What Are the Differences Between a Hashmap and a Hashtable in Java

What are the differences between a HashMap and a Hashtable in Java?

There are several differences between HashMap and Hashtable in Java:

1. Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.

2. Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.

3. One of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.

Since synchronization is not an issue for you, I'd recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.

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

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.

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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.

Difference between HashMap and HashTable purely in Data Structures

In Computing Science terminology, a map is an associative container mapping from a key to a value. In other words, you can do operations like "for key K remember value V" and later "for key K get the value". A map can be implemented in many ways - for example, with a (optionally balanced) binary tree, or a hash table, or even a contiguous array of structs storing the key/value.

A hash table is a structure for storing arbitrary data, and that data does not necessarily consist of a separate key and value. For example, I could have a hash table containing the values { 1, 10, 33, 97 }, which would be their own keys. When there is no value distinct from the key, this is sometimes known as a "set", and with a hash table implementation a "hash set". The defining quality of a hash table is that a hash function calculates an array index from the key data, with different keys tending to yield different indices, allowing constant time access to an array element likely to contain the key. That's an implementation / performance quality, rather than a functional quality like that defining a map.

So, a hash table stores elements, each of which need not consist of distinct key and value components, but if it does then it's also a hash map.

How is Hashtable different to Hashmap

java.util.Hashtable methods are synchronized , java.util.Hashmap methods are not. If you use Hashtable there will be a performance hit as no two threads will be able to access its methods at the same time.
If you care about Thread safety in your app Hashtable is the way to go. if you dont care about thread safety Hashmap is the way to go as it is mor eefficient then hashtable.
also java.util.Hashtable doesnt allow any null keys, where as java.util.HashMap allows one null key.

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