Collective Interview Question for Software Engineer / Developers

Country: India
Interview Type: In-Person

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Hashtable and HashMap both are hash based collection and works on principle of hashing.

Hashtable and HashMap both provide constant time performance for put and get method if objects are distributed uniformly across bucket.

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.

4. If synchronization is not an issue, we can recommend HashMap. If synchronization becomes an issue, we may also look at ConcurrentHashMap.

5. Performance: Since HashMap is not synchronized it perform better than Hashtable.

6. In hashmap you can remove element while iterating, safely. where as it is not possible in hashtables.

7. If i am not wrong, Hashtable is obsolete class and you should be using ConcurrentHashMap in place of Hashtable in Java.

There is a nice discussion going on in StackOverflow, might helpful for others.

Since there is no HashMap in C#.
Dictionary is probably the closest, but not exact, (one e.g. it wont accept null key values.)
System.Collections.Generic.Dictionary implements the System.Collections.Generic.IDictionary interface.

Refer the below thread for more info.

- Srigopal Chitrapu April 29, 2014 | Flag Reply

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