Internal Working of Java HashMap

What is Java HashMap? Working Explained with Example

Java HashMap is a data structure that stores data in key-value pairs, allowing efficient retrieval, insertion, and deletion of values based on their keys. It uses hashing to map keys to buckets, enabling fast access.

How HashMap Works

• Key-Value Pairs: Each entry consists of a unique key and an associated value.
• Hashing: HashMap computes a hash code for each key to determine its storage bucket.
• Buckets and Collisions: If multiple keys map to the same bucket (collision), entries are stored in a linked list or tree within that bucket.
• Uniqueness: Keys must be unique. Adding a value with an existing key overwrites the old value.

How put() and get() Work

• Computes the hash code of the key.
• Determines the bucket index using the hash code.
• If the bucket is empty, the key-value pair is added.
• If the bucket already contains entries, it checks each entry using equals() to see if the key already exists.
• If it exists, the value is updated.
• If not, the new entry is added to the bucket’s linked list.

• Computes the hash code of the key.
• Determines the bucket index.
• Searches the bucket for an entry with the matching key using equals().
• Returns the value if found, otherwise returns null.

Handling Collisions

When two keys have the same hash code, their entries are stored in a linked list (or tree if the list gets too long) within the same bucket. The equals() method is used to distinguish between different keys with the same hash code.

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, String> capitalCities = new HashMap<>();
        capitalCities.put("England", "London");
        capitalCities.put("India", "New Delhi");
        capitalCities.put("Austria", "Vienna");
        capitalCities.put("Norway", "Oslo");
        capitalCities.put("USA", "Washington DC");

        System.out.println(capitalCities.get("India")); // Output: New Delhi
        System.out.println(capitalCities); // Prints all key-value pairs
    }
}