What are the differences between hashtable and hashmap? (Not specific to Java)

Question

During my most recent job interview for a software engineer position, I was asked this questions: what are the differences between hashtable and hashmap? I asked the interviewer if he was specific about Java since in Java hashtable is synchronized and hashmap is not (and actually tons of information to compare hashtable vs hashmap in Java after googling so that's not the answer I am looking for) but he said no and wanted to me to explain the the difference of these two in general.

I was really puzzled and shocked (actually still puzzled now) about this question. IMO, hastable or hashmap is simply a matter of terminology. Actually only Java has both terms and in other languages like C++, they don't even have the term hashtable. During the interview, I just explained the principle of hashing and said that hashmap and hashtable should both be implemented based on this principle and I don't know if there is any difference between these two. The interviewer was definitely not convinced and was looking for other answers and of course I was rejected after that round.

So back to the topic, what could possibly be the differences between hashmap and hashtable in general (not specific to Java) if there is any?

Answer 1

In Computer Science there is a difference due to the wording.

A HashTable is some kind of lookup table using key hashes to lookup the corresponding value in a table like data structure. Thats only one kind of a key-value Mapping. There are different implementations as you are probably aware. Different hashes, hash collusion solutions and table growing strategies and more under the hood. It's only interesting if you need to make your own hash table for whatever reason.

A HashMap is some kind of mapping of key-value pairs with a hashed key. Mapping is abstract as such and it may not be a table. Balanced trees or tries or other data structures/mappings are possible too.

You could simplify and say that a HashTable is the underlying data structure and the HashMap may be utilizing a HashTable.

A Dictionary is yet another abstraction level since it may not use hashes at all - for example with full text binary search lookups or other ways for compares. This is all you can get out of the words without considering certain programming languages.

-- Before thinking too much about it. Can you say - with certainty - that your interviewer had a clue about what he/she was talking about? Did you discuss technical details or did they just listen/ask and sometimes comment? Sometimes interviewers just come up with the most ridicules answers to problems they don't really understand in the first place. Like you wrote yourself, in general it's just Terminology. Software Developers often use the Terms interchangeable, except maybe those who really have differences like in Java.