Difference between Hash Map and Hash Set?

🔹 What is a HashMap?

A HashMap in Java is a data structure that stores data in key-value pairs. It belongs to the java.util package and implements the Map interface.

✅ Key Characteristics:

• Stores data as (key, value) pairs.
• Keys must be unique, but values can be duplicated.
• Allows one null key and multiple null values.
• Not synchronized (not thread-safe by default).
• Offers constant-time performance for basic operations like put() and get().

🧠 Internal Working:

• Internally uses a hash table.
• When you insert a key, its hashCode() is computed to determine the bucket location.
• If two keys have the same hash, a collision occurs, and the map handles it using a linked list or tree (in newer Java versions).

🧪 Example:

🔹 What is a HashSet?

A HashSet is a collection that stores unique elements only. It implements the Set interface and is also part of the java.util package.

✅ Key Characteristics:

• No duplicate elements allowed.
• No key-value mapping—just values.
• Allows one null element.
• Unordered—does not maintain insertion order.
• Internally backed by a HashMap.

🧠 Internal Working:

• Each element in the HashSet is stored as a key in an internal HashMap, with a constant dummy value.
• This is why it ensures uniqueness—because keys in a map must be unique.

🧪 Example:

🧠 Summary: