5 Difference between Hashtable vs HashMap in Java? Answer (original) (raw)

Hashtable vs HashMap in Java

Hashtable and HashMap are two hash-based collections in Java and are used to store objects as key-value pairs. Despite being hash-based and similar in functionality there is a significant difference between Hashtable and HashMap and without understanding those differences if you use Hashtable in place of HashMap then you may run into series of subtle programs which is hard to find and debug. Unlike the Difference between ArrayList and HashMap, Differences between Hashtable and HashMap are more subtle because both are similar kinds of collections. Before seeing the difference between HashMap and Hashtable let's see some common things between HashMap and Hashtable in Java.

Similarities between Hashtable and HashMap in Java

There are a lot of similar things between Hashtable and HashMap in Java which is good to know and these also helps to find exactly what is different between HashMap and Hashtable in Java:

1. Common Parent

Both Hashtable and HashMap implements java.util.Map interface.

2. Common Underlying data structure

Hashtable and HashMap both are hash-based collections and works on the principle of hashing.

3. Common SLA

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

From JDK 4 both Hashtable and HashMap are part of the Java collection framework.

Difference between Hashtable and HashMap in Java

Despite being so similar there are some differences between Hashtable and HashMap in Java which separates them completely, let's have a look :

1. Thread safety

The second important difference between Hashtable and HashMap is performance since HashMap is not synchronized it perform better than Hashtable.

3. Old vs New

4. Synchronized

Hashtable is a synchronized collection but HashMap is not. All methods of Hashtable are synchronized to prevent multithreading issues.

5. Null Key

Since Hashtable is a synchronized collection it doesn't allow Null keys but HashMap does allow null keys and values.

6. Fail fast (Iteraotr vs Enumeration)

Enumeration is used to iterate over keys and values in Hashtable which is not fail-fast, while Iterator is used to go over keys and values in HashMap and its fail-fast.

Here is all the difference between HashMap and Hashtable in the nice tabular format:

That's all about HashMap vs Hashtable in Java. These were some important differences between Hashtable and HashMap in Java. If you know any other difference which is not included here then feels free to add them in the comment section. Remember this is an important question on Java interview and good to prepare it well.