Private methods in an interface are new in Java 9. They allow you to make interface methods (with a body) accessible only to other methods (with a body) in the same interface.
A private method in an interface cannot be overridden anywhere--not by a method of any interface, nor by a method in any class. Its only purpose is to contain functionality that is common between two or more methods with a body, either private or public, in the same interface. It can also be used by one method only in order to make the code easier to understand.
A private interface method must have an implementation. A private interface method not used by other methods of the same interface does not make sense.
A static private interface method can be ...