Java – when to use Set vs. Collection

collectionsjava

Is there any practical difference between a Set and Collection in Java, besides the fact that a Collection can include the same element twice? They have the same methods.

(For example, does Set give me more options to use libraries which accept Sets but not Collections?)

edit: I can think of at least 5 different situations to judge this question. Can anyone else come up with more? I want to make sure I understand the subtleties here.

  1. designing a method which accepts an argument of Set or Collection. Collection is more general and accepts more possibilities of input. (if I'm designing a specific class or interface, I'm being nicer to my consumers and stricter on my subclassers/implementers if I use Collection.)
  2. designing a method which returns a Set or Collection. Set offers more guarantees than Collection (even if it's just the guarantee not to include one element twice). (if I'm designing a specific class or interface, I'm being nicer to my consumers and stricter on my subclassers/implementers if I use Set.)
  3. designing a class that implements the interface Set or Collection. Similar issues as #2. Users of my class/interface get more guarantees, subclassers/implementers have more responsibility.
  4. designing an interface that extends the interface Set or Collection. Very similar to #3.
  5. writing code that uses a Set or Collection. Here I might as well use Set; the only reasons for me to use Collection is if I get back a Collection from someone else's code, or if I have to handle a collection that contains duplicates.

Best Answer

Collection is also the supertype of List, Queue, Deque, and others, so it gives you more options. For example, I try to use Collection as a parameter to library methods that shouldn't explicitly depend on a certain type of collection.

Generally, you should use the right tool for the job. If you don't want duplicates, use Set (or SortedSet if you want ordering, or LinkedHashSet if you want to maintain insertion order). If you want to allow duplicates, use List, and so on.

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