Question

When you perform a lookup in a Hashtable, the key is converted into a hash. Now using that hashed value, does it directly map to a memory location, or are there more steps?

Just trying to understand things a little more under the covers.

And what other key based lookup data structures are there and why are they slower than a hash?

Answer 1

A Hash (aka Hash Table) implies more than a Map (or Associative Array).

In particular, a Map (or Associative Array) is an Abstract Data Type:

…an associative array (also called a map or a dictionary) is an abstract data type composed of a collection of (key,value) pairs, such that each possible key appears at most once in the collection.

While a Hash table is an implementation of a Map (although it could also be considered an ADT that includes a “cost”):

…a hash table or hash map is a data structure that uses a hash function to map identifying values, known as keys […], to their associated values […]. Thus, a hash table implements an associative array [or, map].

Thus it is an implementation-detail leaking out: a HashMap is a Map that uses a Hash-table algorithm and thus provides the expected performance characteristics of such an algorithm. The “leaking” of the implementation detail is good in this case because it provides some basic [expected] bound guarantees, such as an [expected] O(1) — or constant time — get.

Hint: a hash function is important part of a hash-table algorithm and sets a HashMap apart from other Map implementations such as a TreeMap (that uses a red-black tree) or a ConcurrentSkipListMap (that uses a skip list).

Another form of a Map is an Association List (or “alist”, which is common in LISP programming). While association lists are O(n) for get, they can have much less overhead for small n, which brings up another point: Big-Oh describes limiting behavior (as n -> infinity) and does not address the relative performance for a particular [smallish] n:

A description of a function in terms of big O notation usually only provides an upper bound on the growth rate of the function.

Please refer to the links above (including the javadoc) for the basic characteristics and different implementation strategies — anything else I say here is already said there (or in other SO answers). If there are specific questions, open a new SO post if warranted 🙂

Happy coding.

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Here is the source for the HashMap implementation that is in OpenJDK 7. Looking at the put method shows that it a simple chaining as a collision-resolution method and that the underlying “bucket array” will grow by a factor of 2 each resize (which is triggered when the load factor is reached). The load factor and amortized performance expectations — including those of the hashing function used — are covered in the class documentation.