Interface implemented by some enums and mostly other
that have custom "null" behavior in place of Java's built-in
NOTE: There is also an informal "statically polymorphic" contract imposed by this
interface that says an implementor must have a
public static final NULL
member or method.
For enums this simply means having a NULL entry.
NOTE ALSO: Another soft contract imposed by this interface is that the library (in this case SweetBlue)
hosting the implementor must never return an actually
reference of the implementor.