What differences exist between HashMap and Hashtable ?

When answering the interview question about the differences between HashMap and Hashtable in Java, you can highlight several key points:

1. Synchronization and Thread Safety:

   • HashMap is non-synchronized, meaning it is not thread-safe and cannot be shared between multiple threads without proper synchronization code[1][2][3][4][6][7].
   • Hashtable is synchronized, making it thread-safe and suitable for use in multi-threaded environments where it can be shared between threads[1][2][3][4][6][7].

2. Null Keys and Values:

   • HashMap allows one null key and multiple null values, which can be useful in certain scenarios where you need to represent the absence of a value[3][5][6][7].
   • Hashtable does not allow any null key or value, attempting to insert nulls will result in a NullPointerException[3][5][6][7].

3. Performance:

   • Due to the lack of synchronization, HashMap is generally faster and more efficient in scenarios where thread safety is not a concern[3][5][6].
   • Hashtable has a performance cost due to its synchronization feature, which can slow down operations when used in single-threaded applications[5][6].

4. Inheritance and Framework Membership:

   • HashMap is a subclass of AbstractMap and has been a part of the Java Collections Framework since its introduction in JDK 1.2[2][3][5].
   • Hashtable is a legacy class that was originally part of JDK 1.0 and later re-engineered to implement the Map interface to become a member of the collection framework[2][3][5].

5. Iterator Behavior:

   • The iterators returned by HashMap are fail-fast, meaning they will throw a ConcurrentModificationException if the map is modified while ...