HashMap and HashSet in Java

🔑 HashMap and HashSet in Java

Two essential data structures in Java's Collections Framework are the HashMap and HashSet in Java. They offer fast and efficient ways to store and access data, with different use cases based on whether you need key-value pairs or unique elements.

📌 What is a HashMap?

A HashMap stores data in key-value pairs. It allows quick retrieval, insertion, and deletion based on keys.

Internally, it uses a hash table, which ensures near constant-time performance for basic operations.

import java.util.HashMap;

HashMap<String, Integer> map = new HashMap<>();
map.put("Apple", 3);
map.put("Banana", 5);
map.put("Cherry", 2);

System.out.println(map.get("Banana"));  // Outputs: 5

🗃️ What is a HashSet?

A HashSet stores unique elements and does not allow duplicates. It’s ideal when you want to maintain a collection of distinct items without caring about order. Like HashMap, it uses hashing for fast operations.

import java.util.HashSet;

HashSet<String> set = new HashSet<>();
set.add("Java");
set.add("Python");
set.add("Java");  // Duplicate, will be ignored

System.out.println(set.contains("Python")); // Outputs: true
System.out.println(set.size());              // Outputs: 2

⚙️ How They Differ

• HashMap stores key-value pairs, while HashSet stores only unique keys (elements).
• HashMap allows one null key and multiple null values; HashSet allows a single null element.
• HashMap implements the Map interface; HashSet implements the Set interface.

🔧 Common Methods

• HashMap: put(key, value), get(key), remove(key), containsKey(key)
• HashSet: add(element), remove(element), contains(element), size()

💡 When to Use Which?

• Use HashMap when you need to associate values with keys for quick lookup.
• Use HashSet when you need to store unique items and care about membership checking.

🚀 Conclusion

Mastering HashMap and HashSet in Java helps you manage data efficiently. These structures are foundation blocks for many applications, providing fast access and reliable uniqueness where needed.