I am wondering why the Framework doesn’t contain structures as Tree and Graph which are basic collections. Both can be regarded as sub types of
Collection.
This is a good question. I think it simply boils down to scoping. The core features that Collections API provides classes for are:
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iteration order: Lists and sorted maps have specified iteration order, most sets don’t.
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duplicates: Lists allow duplicates, sets do not
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index: List values are indexed by integers, map values are indexed by other objects.
This gets us very far and I assume Joshua Bloch et al argued that more feature rich collections (graphs and trees which require internal relationship between elements, sets with multiplicity, bi-directional maps, …) can be implemented on top of these three core features, and are thus better off in libraries.