The essential idea to remember is that an interface never contains any implementation. The following restrictions are natural consequences of this:
You're not allowed to define any fields in an interface, not even static ones. A field is an implementation detail of a class or structure.
You're not allowed to define any constructors in an interface. A constructor is also considered to be an implementation detail of a class or structure.
You cannot specify an access modifier for any method. All methods in an interface are implicitly public.