#1 Features of Java HashMap

HashMap is one of the most commonly used classes in Java Collections Framework. It stores data in key-value pairs and provides fast data access.

Example:

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

map.put(1, "Java");
map.put(2, "Python");

System.out.println(map.get(1));

Output:

Java

1. Stores Data in Key-Value Pairs

Each element in a HashMap consists of:

• Key

• Value

Example:

map.put(101, "Anik");

• 101 → Key

• "Anik" → Value

2. Unique Keys

HashMap does not allow duplicate keys.

map.put(1, "Java");
map.put(1, "Python");

The old value gets replaced.

Output:

{1=Python}

3. Allows Duplicate Values

Different keys can have the same value.

map.put(1, "Java");
map.put(2, "Java");

Valid in HashMap.

4. Fast Performance

Operations like:

• put()

• get()

• remove()

work in approximately O(1) time complexity.

This makes HashMap very efficient.

5. Allows One Null Key

HashMap allows:

• One null key

• Multiple null values

Example:

map.put(null, "Java");
map.put(2, null);

6. Unordered Collection

HashMap does not maintain insertion order.

Example:

map.put(3, "C");
map.put(1, "Java");
map.put(2, "Python");

Output order may vary.

7. Not Synchronized

HashMap is not thread-safe.

It is faster than synchronized collections because multiple threads can access it simultaneously.

For thread safety, Java provides:

ConcurrentHashMap

8. Part of Java Collections Framework

HashMap belongs to:

java.util

Package.

Import statement:

import java.util.HashMap;

9. Uses Hashing Internally

HashMap uses a hashing mechanism to store and retrieve elements efficiently.

Internally it uses:

• Hash function

• Buckets

• Hash codes

10. Dynamic in Size

HashMap automatically grows when more elements are added.

No need to define fixed size manually.

A phone contacts app:

• Name → Key

• Phone Number → Value