The Difference Between HashMap and Hashtable in Java

In Java, both HashMap and Hashtable are widely used classes that implement the Map interface and store key-value pairs. However, despite their similar purpose, there are several important differences between the two. Understanding these differences can help developers choose the right class for their specific use cases, particularly when dealing with concurrency, performance, and thread-safety.

This article highlights the key differences between HashMap and Hashtable in Java.

1. Thread Safety

• HashMap:
  • HashMap is not thread-safe. This means that multiple threads can access and modify the map simultaneously, which may lead to data inconsistencies or corruption if proper synchronization is not implemented externally.
  • To make a HashMap thread-safe, you can use Collections.synchronizedMap() or opt for ConcurrentHashMap, which provides better thread safety.
• Hashtable:
  • Hashtable, on the other hand, is synchronized and thread-safe by default. Every method of Hashtable is synchronized, which ensures that only one thread can access the map at any given time.
  • While this makes Hashtable thread-safe, it may also result in performance overhead due to synchronization, especially in a multi-threaded environment where many threads need to access the map concurrently.

Summary: HashMap is not thread-safe, while Hashtable is thread-safe by default.

2. Null Keys and Null Values

• HashMap:
  • HashMap allows one null key and multiple null values. This means that you can store a null value or even a null key without any issues.
  java

  HashMap<String, String> map = new HashMap<>();
map.put(null, "value"); // Null key is allowed
map.put("key", null); // Null value is allowed

• Hashtable:
  • Hashtable does not allow null keys or null values. If you attempt to insert a null key or null value, it will throw a NullPointerException.
  java

  Hashtable<String, String> map = new Hashtable<>();
map.put(null, "value"); // Throws NullPointerException
map.put("key", null); // Throws NullPointerException


Summary: HashMap allows null keys and values, while Hashtable does not.

3. Performance

• HashMap:
  • Since HashMap is not synchronized, it generally provides better performance in non-concurrent environments. The absence of synchronization overhead makes it faster when used in a single-threaded context or when thread safety is managed externally.
• Hashtable:
  • Hashtable is slower than HashMap due to the built-in synchronization on every method. The synchronization overhead can be significant in environments with high concurrency, which may cause performance bottlenecks when multiple threads try to access the map simultaneously.

Summary: HashMap generally provides better performance than Hashtable due to the lack of synchronization overhead.

4. Iterators

• HashMap:
  • HashMap uses the Iterator interface to iterate over the elements in the map. The Iterator returned by HashMap is fail-fast, meaning that if the map is modified while it is being iterated, a ConcurrentModificationException will be thrown.
  java

  Iterator<Map.Entry<String, String>> iterator = map.entrySet().iterator();

• Hashtable:
  • Hashtable uses the Enumerator interface for iteration, which is an older approach. Although Enumerator does not throw a ConcurrentModificationException (unlike Iterator), it is considered less modern and less flexible.

Summary: HashMap uses Iterator (fail-fast) while Hashtable uses Enumerator for iteration.

5. Legacy Status

• HashMap:
  • HashMap was introduced in Java 1.2 as part of the Java Collections Framework and is the modern, preferred implementation for storing key-value pairs. It is widely used today in most applications.
• Hashtable:
  • Hashtable was part of the original version of Java (pre-Java 1.2) and was later retrofitted into the Collections Framework in Java 1.2. Despite being thread-safe, it is now considered a legacy class, and it is generally recommended to use HashMap or other modern classes like ConcurrentHashMap instead.

Summary: HashMap is a part of the modern Java Collections Framework, while Hashtable is considered a legacy class.

6. Use Cases

• HashMap:
  • Use HashMap when:
    • Thread safety is not a concern or is managed externally.
    • You need better performance in a single-threaded environment.
    • You need to store null keys or values.
• Hashtable:
  • Use Hashtable when:
    • You specifically need thread-safety and are working in a legacy system.
    • You are maintaining old code where Hashtable was used before the introduction of more efficient concurrent collections.

Summary: HashMap is preferred for most use cases due to its better performance, flexibility, and support for null keys/values. Hashtable is mostly used in legacy applications or in scenarios requiring built-in thread safety (though other modern thread-safe classes like ConcurrentHashMap are better options).

While both HashMap and Hashtable serve the same basic purpose of storing key-value pairs in an unordered fashion, they differ in several key areas, including thread safety, performance, support for null values, and modern usage.

• HashMap is generally the better choice for most modern applications due to its performance advantages and support for null values.
• Hashtable is considered outdated, with its synchronization overhead and legacy status, making it less suitable for new projects.

In many cases, when thread safety is required, developers opt for ConcurrentHashMap rather than Hashtable, as it provides more efficient and scalable thread-safe operations.