Definition Hashing is a technique to convert data into a fixed-size integer (hash code) using a hash function. This hash code decides where data will be stored in memory.

• Used in HashMap, HashSet, Hashtable, etc.
• Provides O(1) average time complexity for search, insert, and delete.

How it works:

1. Object → Hash Function → Hash Code
2. Hash Code → Index in an array (bucket)
3. If collision happens → handled using chaining (LinkedList/Tree)

Why use Hashing?

• Faster search compared to arrays or lists.
• Used in databases, caches, compilers, and cryptography.

Example

String s = "Hello";
System.out.println(s.hashCode()); 

Definition

• Stores unique elements only.
• Unordered collection (no guarantee of order).
• Internally uses HashMap (values are stored as Object constants).

Important Methods

• add(element)
• remove(element)
• contains(element)
• size()
• clear()

Example

import java.util.HashSet;

HashSet<Integer> set = new HashSet<>();
set.add(10);
set.add(20);
set.add(20); // duplicate ignored
System.out.println(set); // [20, 10]

Use Cases

• Removing duplicates
• Storing unique items (roll numbers, IDs)

Definition HashMap<K, V> stores key-value pairs.

• Keys must be unique.
• Values can be duplicated.
• Provides O(1) average time for operations.

Important Methods

• put(key, value)
• get(key)
• remove(key)
• containsKey(key)
• containsValue(value)
• keySet()
• values()
• entrySet()

Example

import java.util.HashMap;

HashMap<Integer, String> map = new HashMap<>();
map.put(1, "Gourab");
map.put(2, "Rahul");
map.put(3, "Ananya");

System.out.println(map.get(2)); // Rahul
map.remove(1);

for (Integer key : map.keySet()) {
    System.out.println(key + " -> " + map.get(key));
}

Use Cases

• Dictionary (word → meaning)
• Student ID → Name
• Cache storage
• Configuration storage

• Hashing: Converts object → hash code → bucket index (fast operations).
• HashSet: Stores only unique values (internally uses HashMap).
• HashMap: Stores key-value pairs (unique keys, duplicate values allowed).