HashMap internal mechanism in java

HashMap is an implementation of the Map interface in Java, which provides key-value pair storage and retrieval. It uses a hash table data structure to store and organize the key-value pairs. Here’s an overview of the internal mechanism of HashMap:

   +-------------------+
   |      HashMap      |
   +-------------------+
             |
   +-------------------+
   |      Entries      |
   +-------------------+
             |
   +-------------------+
   |       Array       |
   +-------------------+
             |
   +-------------------+
   |     Buckets       |
   +-------------------+
             |
   +-------------------+
   |   Linked List     |
   +-------------------+
             |
   +-------------------+
   | Key-Value Pairs   |
   +-------------------+

+-------------------+

| HashMap |

+-------------------+

| Entries |

+-------------------+

| Array |

+-------------------+

| Buckets |

+-------------------+

| Linked List |

+-------------------+

| Key-Value Pairs |

+-------------------+

Hashing: When a key-value pair is added to a HashMap, the hash code of the key is computed using the key’s hashCode() method. The hash code is an integer value that determines the index in the underlying array where the key-value pair will be stored.
Index Calculation: The hash code is then transformed into a valid index within the range of the underlying array using a hashing algorithm. This transformation typically involves taking the modulus (%) of the hash code with the size of the array to ensure it fits within the array bounds.
Collision Handling: Since different keys can have the same hash code or the transformed index, collisions can occur. HashMap uses a technique called “chaining” to handle collisions. Each index in the underlying array contains a linked list of key-value pairs that have the same hash code or transformed index. New entries with the same index are appended to the existing linked list.
Entry Structure: Each key-value pair in HashMap is represented by an instance of the Entry class, which contains the key, value, hash code, and a reference to the next entry in the linked list. The linked list structure allows multiple key-value pairs with the same index to coexist.
Load Factor and Rehashing: HashMap has a load factor, which determines the threshold for resizing the underlying array. The load factor is a measure of how full the hash table is allowed to be before it is resized. When the number of key-value pairs exceeds the load factor multiplied by the current capacity of the hash table, the hash table is resized, and the entries are rehashed to distribute them across the new larger array. This process is called rehashing.
Retrieval and Modification: When retrieving a value from the HashMap using a key, the hash code of the key is computed, and the corresponding index in the array is determined. The linked list at that index is then traversed to find the entry with a matching key using the equals() method. Once found, the value associated with the key is returned.
Iterating over Entries: To iterate over the entries in a HashMap, the underlying array is traversed, and each linked list is iterated to access the key-value pairs. The order of iteration is not guaranteed and may not reflect the order in which the entries were added.

The internal mechanism of HashMap allows for efficient storage and retrieval of key-value pairs based on their hash codes. By appropriately handling collisions and resizing the underlying array when needed, HashMap provides fast access to values based on their associated keys.

Here’s an example that demonstrates the internal mechanism of HashMap:

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        // Create a HashMap
        HashMap<String, Integer> hashMap = new HashMap<>();

        // Add key-value pairs to the HashMap
        hashMap.put("apple", 10);
        hashMap.put("banana", 20);
        hashMap.put("cherry", 30);

        // Retrieve values from the HashMap
        int appleValue = hashMap.get("apple");
        int cherryValue = hashMap.get("cherry");

        System.out.println("Value of 'apple': " + appleValue);
        System.out.println("Value of 'cherry': " + cherryValue);

        // Print the HashMap
        System.out.println("HashMap: " + hashMap);
    }
}

import java.util.HashMap;

public class HashMapExample {

public static void main(String[] args) {

// Create a HashMap

HashMap<String, Integer> hashMap = new HashMap<>();

// Add key-value pairs to the HashMap

hashMap.put("apple", 10);

hashMap.put("banana", 20);

hashMap.put("cherry", 30);

// Retrieve values from the HashMap

int appleValue = hashMap.get("apple");

int cherryValue = hashMap.get("cherry");

System.out.println("Value of 'apple': " + appleValue);

System.out.println("Value of 'cherry': " + cherryValue);

// Print the HashMap

System.out.println("HashMap: " + hashMap);

}

In this example, we create a HashMap called hashMap that stores String keys and Integer values. We add three key-value pairs to the HashMap using the put() method. The keys are “apple”, “banana”, and “cherry”, and the corresponding values are 10, 20, and 30, respectively.

We then retrieve the values associated with the keys “apple” and “cherry” using the get() method. The retrieved values, appleValue and cherryValue, are printed to the console.

Finally, we print the entire HashMap using the toString() method, which displays the contents of the HashMap. The order of the key-value pairs may not be the same as the order in which they were added, as HashMap does not guarantee a specific order.

Output:

Value of 'apple': 10
Value of 'cherry': 30
HashMap: {cherry=30, banana=20, apple=10}

Value of 'apple': 10

Value of 'cherry': 30

HashMap: {cherry=30, banana=20, apple=10}

This example demonstrates how the HashMap internally uses hashing, index calculation, collision handling, and entry structure to store and retrieve key-value pairs efficiently.