What are the differences between HashMap and Hashtable?

Table of Contents

• Introduction
• Key Differences Between HashMap and Hashtable
  • 1. Synchronization
  • 2. Null Keys and Null Values
  • 3. Performance
  • 4. Iterator vs Enumerator
  • 5. Legacy Status
  • 6. Subclassing
  • 7. Thread-Safety Alternatives
• Practical Example: HashMap vs Hashtable
• Conclusion

Introduction

In Java, both **HashMap** and **Hashtable** are used to store key-value pairs, but they have distinct differences in terms of synchronization, null handling, and performance. While both are part of the Java Collections Framework and implement the **Map** interface, their behavior differs significantly.

This article outlines the major differences between HashMap and Hashtable to help you decide which to use in different scenarios.

Key Differences Between HashMap and Hashtable

1. Synchronization

One of the biggest differences between HashMap and Hashtable is their approach to synchronization.

• HashMap: It is not synchronized, meaning it is not thread-safe by default. If you are using HashMap in a multithreaded environment and need thread safety, you have to manually synchronize the map using Collections.synchronizedMap() or by using concurrency tools like **ConcurrentHashMap**.
• Hashtable: It is synchronized, meaning it is thread-safe and can be used in multithreaded environments without additional synchronization. However, its synchronization comes at the cost of performance, as every method call is synchronized.

Example:

2. Null Keys and Null Values

Another difference between the two is how they handle null keys and null values.

• HashMap: It allows one null key and multiple null values. This makes it more flexible when working with data that may contain null values or an empty key.
• Hashtable: It does not allow null keys or null values. If you try to insert a null key or value into a Hashtable, it will throw a NullPointerException.

Example:

3. Performance

Due to the differences in synchronization, HashMap generally offers better performance compared to Hashtable in single-threaded applications.

• HashMap: Since it is not synchronized, it tends to perform better in environments where thread safety is not a concern.
• Hashtable: The overhead of synchronization can significantly slow down performance, especially in single-threaded applications. The synchronization is applied to every method call, even if it’s not needed.

4. Iterator vs Enumerator

Both HashMap and Hashtable provide ways to iterate through their elements, but they use different methods.

• HashMap: Uses **Iterator** to traverse the map. The Iterator is part of the modern Java Collections Framework and is considered the preferred approach for iterating over maps.
• Hashtable: Uses the older **Enumerator** to traverse the map. While it is still available, it is considered outdated and less flexible than Iterator.

Example:

5. Legacy Status

• HashMap: It is part of the Java Collections Framework (introduced in Java 1.2), which is the modern API for working with collections in Java.
• Hashtable: It is considered legacy (introduced in Java 1.0) and was replaced by the more flexible HashMap class in later versions. It is still maintained for compatibility reasons but is not recommended for use in new applications.

6. Subclassing

• HashMap: It does not extend any other collection directly but implements the Map interface, making it a part of the modern collection hierarchy.
• Hashtable: It extends the **Dictionary** class, which is considered obsolete. The Dictionary class was part of the original Java version but has been superseded by more modern collection classes like Map.

7. Thread-Safety Alternatives

While Hashtable offers built-in thread safety, it's now generally recommended to use more advanced alternatives, such as:

• ConcurrentHashMap: This class is designed for concurrent access and offers better performance than Hashtable because it uses a more fine-grained approach to synchronization (lock splitting).

Practical Example: HashMap vs Hashtable

Let’s look at an example where we compare the use of HashMap and Hashtable.

Output:

Conclusion

Here’s a quick summary of the differences between HashMap and Hashtable:

Feature	HashMap	Hashtable
Thread Safety	Not synchronized (non-thread-safe)	Synchronized (thread-safe)
Null Values	Allows one null key and multiple null values	Does not allow null keys or values
Performance	Faster (in single-threaded environments)	Slower (due to synchronization)
Iterator	Uses Iterator (part of Java Collections Framework)	Uses Enumerator (legacy)
Legacy Status	Part of the modern Collections Framework	Legacy class, considered outdated
Synchronization Alternatives	ConcurrentHashMap for better performance	No direct alternatives

HashMap is generally preferred for modern applications, as it is more flexible, provides better performance, and integrates well with the Java Collections Framework. Hashtable, while still used in legacy systems, is typically avoided in favor of newer, more efficient concurrency solutions like **ConcurrentHashMap**.