What are the differences between HashMap and Hashtable in terms of implementation and characteristics?

HashMap and Hashtable are implementations of the Map interface in Java, but they have some differences:

• Synchronization: Hashtable is synchronized (thread-safe), which makes it slower in a multithreaded environment. HashMap is not synchronized, so it is faster but not safe for concurrent access without external synchronization.

• Null allowance: HashMap allows one null key and multiple null values. Hashtable does not permit null keys or null values.

• Inheritance: Hashtable is an older class, inherited from Dictionary, while HashMap inherits from AbstractMap.

• Usage: In modern applications, HashMap is preferred, and for synchronization needs, ConcurrentHashMap or external synchronization of HashMap should be used.

Example of using HashMap:

Map<String, Integer> map = new HashMap<>();
map.put("key", 1);
Integer value = map.get("key");