A detailed explanation of the differences between a hashtable and a hashmap.

The Hashtable and HashMap are two commonly used classes in Java for storing key-value pairs, with the following differences:

1. Thread safety: Hashtable is thread safe, while HashMap is not. All methods of Hashtable are synchronized, ensuring safety in a multi-threaded environment. On the other hand, HashMap requires external synchronization control in a multi-threaded environment to prevent data inconsistency.
2. In Hashtable, storing null keys and values is not allowed and will result in a NullPointerException. However, in HashMap, it is permitted to store one null key and multiple null values since both key and value can be null.
3. Initial capacity and resizing mechanism: The initial capacity of Hashtable is 11 with a load factor of 0.75. When the size of the Hashtable exceeds the load factor multiplied by the capacity, it will automatically resize to double the original size plus one. Meanwhile, HashMap has an initial capacity of 16 with a load factor of 0.75. When the size of HashMap exceeds the load factor multiplied by the capacity, it will automatically resize to double the original size.
4. Iterators: Hashtable uses Enumeration to implement its iterator, while HashMap uses Iterator. When iterating through Hashtable, deletion operation is not supported and will throw a ConcurrentModificationException. However, deletion operation is supported when iterating through HashMap.
5. Inheritance relationships: Hashtable is a subclass of the Dictionary class, while HashMap is a subclass of the AbstractMap class.

In general, if using in a single-threaded environment, you can choose HashMap for higher performance. If using in a multi-threaded environment, you must use Hashtable or convert HashMap to a thread-safe version using the Collections.synchronizedMap method.