Exact usage of System.gc

hi i am new to java prgm.i had used system.gc in my prgm to destroy obj.
but after using system.gc

like..

system.gc();
obj.getsome();
this will call the method getsome

and

obj=null;
obj.getsome();
won't work

how exactly i have to use system.gc

Traditionally you never need to call System.gc(). In general it is a hint to the JVM, and is not guaranteed to actually do anything.

In your second call, you'll get a NullPointerException as the Object obj is now null.

What are you trying to solve? A little more information would be helpful.