title: when abstract class doesn't contain any abstract method
body: <p>I am still confused when abstract class doesn't contain any abstract method, what a purpose of it? why don't use regular class rather than abstract class if it doesn't contain any abstract method ? In fact, I was saw this situation is applied on java and libgdx library or perhaps for every library. 
  So, because this situation,  I was thinking  is it very important to know why use abstract class without abstract method rather than regular class.  </p>