Implementation principle of hashmap in java

HashMap is implemented using a hash table and maps keys to slots through hash functions to achieve fast access. Conflict handling uses techniques such as zippers, open addressing, and buckets. The load factor controls the ratio of the number of elements to the number of buckets. If it is too high, conflicts will increase. HashMap will automatically expand to reduce conflicts. It is not thread-safe by default and requires using ConcurrentHashMap instead.

The implementation principle of HashMap

HashMap is a commonly used data structure in Java, used to store key-value pairs . It is implemented based on a hash table and maps a key to a slot through a hash function to quickly access elements.

Hash function

The hash function converts the key into an integer that represents the key's position in the hash table. HashMap uses the hashCode() method to generate a hash code, and then maps it to a slot through modulo operation.

Conflict handling

When two keys hash to the same slot, a conflict occurs. HashMap uses the following techniques to handle conflicts:

• Zipper method: Save conflicting elements in a linked list.
• Open addressing: Find the next available slot in the hash table and insert the element into it.

Buckets

The hash table is divided into multiple buckets, and each bucket is a linked list or array. Conflicting elements are stored in the same bucket.

Load factor

Load factor refers to the ratio of the number of elements stored in the hash table to the number of buckets. If the load factor is too high, the hash table becomes inefficient because collisions increase. HashMap allows the user to set the load factor, the default value is 0.75.

Expansion

When the load factor reaches the preset threshold, HashMap will automatically expand. It creates a larger hash table and rehashes the elements into the new table. Sizing helps reduce collisions and improves hash table efficiency.

Thread Safety

By default, HashMap is not thread-safe. In order to use HashMap in a multi-threaded environment, you need to use ConcurrentHashMap, which is a thread-safe HashMap implementation. It uses concurrent data structures to handle concurrent access.

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