Could the interfaces contain several definitions of the same constant?

A good answer might be:

No. To ensure consistancy, a constant should be defined only once, in one interface.

However, it is OK if two interfaces ask for the same method. A class that implements the interfaces only needs to provide one complete method definition to satisfy both interfaces.

An interface can be made public. In fact, this is usually what is done. When a class or interface is public it must be the only public class or interface in the file that contains it. (These notes have been avoiding public classes so that the "copy paste save and run" method can be used with just one file.)

A public interface can be implemented by any class in any file. Many graphical user interface components are described with public interfaces. The classes you create must implement them to work with the GUI features of Java.

QUESTION 19:

Can an interface be made private?