Hash Tables

Hash tables, also known as hash maps, are a type of data structure that provides efficient insertion, deletion, and search operations. They are based on the concept of hashing, which involves mapping keys to indices in an array.

In a hash table, data is stored in key-value pairs. When you insert an element into a hash table, the key is hashed using a hash function to generate an index. This index is then used to store the value in the array.

One of the key advantages of hash tables is their constant-time complexity for average-case operations. This means that the time taken to perform operations like insertion, deletion, and search is independent of the size of the hash table. However, in the worst-case scenario, the time complexity can be linear if there are many hash collisions.

To handle hash collisions, hash tables use techniques like chaining or open-addressing. In chaining, each array index stores a linked list of key-value pairs that have collided. In open-addressing, if a hash collision occurs, the algorithm finds the next available slot in the array to store the key-value pair.

Here's an example of a hash table implementation in Java:

1import java.util.HashMap;
2
3public class Main {
4    public static void main(String[] args) {
5        // Create a hash table
6        HashMap<String, Integer> hashMap = new HashMap<>();
7
8        // Insert key-value pairs
9        hashMap.put("Alice", 25);
10        hashMap.put("Bob", 30);
11        hashMap.put("Charlie", 35);
12
13        // Retrieve values
14        int age = hashMap.get("Bob");
15        System.out.println("Bob's age is " + age);
16
17        // Update values
18        hashMap.put("Charlie", 40);
19        age = hashMap.get("Charlie");
20        System.out.println("Charlie's age is " + age);
21
22        // Delete a key-value pair
23        hashMap.remove("Alice");
24        System.out.println("Alice's information has been deleted.");
25    }
26}