Certainly, when discussing the differences between HashMap and Hashtable in Java, there are several key points to consider:
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][5][6].
Hashtable, on the other hand, is synchronized. It is thread-safe and can be shared between multiple threads, as each method in Hashtable is synchronized[1][2][3][4][5][6].
Null Keys and Values:
HashMap allows one null key and multiple null values, which provides more flexibility in storing null entries[2][4][5][6].
Hashtable does not allow any null key or null value. Attempting to insert nulls into a Hashtable will result in a NullPointerException[2][4][5][6].
Performance:
Due to its non-synchronized nature, HashMap is generally faster and has higher performance in scenarios where thread safety is not a concern[4][5][6].
Hashtable's synchronized methods mean that it has a performance cost, which can make it slower compared to HashMap when used in a single-threaded context[4][5][6].
Legacy Status:
Hashtable is considered a legacy class as it was introduced in JDK 1.0. From JDK 1.2, it was re-engineered to implement the Map interface to become a member of the Java Collections Framework[2][5].
HashMap is a newer class, introduced in JDK 1.2, and was designed to be a part of the Java Collections Framework from the beginning[2][5].
Iterators:
The iterators returned by HashMap are fail-fast, meaning they will throw a ConcurrentModificationException if the map is modified while iterating over it[2][4].
Hashtable provides enumerations and iterators, which are not fail-fast[2].
