CODEWITHSUNDEEP
javainterfaceset
user496949
user496949

Reputation: 86135

What's the difference between HashSet and Set?

Saw the code snippet like 

Set<Record> instances = new HashSet<Record>();

I am wondering if Hashset is a special kind of set. Any difference between them?

Upvotes: 112

Views: 147782

Answers (7)

Anonymous User
Anonymous User

Reputation: 1

**

  • Set:

** It is an interface which is a subtype of Collection interface, just like LIST and QUEUE.

Set has below 3 subclasses, it is used to store multiple objects without duplicates.

  1. HashSet
  2. LinkedHashSet
  3. TreeSet(which implements SortedSet interface)

**

  • HashSet:

**

Can use one NULL value(as Duplicate is not allowed), data is stored randomly as it does not maintain sequence.

Upvotes: -1

Hemlata Gehlot
Hemlata Gehlot

Reputation: 361

HashSet is a class derived from Set interface. As a derived class of Set, the HashSet attains the properties of Set. Important and the most frequently used derived classes of Set are HashSet and TreeSet.

Upvotes: 1

MeBigFatGuy
MeBigFatGuy

Reputation: 28578

The question has been answered, but I haven't seen the answer to why the code mentions both types in the same code.

Typically, you want to code against interfaces which in this case is Set. Why? Because if you reference your object through interfaces always (except the new HashSet()) then it is trivial to change the implementation of the object later if you find it would be better to do so because you've only mentioned it once in your code base (where you did new HashSet()).

Upvotes: 21

Erik
Erik

Reputation: 91270

A Set represents a generic "set of values". A TreeSet is a set where the elements are sorted (and thus ordered), a HashSet is a set where the elements are not sorted or ordered.

A HashSet is typically a lot faster than a TreeSet.

A TreeSet is typically implemented as a red-black tree (See http://en.wikipedia.org/wiki/Red-black_tree - I've not validated the actual implementation of sun/oracle's TreeSet), whereas a HashSet uses Object.hashCode() to create an index in an array. Access time for a red-black tree is O(log(n)) whereas access time for a HashSet ranges from constant-time to the worst case (every item has the same hashCode) where you can have a linear search time O(n).

Upvotes: 141

Umesh Kacha
Umesh Kacha

Reputation: 13676

Set is a parent interface of all set classes like TreeSet, LinkedHashSet etc.

HashSet is a class implementing Set interface.

Upvotes: 5

vaugham
vaugham

Reputation: 1851

The HashSet is an implementation of a Set.

Upvotes: 44

gmw
gmw

Reputation: 437

Set is the general interface to a set-like collection, while HashSet is a specific implementation of the Set interface (which uses hash codes, hence the name).

Upvotes: 12

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