Yes we can send arequest to garbage collection by writing a 
simple line of code:

System.gc();

OR

Runtime.gc();

But it is simply a request not a order to JVM
because Garbage collection is JVM dependent

It's Runtime.getRuntime().gc()

As far as I know,it is not required to call the garbage 
collector.
It automatically and periodically removes the unused 
objects by an algorithm known as "Mark and Sweep".

However, if we intentionally want to call it then the above 
suggestions are perfect.
Anyone who feels that it is incorrect, please feel free to 
correct me.