When does Garbage collection occur in java?

Java

No one can force garbage collector to free memory, A Java object is subjected to garbage collection when it can not be reached by the program in which it is being used. Programmer can request for garbage collection by System.gc() but JVM doesn't guarantee that is will start immediately

Actually, the specific method for GC and how it will be run is dependent on the JVM implementation being used.

For instance, Sun (now Oracle)'s Hotspot JVM has several different GC designs, one of which (the RealTime engine) can be immediately triggered via a System.gc() call. Each type of GC will have different triggers and thresholds as to when it will be run, which allows the user to pick the GC option that they think will be least disruptive and most effective for their particular application.

You must read the JVM documentation for your particular VM to see what available GC options there are, how each GC method works, and what the various thresholds are.