free memory space alloted to these 
objects.However, it is not guaranteed that all the idle 
objects wil be destroyed every time garbage collection 
process gets executed.A user can also manually call the 
garbage collection to take place by executing System.gc() 
method.

Another way to call garbage collector :-
Runtime rt = Runtime.getRuntime();
rt.gc();

Variable/object 
1. is eligible for garbage collection when no object refers
to it. 
2. Is eligible when its reference is set to null  i.e
objName=null.
3. referred by method variables or local variables are
eligible for garbage collection  when they go out of scope.

As in Java its happen automatically, but if you want to do 
this force fully you can do this by the use of 
keyword "finalize".