What is the basic concept of a hashmap?

HashMap is a data structure in Java that is implemented based on a hash table and is used to store key-value pairs. It allows for storing null keys and values, ensures the uniqueness of keys, but allows for duplicate values. HashMap determines the storage location of key-value pairs by calculating the hash code of the key, enabling fast lookup and insertion operations.

The basic concepts include:

1. Key-Value pairs: The data stored in a HashMap exists in the form of key-value pairs, where each pair includes a key and a value.
2. HashMap uses a hash table internally to store key-value pairs, with the storage location of each pair determined by calculating the hash code of the key.
3. Hash code: Each key generates a hash code by calling the hashCode() method, which is used to determine the storage position of the key-value pair in the hash table.
4. Conflict resolution: When different keys generate the same hash code, a hash conflict occurs. HashMap uses chaining to resolve conflicts, in which a linked list or red-black tree is stored in the same hash bucket.
5. Capacity Expansion: When the number of key-value pairs stored in a HashMap exceeds the product of the load factor and capacity, the HashMap will automatically expand by increasing its capacity and re-computing the hash codes.
6. The iteration order of HashMap is unrelated to the order of element insertion and is unpredictable.