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All Collections implements interface Collection, these collection have specific abstract hierarchy e.g.
But there are also corresponding interfaces like Collection, List, Set. These interface seem to me kind of redundant.
Why are they here ? Is is just convention or is there a reason to just implement interface and not extend the abstract class.
The interfaces are there because it’s good to be able to assign a type to a variable or parameter without imposing an implementation.
For instance if I create a persistent entity to use in Hibernate, and it has a collection of things, I want to assign a type of List or Set to that. Hibernate will swap out whatever list I initialize it to with its own, with some Hibernate-specific implementation that does lazy-loading or whatever else it needs to. The Hibernate developers may not want to be constrained by having to extend the abstract class.
The abstract class exists as a convenience for implementers. The interface is a contract used by clients.