Can you explain the process of rehashing in a HashMap?

Category : Java | Sub Category : Java Interview questions | By Prasad Bonam Last updated: 2023-08-03 19:23:18 Viewed : 56

Rehashing is the process of resizing and reorganizing the internal hash table of a HashMap when its load factor exceeds a predefined threshold. This process is essential to maintain a good balance between space and time complexity in the HashMap and to ensure efficient operations.

The steps involved in rehashing in a HashMap are as follows:

  1. Load Factor Check: When a new key-value pair is added to the HashMap, it first checks whether the number of stored entries (size) exceeds the load factor threshold. The load factor is the ratio of the number of entries to the number of buckets in the hash table.

  2. Resize the Internal Array: If the load factor exceeds the threshold, the HashMap initiates the rehashing process. It creates a new internal array (hash table) with an increased capacity, typically doubling the current capacity.

  3. Rehashing Key-Value Pairs: The HashMap iterates through all the existing key-value pairs (entries) in the original hash table. For each entry, it calculates the new bucket index based on the increased capacity of the new hash table. This is done by computing the new hash code for the key using the updated capacity.

  4. Redistributing Entries: After calculating the new bucket index for each entry, the HashMap redistributes them into the new hash table. Collisions may occur during this process, where multiple entries end up in the same bucket due to the updated hash codes.

  5. Resizing Completed: Once all the entries have been rehashed and redistributed to the new hash table, the resizing process is complete. The old hash table is discarded, and the HashMap now uses the new hash table with the updated capacity.

  6. Continued Operations: After rehashing, the HashMap can continue adding new key-value pairs as usual. The new, larger hash table provides more buckets and reduces the likelihood of collisions, leading to better performance.

Its important to note that rehashing can be an expensive operation, especially for large HashMaps, as it involves iterating through all existing entries and redistributing them. Javas standard HashMap implementation employs a load factor of 0.75, which means that rehashing is triggered when the number of entries reaches 75% of the current capacity. This load factor strikes a balance between reducing the frequency of resizing and avoiding excessive memory consumption.

Rehashing ensures that the HashMap maintains an optimal balance between memory usage and performance, allowing it to efficiently handle a dynamic set of key-value pairs with minimal collisions and fast retrieval times.

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