What are the differences between System.gc() and Runtime.gc()?

There is no difference between these two methods.

Both methods suggest that the Java Virtual Machine expend effort toward recycling unused objects in order to make the memory they currently occupy available for quick reuse. When control returns from the method call, the Java Virtual Machine has made a best effort to reclaim space from all discarded objects.

The System.gc() is a static method so it's a little bit more convenient to use. The call System.gc() is effectively equivalent to the call:

 Runtime.getRuntime().gc()