This week's book giveaways are in the Refactoring and Agile forums. We're giving away four copies each of Re-engineering Legacy Software and Docker in Action and have the authors on-line! See this thread and this one for details.
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: