How Hashmap works internally in java

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Could you provide a detailed explanation of the internal workings of a HashMap in Java, including the data structures and algorithms involved in its implementation?
Oct 11, 2023 in Java by Saniya
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A popular data structure for storing key-value pairs in Java is a hash map. On the basis of their keys, it provides quick access to values. We can examine its implementation details to learn how it functions internally:

1. Buckets and Hashing:  A HashMap fundamentally stores key-value pairs in an array of "buckets". It generates the key's hash code when you input a key-value pair to decide which bucket the pair belongs in.

2. Calculating a Hash Code: The value of the key is represented numerically by the hash code. This code is generated by using the key object's 'hashCode()' function in Java.The hash code is subsequently translated into a usable index within the array of buckets by means of an algorithm.

3. Collision Handling: When two separate keys generate the same hash code, a hash collision occurs. This causes an effort to group several key-value pairs into one bucket. The majority of contemporary HashMap implementations hold several key-value pairs in the same bucket using a linked list or a more sophisticated data structure (such as a balanced tree) to prevent collisions.

4. Load Factor: A load factor is a measure of how full the HashMap is. It is a ratio of the number of stored entries to the number of available buckets. When the load factor exceeds a certain threshold, the HashMap is resized, creating more buckets and rehashing existing entries. This operation ensures that the HashMap remains efficient in terms of access time.

5. Retrieval: When you want to retrieve a value from the HashMap, it calculates the hash code of the provided key. It then identifies the bucket where the key might be located.If multiple key-value pairs exist in the same bucket due to collisions, it searches through the linked list (or other data structure) to find the correct key and retrieve the associated value.

6. Complexity: On average, the time complexity for retrieval, insertion, and deletion in a well-designed HashMap is O(1). However, in the worst case, if there are many collisions and the HashMap is not resized, the complexity can degrade to O(n) for these operations.

Understanding the internal workings of a HashMap is essential for Java developers, as it helps in making informed decisions about when and how to use HashMaps, as well as optimizing their performance when dealing with large datasets or in applications where efficiency is critical.

answered Oct 11, 2023 by anonymous
• 3,320 points

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