interface 本身就是abstract的，只不過沒有明確的規定出來，說一定要把abstract這個關鍵字寫上。
所以abstract interface 就是interface，兩者根本沒有區別。
在《java in a nutshell》里，
“All methods of an interface are implicitly abstract, even if the abstract modifier is omitted. ”
在《thinking in java》里，
“ the abstract keyword, which allows you to create one or more methods in a class that have no definitions
—you provide part of the interface without providing a corresponding implementation, which is created by inheritors.
The interface keyword produces a completely abstract class, one that provides no implementation at all.”
所以結論就是 abstract interface 就是interface，兩者根本沒有區別。