HashMap vs Hashtable: Key Differences and Performance Comparison

In computer science, techniques like hashing are used to store and retrieve data more promptly and productively. When working with key-value pairs in Java, the two most commonly used data structures are HashMap and Hashtable. Both data structure has similarities, but they differ significantly in terms of Synchronization, Null values, and Efficiency. So in this article, we will explore the key differences between HashMap vs Hashtable and compare their performances in different terms.

What are Hashtable and HashMap?

Before diving into the difference between HashMap vs Hashtable, let's just understand what they are. So, a hashtable is a type of data structure that applies a hash function to link keys with index values in an array. It keeps key-value pairs in a bucket, which is based on the hash code of the key to decide the bucket's index. Hash Tables may also be called hash maps, maps, dictionaries, or associative arrays. In a Hash Table, each key is unique, although the same value may appear more than once.

A Hash Map is a type of data structure designed to hold pairs of keys and values. It applies a hash function to calculate an index in an array of slots or buckets, allowing the retrieval of the required value. Hash maps are part of the Java. util package, a collection of Java frameworks, for fast retrieval, deletion, and insertion operations.

Difference between HashMap vs hashtable

There are quite a few differences between HashMap and Hashtable in terms of Data Structure, performance, null key/value support, internal implementation, Collision Handling, and Thread Safety. Let's have a brief study of these terms:

1. Data Structure

The primary difference between HashMap and Hashtable lies in their basic data arrangements. Hash Maps are designed with an array containing linked lists, with each item in the array representing a linked list of key-value pairs. In contrast, Hash Tables utilize an array of buckets, where every bucket holds a key-value pair.

2. Performance

In terms of performance, both data structures offer average-time complexity for searching, inserting, and deleting in constant time. However, performance may decline when collisions occur. In Java Hashtable vs HashMap, collisions are handled through chaining, where multiple key-value pairs are stored in the same bucket using a linked list. In contrast, Hash Tables use open addressing, where a secondary hash function selects a different bucket for the key-value pair.

3. Null Key/ Value Support

The difference between HashMap vs Hashtable null keys and values is that the Hashtable

keys are stored in sorted order. Does not allow having one null key and multiple null values. Whereas in a HashMap are not stored in any particular order and allow to have one null key and multiple null values.

4. Internal Implementation

In terms of internal implementation, HashMap vs Hashtable rely on employing buckets it have synchronized methods. HashMap buckets are determined by a hash function.

5. Collision Handling

HashTables vs HashMaps in collision handling can experience collisions. The key distinction in how collisions are managed depends on the technique applied to fix them. In HashMaps, collisions are managed through chaining, enabling the storage of several key-value pairs within the same bucket through a linked list.

In contrast, Hash Tables address collisions with open addressing. This method chooses another bucket for the key-value pair by utilizing a secondary hash function.

6. Thread Safety

Hash tables are often safe for threads, indicating that various threads can use them at the same time without damaging the data. On the other hand, hash maps are usually not safe for threads on their own, so extra measures must be taken to avoid any data damage.

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Example of Hashtable and HashMap in Java

As we know earlier, when we are working with key-value pairs in Java, the two most commonly used data structures are hash table vs hash map:

Hashtable Example in Java

import java.util.Hashtable;

HashMap Example in Java

import java.util.HashMap;

Conclusion

In conclusion, HashMap vs Hashtable are both used to store data in key-value pairs; however, they differ greatly in how they handle synchronization, null values, and their overall efficiency. For applications that run on a single thread, HashMap is the best option because it is quick and adaptable. On the other hand, a Hashtable ensures safety for threads but sacrifices some speed. In environments with multiple threads, ConcurrentHashMap is a superior option compared to Hashtable. Recognizing these distinctions will aid you in selecting the most suitable data structure for your application's requirements.