HashMap and Hashtable

The key differences between HashMap and Hashtable in Java are mainly in terms of synchronization, null handling, and performance. This is the brief comparison table:

Feature	HashMap	Hashtable
Synchronization	Not synchronized (thread-unsafe)	Synchronized (thread-safe)
Null keys/values	Allows one null key, multiple null values	Does not allow null keys/values
Performance	Faster (no overhead of synchronization)	Slower (due to synchronization)
Introduced	Java 1.2 (Collections Framework)	Java 1.0 (Legacy class)
Iterator behavior	Fail-fast	Fail-safe
Preferred for	Single-threaded or custom-synchronized	Thread-safe (use ConcurrentHashMap instead in most cases)

Here’s a detailed comparison:

1. Thread Safety

• HashMap:
  • Not synchronized and hence not thread-safe. This means multiple threads can access and modify a HashMap concurrently, leading to inconsistent behavior if proper external synchronization is not used.
  • For thread-safe operations, you can wrap a HashMap using Collections.synchronizedMap() or use ConcurrentHashMap.
• Hashtable:
  • Synchronized, which means it is thread-safe for multi-threaded environments. All its methods are synchronized, which slows down performance in single-threaded environments due to unnecessary locking.

2. Null Keys and Values

• HashMap:
  • Allows one null key and multiple null values. It does not throw an exception when you insert null keys or values.
• Hashtable:
  • Does not allow null keys or values. Trying to insert a null key or value will throw a NullPointerException.

3. Legacy vs Modern

• HashMap:
  • Introduced in Java 1.2 as part of the Java Collections Framework, which offers more modern data structures.
• Hashtable:
  • A legacy class from Java 1.0 and is part of the earlier version of Java. While it has been retrofitted to implement the Map interface, it is generally considered outdated and less commonly used today.

4. Performance

• HashMap:
  • Because it is not synchronized, HashMap generally provides better performance in single-threaded or externally synchronized environments.
• Hashtable:
  • Due to its built-in synchronization, Hashtable is slower compared to HashMap, especially in multi-threaded environments where synchronization is unnecessary or can be handled more efficiently by other means (e.g., ConcurrentHashMap).

5. Iterators

• HashMap:
  • Uses fail-fast iterators, meaning that if the map is structurally modified after the iterator is created (except through the iterator’s own remove() method), it will throw a ConcurrentModificationException. This prevents inconsistent iteration.
• Hashtable:
  • Uses enumerators, which are fail-safe and do not throw exceptions if the Hashtable is modified during iteration, but the behavior of the iteration might not reflect the changes accurately.

6. Substitute for Synchronization

• HashMap:
  • If synchronization is required, it’s better to use ConcurrentHashMap, which provides a more efficient way of synchronizing in multi-threaded scenarios by locking only portions of the map rather than the entire structure.
• Hashtable:
  • Due to its synchronous nature, it is less efficient compared to ConcurrentHashMap, which is the preferred choice for thread-safe operations in modern Java.

7. Default Capacity and Load Factor

• HashMap:
  • Default capacity is 16 and the default load factor is 0.75, meaning the capacity will double once 75% of the bucket array is filled.
• Hashtable:
  • Default capacity is 11 and the load factor is 0.75, but resizing works differently, rehashing when capacity is reached.

8. Inheritance

• HashMap:
  • Extends the AbstractMap class and implements the Map interface.
• Hashtable:
  • Extends the Dictionary class (an older class) and also implements the Map interface.

Which One to Use?

• Use HashMap when thread safety is not required or when you can handle synchronization externally.
• Use ConcurrentHashMap when you need thread safety but want better performance than Hashtable.
• Avoid using Hashtable unless you are maintaining older, legacy code that specifically requires it.