Hashmap vs. Hashtable

Attribute	Hashmap	Hashtable
Implementation	Hashmap is implemented as a hash table with an array of buckets.	Hashtable is implemented as a hash table with an array of buckets.
Thread-Safety	Not synchronized, not thread-safe.	Synchronized, thread-safe.
Null Keys and Values	Allows null keys and values.	Does not allow null keys or values.
Performance	Generally faster due to lack of synchronization.	Slower due to synchronization overhead.
Iterators	Fail-fast iterator.	Fail-fast iterator.
Ordering	Does not guarantee any specific order.	Does not guarantee any specific order.
Inheritance	Extends AbstractMap class and implements Map interface.	Extends Dictionary class and implements Map interface.
Introduced In	Introduced in Java 1.2.	Introduced in Java 1.0.

Introduction

When it comes to working with key-value pairs in Java, two commonly used classes areHashMap andHashtable. Both of these classes provide similar functionality, but they have some important differences that developers should be aware of. In this article, we will explore the attributes ofHashMap andHashtable and discuss their similarities and differences.

Similarities

Before diving into the differences, let's first highlight the similarities betweenHashMap andHashtable. Both classes are part of the Java Collections Framework and implement theMap interface. This means that they both allow you to store and retrieve key-value pairs, and they provide methods to manipulate and query the data.

BothHashMap andHashtable use hashing techniques to store and retrieve elements efficiently. They internally use an array to store the elements, and the keys are hashed to determine the index where the value is stored. This allows for fast access to the values based on their keys.

Another similarity is that both classes allow null values and null keys. This means that you can store null as a value or as a key in bothHashMap andHashtable. However, it is important to note that while null values are allowed, using null as a key inHashtable will result in aNullPointerException.

BothHashMap andHashtable are not thread-safe by default. This means that if multiple threads access and modify the map concurrently, it can lead to unexpected behavior or data corruption. However, there are ways to make them thread-safe, which we will discuss later in this article.

Lastly, both classes provide similar methods to add, remove, and retrieve elements from the map. They both have methods likeput(key, value) to add a key-value pair,get(key) to retrieve the value associated with a key, andremove(key) to remove a key-value pair from the map.

Differences

WhileHashMap andHashtable share many similarities, there are some important differences that set them apart. Let's explore these differences in more detail.

1. Synchronization

One of the key differences betweenHashMap andHashtable is their synchronization behavior. As mentioned earlier, both classes are not thread-safe by default. However,Hashtable is synchronized, which means that it provides built-in thread-safety. This makesHashtable suitable for use in multi-threaded environments where multiple threads may access and modify the map concurrently.

On the other hand,HashMap is not synchronized by default. This means that if multiple threads access and modify aHashMap concurrently, it can lead to data corruption or unexpected behavior. However, you can make aHashMap thread-safe by using theCollections.synchronizedMap() method, which wraps theHashMap and provides synchronization.

It is important to note that the synchronization inHashtable comes with a performance cost. The synchronized methods inHashtable introduce overhead, which can impact the performance in single-threaded scenarios. In contrast, the unsynchronized nature ofHashMap makes it more efficient in single-threaded environments.

2. Iteration Order

Another difference betweenHashMap andHashtable is the iteration order of their elements. TheHashMap class does not guarantee any specific order of its elements. The order in which the elements are returned during iteration may vary and is not predictable. This is becauseHashMap does not maintain the insertion order of its elements.

On the other hand,Hashtable does guarantee a specific iteration order. The elements in aHashtable are iterated in the same order in which they were inserted into the map. This can be useful in scenarios where the order of elements is important.

It is worth mentioning that if you need a predictable iteration order inHashMap, you can use theLinkedHashMap class, which extendsHashMap and maintains the insertion order of its elements.

3. Performance

When it comes to performance,HashMap andHashtable have some differences. As mentioned earlier,HashMap is generally more efficient in single-threaded scenarios due to its unsynchronized nature. The absence of synchronization overhead makesHashMap faster in situations where thread-safety is not a concern.

On the other hand,Hashtable incurs the synchronization overhead, which can impact its performance in single-threaded scenarios. However, in multi-threaded environments where thread-safety is required, the built-in synchronization ofHashtable can be beneficial.

It is important to note that the performance difference betweenHashMap andHashtable may not be significant in many applications. The choice between the two should be based on the specific requirements of your application and the expected usage patterns.

4. Null Keys

As mentioned earlier, bothHashMap andHashtable allow null values. However, there is a difference in how they handle null keys. InHashMap, null keys are allowed, and you can store and retrieve null as a key without any issues.

On the other hand,Hashtable does not allow null keys. If you attempt to store null as a key in aHashtable, it will throw aNullPointerException. This is an important distinction to keep in mind when choosing betweenHashMap andHashtable based on your specific use case.

5. Legacy

One final difference worth mentioning is the legacy status ofHashtable.Hashtable has been part of Java since the early versions, whileHashMap was introduced in Java 1.2 as part of the Collections Framework. This means thatHashtable has been around for a longer time and is considered a legacy class.

WhileHashtable is still supported and can be used in modern Java applications, it is generally recommended to useHashMap due to its improved performance and flexibility. The introduction ofHashMap was a step towards providing a more efficient and flexible alternative toHashtable.

Conclusion

In conclusion, bothHashMap andHashtable are useful classes for working with key-value pairs in Java. They share many similarities, such as implementing theMap interface and using hashing techniques for efficient storage and retrieval. However, they also have important differences in terms of synchronization, iteration order, performance, handling of null keys, and legacy status.

When choosing betweenHashMap andHashtable, it is important to consider the specific requirements of your application. If thread-safety is a concern and you need built-in synchronization,Hashtable can be a suitable choice. On the other hand, if you prioritize performance and flexibility,HashMap is generally recommended. Additionally, if you require a predictable iteration order, you can consider usingLinkedHashMap.

Ultimately, the choice betweenHashMap andHashtable depends on the specific needs of your application and the trade-offs you are willing to make. Understanding the attributes and differences of these classes will help you make an informed decision and write more efficient and reliable code.