What is the difference between a hash table and hashmap?

Answer 1

Hash tables and hashmaps are basically the same: they store data using key-value pairs. The main difference is that hash tables are thread-safe (suitable for multiple processes) but slower, while hashmaps are faster but not thread-safe. Hash tables also don't allow null values, but hashmaps do. Most people use the terms interchangeably, though!

Answer 2

Main difference is that a hashtable is synchronized and allows multiple threads to work with it while a hashmap is not synchronized and you have to do your own synchronization. The synchronization for hash tables slows it down if working in a single threaded environment. Hashtables don’t allow null keys or values while a Hashmap allows one null key and multiple values. It’s preferred to use a HashMap or a ConcurrentHashMap rather than a Hashtable for performance reasons.

Answer 3

HashMap and Hashtable are very similar key-value data structures in Java.

Hashtables are generally heavier and slower because it has built-in protection that only allows its values to be accessed one thread at a time.

HashMaps are modern, faster, and allows null values. If you need thread-safety like Hashtable provides, you can simply use a ConcurrentHashMap which is designed to handle it for you.

Why is Hashtable still in Java if there are better alternatives?

Java values backward compatibility, so old features like Hashtable still exist to keep old programs functional. That said, now you know what to use in modern development.

Answer 4

Features: Synchronization (execution thread safety)	HashMap: Not synchronized (not thread-safe)	HashTable: Synchronized (thread-safe)
Null values	Allows one null key and multiple null values	Does not allow null keys or values
Iteration	Uses Iterator	Uses Enumerator
Performance	Generally faster	Generally slower

Which one to use?

1. For thread safety, use ConcurrentHashMap instead of Hashtable. ConcurrentHashMap provides better performance than Hashtable while still being thread-safe.
2. If you don't need thread safety, use HashMap. It is generally faster and more flexible than Hashtable.

Hashtable is a legacy class in Java.

HashMap and ConcurrentHashMap are generally preferred for new code.

In Java, HashMap and Hashtable are both classes that implement the Map interface.

This means they store data in key-value pairs.

They both use a hash table as their underlying data structure, key differences are:

Synchronization:

1. Hashtable is synchronized: Only one thread can access aHashtableat a time. Hashtable is thread-safe and less efficient in single-threaded applications.
2. HashMap is not synchronized: Multiple threads can access aHashMapsimultaneously. HashMap is more efficient in single-threaded applications, but not thread-safe. If multiple threads access a HashMapconcurrently, and one or more threads modify the map structurally, then the map must be synchronized externally.  

Performance:

1. HashMap is generally faster than Hashtable:HashMap is not synchronized, it has less overhead and is faster thanHashtable, especially in single-threaded applications.

The two process Null values differently & use different enumerators/iterators.

Quiz Time ;)

1. What does a Hashtable use for Iteration?
2. How about a HashMap?
3. Why does a Hashtable use a lowercase "t" whereas a HashMap uses an uppercase "M"?