What is the underlying implementation principle of Java HashMap?

The underlying implementation principle of HashMap is based on a Hash Table. Specifically, HashMap uses an array to store data, with each array element referred to as a bucket, and each key-value pair in the HashMap referred to as an entry.

When we insert a key-value pair into a HashMap, the HashMap will place that pair into the corresponding bucket based on the hash value of the key. HashMap uses the hash value of the key to determine the index of the bucket, and then stores the key-value pair in the bucket at that index. If different keys have the same hash value, causing a hash collision, the HashMap will use data structures like linked lists or red-black trees to resolve the collision.

When we need to find or retrieve the value corresponding to a key, HashMap will use the key’s hash value to locate the corresponding bucket, and then search for the value associated with that key within the bucket. If there is a hash collision, HashMap will traverse the linked list or red-black tree to find the corresponding key-value pair.

When the load factor of a HashMap exceeds a certain threshold, the HashMap undergoes a resizing operation, meaning the array size is adjusted to reduce the likelihood of hash collisions. During resizing, the HashMap redistributes the key-value pairs to new buckets to maintain the performance of the hash table.

In essence, the underlying implementation of HashMap is based on a hash table, utilizing arrays and linked lists or red-black trees to store key-value pairs. Keys are mapped to corresponding buckets using a hash algorithm, and collisions are resolved through linked lists or red-black trees.