String.Compare an example of abstraction or encapsulation?

Yes it is.

As String class gives you a method called compare to do comparison, it does not show you how the comparison is done
Whether it calls a method to convert string into chararray and iterated through the chararay, or calls a method to convert characters into ASCII codes and compare the ASCII value

You do not know how it handles these things

So this is good example of Abstraction and Encapsulation.

In the end - 

 

 Nobody should know how it is done.