# HashMap (Condensed Guide)

## What Is HashMap?

`HashMap` is a **Map implementation** that stores **key-value pairs**.

It:
- Uses **keys to quickly retrieve values**
- Does **not maintain ordering**
- Allows **one null key**
- Allows **multiple null values**

```java
import java.util.HashMap;
import java.util.Map;

Map<String, Integer> scores = new HashMap<>();

Map
 ↓
HashMap

⚠ Map is not part of the Collection hierarchy, but it is part of the Java Collections Framework.

HashMap stores data using hashing.

Steps when storing data:

1. The key’s hashCode() is calculated
2. The hash determines a bucket location
3. The value is stored at that location

Because of hashing:

• Lookups are extremely fast
• Keys must implement proper equals() and hashCode()

Worst case (rare collisions) → O(n)

map.put("Alice", 90);
map.put("Bob", 85);

map.get("Alice");

map.remove("Bob");

map.containsKey("Alice");

map.size();

for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + " -> " + entry.getValue());
}

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

• Expecting HashMap to maintain order
• Using mutable objects as keys
• Forgetting equals() and hashCode() rules

Use HashMap when you need:

• Fast key lookups
• Data associations
• Frequency counting
• Caching or indexing

Common examples:

• Word frequency counters
• User ID → profile lookup
• Student name → grade mapping

Map<String, Integer> wordCount = new HashMap<>();

wordCount.put("java", 3);
wordCount.put("python", 2);

System.out.println(wordCount.get("java"));

Output:

3

• Count word frequencies in a sentence
• Implement a phone directory
• Find the first non-repeating character
• Solve the Two-Sum problem

HashMap = fast key-value storage using hashing.

Key characteristics:

• O(1) lookups
• No ordering
• Unique keys

Understanding HashMap is essential for solving many algorithm problems efficiently.