Difference between map and hashmap

Both Map and HashMap are collection interfaces in Java, with HashMap being an implementation class of the Map interface. The differences between them are as follows:

1. Inheritance relationship: Map is an interface, while HashMap is a class that implements the Map interface.
2. Thread safety: The Map interface does not guarantee thread safety, and HashMap is not thread-safe. If HashMap is used in a multi-threaded environment, synchronization mechanisms must be implemented to ensure thread safety.
3. Allowing key-value pairs to be null: The Map interface allows both keys and values to be null, and HashMap also allows keys and values to be null. However, since the keys in HashMap are unique, there can only be one key that is null.
4. Order: The Map interface does not guarantee the order of key-value pairs, nor does HashMap. If ordered key-value pairs are needed, LinkedHashMap class can be used.
5. Efficiency: The underlying implementation of HashMap is based on a hash table, which maps keys to array indexes through a hash function, allowing for quick insertion, deletion, and lookup operations. As a result, the average time complexity of HashMap’s insert, delete, and lookup operations is O(1).

In conclusion, Map is an interface, and HashMap is a concrete implementation of the Map interface. HashMap is not thread-safe and allows null keys and values. It is implemented using a hash table, providing efficient insertion, deletion, and lookup operations.