the Set interface is just such a restricted marker interface. It is applicable only to Collection subtypes, but it adds no methods beyond those defined by Collection. It is not generally considered to be a marker interface because it refines the contracts of several Collection methods, including add, equals, and hashCode. But it is easy to imagine a marker interface that is applicable only to subtypes of some particular interface and does not refine the contracts of any of the interface’s methods. Such a marker interface might describe some invariant of the entire object or indicate that
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> It is easy to imagine a marker interface that is applicable only to subtypes of some particular interface and does not refine the contracts of any of the interface’s methods. Such a marker interface might describe some invariant of the entire object or indicate that instances are eligible for processing by a method of some other class
What about RandomAccess, which marks that a list supports random access in O(1) - why not give that as an example rather than leave it to our imagination to come up with one?
