The differences between HashMap and Hashtable in Java are significant and revolve around synchronization, null handling, and performance. Here are the key distinctions:
Synchronization and Thread Safety
HashMap: It is not synchronized, meaning it is not thread-safe. Multiple threads can access and modify a HashMap concurrently, which can lead to inconsistent data if not properly synchronized externally. This makes HashMap faster in single-threaded environments due to the lack of synchronization overhead[1][2][3][4][5].
Hashtable: It is synchronized, meaning it is thread-safe. Only one thread can access or modify a Hashtable at a time, which prevents data inconsistency in multi-threaded environments but also makes it slower due to the synchronization overhead[1][2][3][4][5].
Null Handling
HashMap: Allows one null key and multiple null values. This flexibility can be useful in certain scenarios where null values are expected or need to be handled[1][2][3][4][5].
Hashtable: Does not allow any null key or value. Attempting to insert a null key or value will result in a NullPointerException[1][2][3][4][5].
Performance
HashMap: Generally faster than Hashtable because it is not synchronized. The absence of synchronization makes HashMap more efficient in single-threaded applications[1][2][3][4][5].
Hashtable: Slower compared to HashMap due to the synchronization overhead. This makes Hashtable less efficient in single-threaded applications[1][2][3][4][5].
Iteration and Fail-Fast Behavior
HashMap: Uses an iterator that is fail-fast, meaning it will throw a ConcurrentModificationException if the map is modified while iterating over it[1][2][3][4][5].
Hashtable: Uses an enumerator that is not fail-fast, meaning it does not throw an exception if the map is modified during iteration[1][2][3][4][5].
Legacy and Modern Alternatives
HashMap: Introduced in Java 1.2 and is part of the Java Collections Framework. It is a modern implementation and is generally preferred for new applications[1][2][3][4][5].
Hashtable: A legacy class that has been part of Java since the early ve...
