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Ashutosh Kumar Verma

Ashutosh Kumar Verma

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Hi! This is Ashutosh Kumar Verma. I am a software developer at MindStick Software Pvt Ltd since 2021. I have added some new and interesting features to the MindStick website like a story section, audio section, and merge profile feature on MindStick subdomains, etc. I love coding and I have good knowledge of SQL Database.

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Ashutosh Kumar Verma

Ashutosh Kumar Verma

19-Jul-2024

Java HashMap

In Java, HashMap is a data structure that implements the Map interface and stores key-value pairs. It uses hash tables to store elements, allowing you to quickly retrieve values ​​based on their keys.

 

Working of HashMap

Hashing

  • When you put a key-value pair into a HashMap using put(key, value), the HashMap calculates the hash code of the key. This hash code is used to determine the bucket (or index) in which the key value pair will be stored.


Bucket Storage

  • The HashMap is divided into blocks called buckets. Each bucket can contain multiple key-value pairs. The index of the bucket is determined by applying the hash function to the hash code of the key.
  • If multiple keys end up with the same index (hash collision), HashMap uses a linked list (or a balanced tree in other Java versions for better performance) to store more information in a bucket.


Retrieval

  • When you retrieve a value from the HashMap with get(key), the HashMap recalculates the hash code of the key to find a matching bucket.
  • It then scans the entries in the bucket to find the exact key-value pair (using the equals() method to compare).


Performance

  • HashMap provides a constant time operation (O(1)) for the get() and put() operations, assuming that the hash operation disperses the items efficiently in the bucket.
  • However, in the worst-case scenario (due to a hash clash), the assignments can go down to O(n), where n is the number of entries in the bucket.

Example-

import java.util.HashMap;
public class Example {
   public static void main(String[] args) {
       // Create a HashMap with Integer keys and String values
       HashMap<Integer, String> hashMap = new HashMap<>();
       // Adding key-value pairs to the HashMap
       hashMap.put(1, "One");
       hashMap.put(2, "Two");
       hashMap.put(3, "Three");
       // Retrieving values from the HashMap
       System.out.println(hashMap.get(2));  // Output: Two
       // Iterating over the HashMap
       for (Integer key : hashMap.keySet()) {
           String value = hashMap.get(key);
           System.out.println(key + " => " + value);
       }
   }
}

In the example above

AHashMap is used to store mappings from integers (keys) to strings (values). The put() method adds entries, get() retrieves key-based values, and keySet() allows iterating over all keys.

 

Also, Read: What is the difference between wait() and sleep() in Java?

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