Difference Between Hashset and Hashmap

Difference between HashSet and HashMap?

They are entirely different constructs. A HashMap is an implementation of Map. A Map maps keys to values. The key look up occurs using the hash.

On the other hand, a HashSet is an implementation of Set. A Set is designed to match the mathematical model of a set. A HashSet does use a HashMap to back its implementation, as you noted. However, it implements an entirely different interface.

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Hashtable, HashMap, HashSet , hash table concept in Java collection framework

Java's Set and Map interfaces specify two very different collection types. A Set is just what it sounds like: a collection of distinct (non-equal) objects, with no other structure. A Map is, conceptually, also just what it sounds like: a mapping from a set of objects (the distinct keys) to a collection of objects (the values). Hashtable and HashMap both implement Map, HashSet implements Set, and they all use hash codes for keys/objects contained in the sets to improve performance.

Hashtable and HashMap

Hashtable is a legacy class that almost always should be avoided in favor of HashMap. They do essentially the same thing, except most methods in Hashtable are synchronized, making individual method calls thread-safe.¹ You have to provide your own synchronization or other thread safety mechanism if you are using multiple threads and HashMap.

The problem with Hashtable is that synchronizing each method call (which is a not-insignificant operation) is usually the wrong thing. Either you don't need synchronization at all or, from the point of the view of the application logic, you need to synchronize over transactions that span multiple method calls. Since it was impossible to simply remove the method-level synchronization from Hashtable without breaking existing code, the Collections framework authors needed to come up with a new class; hence HashMap. It's also a better name, since it becomes clear that it's a kind of Map.

Oh, if you do need method-level synchronization, you still shouldn't use Hashtable. Instead, you can call Collections.synchronizedMap() to turn any map into a synchronized one. Alternatively, you can use ConcurrentHashMap, which, according to the docs: "obeys the same functional specification as Hashtable" but has better performance and additional functionality (such as putIfAbsent()).

¹ _{There are other differences (less significant, in my view) such as HashMap supporting null values and keys.}

HashSet

In terms of functionality, HashSet has nothing to do with HashMap. It happens to use a HashMap internally to implement the Set functionality. For some reason, the Collections framework developers thought it would be a good idea to make this internal implementation detail part of the public specification for the class. (This was an error, in my view.)

Java hashmap vs hashset performance

HashMap vs Vector: Inserting in HashMap is way more costlier than inserting in Vector. Although both are amortized constant time operations, but the HashMap performs a number of other operations internally (like generating hashCode, checking collissions, resolving collissions, etc), whereas the Vector just inserts the element at the end (increasing the size of the structure, if required).

HashMap vs HashSet: HashSet internally uses HashMap. So, there shouldn't be any performance difference whatsoever if you use them for the same purpose. Ideally, both of these have different purposes, so the discussion regarding which is better is useless.

Since, you need B,C,D as value for A as key, you should definitely stick to HashMap. If you really want to just compare the performance, put "null" instead of 0.0 as the value for all the keys (because that is what HashSet uses while putting the keys in its backed HashMap).

Update: HashSet uses a dummy constant value (static final) to insert in the HashMap, and not null. Sorry about that. You can replace your 0.0 with any constant and the performance should be similar to HashSet.

What's the difference between HashSet and Set?

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).

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