Can you explain how HashSet handles collisions?

Category : Java | Sub Category : Java Interview questions | By Prasad Bonam Last updated: 2023-08-03 19:12:56 Viewed : 54

HashSet uses a technique called chaining to handle collisions. A collision occurs in HashSet when two elements have the same hash code but are not equal according to the equals() method. To address this situation, HashSet employs a linked list data structure internally.

Here is how HashSet handles collisions step-by-step:

  1. Computing the hash code: When an element is added to the HashSet, its hash code is computed using the hashCode() method. This hash code determines the index (bucket) in the internal array where the element should be placed.

  2. Checking for collision: HashSet checks if there is already an element at the calculated index. If the index is empty, the new element is simply added at that index.

  3. Collision detected: If the index is not empty, a collision has occurred. In this case, HashSet uses the equals() method to compare the new element with the existing element(s) at that index.

  4. Chaining: If the equals() method returns false, it means that the new element is not equal to the existing element(s) at that index, and it is a collision. To handle this, HashSet uses a linked list to store all elements with the same hash code at that index.

    • The new element is added to the linked list, preserving the order of insertion.
    • Subsequent elements with the same hash code will also be added to the linked list.
  5. Searching for an element: When searching for an element in the HashSet, the hash code of the element to find is first computed. Then, the bucket (index) corresponding to that hash code is accessed in the internal array.

  6. Searching within the linked list: If there are multiple elements at that bucket (due to collisions), HashSet uses the equals() method to search for the desired element within the linked list. It iterates through the linked list and compares each element with the one being searched for using the equals() method.

  7. Retrieving or removing elements: When retrieving or removing an element, the same process is followed. The hash code of the element is computed, and the corresponding bucket is accessed. If multiple elements are found at that bucket, the linked list is traversed to find the correct element.

Its important to note that the performance of HashSet is highly dependent on the quality of the hashCode() method implementation for the objects being stored. A good hashCode() implementation can distribute elements evenly across the internal array, reducing the number of collisions and improving the efficiency of HashSet operations. However, in the worst-case scenario, when all elements have the same hash code, HashSets performance may degrade to linear time due to long linked lists.

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