In the world of Java programming and data structures, one of the most common interview questions is: “What is Hashing?”.
🔹 What is Hashing?
Hashing is a technique used to convert data into a fixed-size value, known as a hash, using a hash function. This process enables fast data retrieval, efficient storage, and quick lookups.
In Java, hashing is the backbone of popular collections like:
- HashMap
- HashSet
- LinkedHashMap
🔹 How Hashing Works in Java
When you insert a key-value pair into a HashMap, Java internally:
- Calls the hashCode() method on the key.
- Computes the hash and places the key-value pair into the correct bucket (memory location).
- Uses equals() to verify the actual key during retrieval.
👉 Example:
Map<String, Integer> map = new HashMap<>();
map.put("Java", 1);
map.put("Python", 2);
Here, "Java" and "Python" keys are hashed and placed in buckets for quick access.
🔹 Collisions in Hashing
Sometimes, different keys can generate the same hash value. This situation is called a collision.
Java resolves collisions using techniques like chaining or open addressing.
🔹 Importance of hashCode() and equals()
When working with custom objects in hash-based collections, it’s crucial to override:
- hashCode() → ensures the object is stored in the right bucket.
- equals() → ensures two logically equal objects are treated as equal.
✅ Rule: If two objects are equal according to equals(), they must return the same hashCode().
📌 Conclusion
Hashing in Java makes data access fast, reliable, and efficient. By understanding hashCode(), equals(), and collisions, developers can build optimized applications using HashMap and HashSet.
