Which data structure used by HashSet?

Question

I have seen internal data structure used by hashset is HashTable on many websites but when i saw HashSet.class (after decompile) it is using HashMap.Now iam confused please clear my confusion.HashSet uses which data structure ?

Also tell me internal data structure used by linkedhashset, treeset, hashmap, hashtable, linkedhashmap, treemap.

Yes, a HashSet is backed by a HashMap. The Javadoc even says so. You should peruse the Javadoc for these classes; it would answer most of these questions. — Makoto
– Makoto, Commented Jun 21, 2014 at 18:36
@GeorgeNetu: No, I don't see that as a duplicate question at all. — Makoto
– Makoto, Commented Jun 21, 2014 at 18:41
@Makoto I would agree that the question is, but the "I mean implementation wise.....?" in the question is ignored in most every answer so .. I don't think it's a "suitable" duplicate. — user2864740
– user2864740, Commented Jun 21, 2014 at 18:42
stackoverflow.com/questions/12431581/… , stackoverflow.com/questions/20217414/… , stackoverflow.com/questions/11680751/… — user2864740
– user2864740, Commented Jun 21, 2014 at 18:43

Jigar Joshi · Accepted Answer · 2014-06-21 18:40:04Z

2

HashSet internally stores values as part of HashMap's key and it puts a dummy value as value

Check the source code, relevant portions extracted:

// Dummy value to associate with an Object in the backing Map
private static final Object PRESENT = new Object();
private transient HashMap<E,Object> map;

public boolean add(E e) { 
 return map.put(e, PRESENT)==null;
}

edited Jun 21, 2014 at 18:40

answered Jun 21, 2014 at 18:37

Jigar Joshi

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2 Comments

Makoto Over a year ago

Yes, this is correct, but it barely feels like an answer.

user2864740 Over a year ago

@Makoto Such is the problem with such questions :|

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