Do not forget the public reserved word when declaring a method from an interface. Although
interfaces cannot have instance variables, they can have constants. When declaring a constant in an
interface, you can (and should) omit the reserved words public static final, because all
variables in an interface are automatically public static final.
You can never construct an object of an interface type:
Measurable meas = new Measurable(); // Error
Interfaces aren’t classes. There are no objects whose types are interfaces.
Coming back to the question “what is the type of x?” Now we can use the interface type
Measurable to declare x and maximum.