What are hashtables and hashmaps and their typical use cases?

Question

I have recently run across these terms few times but I am quite confused how they work and when they are usualy implemented?

Answer 1

lassevk's answer is very good, but might contain a little too much detail. Here is the executive summary. I am intentionally omitting certain relevant information which you can safely ignore 99% of the time.

There is no important difference between hash tables and hash maps 99% of the time.

Hash tables are magic

Seriously. Its a magic data structure which all but guarantees three things. (There are exceptions. You can largely ignore them, although learning them someday might be useful for you.)

1) Everything in the hash table is part of a pair -- there is a key and a value. You put in and get out data by specifying the key you are operating on.

2) If you are doing anything by a single key on a hash table, it is blazingly fast. This implies that put(key,value), get(key), contains(key), and remove(key) are all really fast.

3) Generic hash tables fail at doing anything not listed in #2! (By "fail", we mean they are blazingly slow.)

When do we use hash tables?

We use hash tables when their magic fits our problem.

For example, caching frequently ends up using a hash table -- for example, let's say we have 45,000 students in a university and some process needs to hold on to records for all of them. If you routinely refer to student by ID number, then a ID => student cache makes excellent sense. The operation you are optimizing for this cache is fast lookup.

Hashes are also extraordinarily useful for storing relationships between data when you don't want to go whole hog and alter the objects themselves. For example, during course registration, it might be a good idea to be able to relate students to the classes they are taking. However, for whatever reason you might not want the Student object itself to know about that. Use a studentToClassRegistration hash and keep it around while you do whatever it is you need to do.

They also make a fairly good first choice for a data structure except when you need to do one of the following:

When Not To Use Hash Tables

Iterate over the elements. Hash tables typically do not do iteration very well. (Generic ones, that is. Particular implementations sometimes contain linked lists which are used to make iterating over them suck less. For example, in Java, LinkedHashMap lets you iterate over keys or values quickly.)

Sorting. If you can't iterate, sorting is a royal pain, too.

Going from value to key. Use two hash tables. Trust me, I just saved you a lot of pain.