I looked more into my arguments about "contains" relationship b/w interfaces and I found
this group discussion Please refer to the "Oreilly's Nutshell" quote there which says,
"
Nested interfaces are implicitly static ([snip]) and so are always
top level. A nested top-level class or interface behaves just like
a "normal" class or interface that is a member of a package. [snip]
Nested top-level classes and interfaces are typically used as a
convenient way to group related classes.
"
Regards
Maulin