The following function is used to compute memory usage.
I do not understand how to understand "Runtime.getRuntime().totalMemory()"? In specific, how to understand the relationships connecting "Runtime", "getRuntime()" and "totalMemory()"?
Runtime is an abstract class. There is no direct way to instantiate it. It has a static method to get an instance of some unknown subclass. This method is called getRuntime(). Therefore, Runtime.getRuntime() returns an instance of a subclass of Runtime.
totalMemory() is an instance method that you call on that instance. Your single line is similar to this: