This content originally appeared on DEV Community and was authored by Mohammed mhanna
If you’ve ever needed to store key–value pairs (like usernames and passwords, or IDs and names) — you’ve already been thinking about a HashMap.
It’s one of the most powerful and widely used data structures in Java.
Let’s break it down completely 👇
🧠What Is a HashMap?
A HashMap in Java is a part of the Collections Framework that stores data as key-value pairs.
It allows fast access, insertion, and deletion based on keys.
âś… Keys are unique
âś… One null key is allowed
âś… Multiple null values are allowed
âś… Not thread-safe by default
import java.util.HashMap;
public class HashMapDemo {
public static void main(String[] args) {
HashMap<Integer, String> users = new HashMap<>();
users.put(101, "Mohammed");
users.put(102, "Ali");
users.put(103, "Omar");
System.out.println(users.get(102)); // Output: Ali
}
}
⚙️ How HashMap Works Internally
At its core, a HashMap uses an array of buckets.
Each bucket holds a linked list (or a tree since Java 8) of entries that share the same hash.
When you put(key, value):
- Java calculates the hash code of the key.
- It finds the correct bucket index in the array.
- It stores the key–value pair there.
- If another entry has the same hash, it’s linked in the same bucket (collision).
When you get(key):
- Java finds the hash bucket for the key.
- Then checks for equality using equals() to find the correct value.
HashMap<String, Integer> scores = new HashMap<>();
scores.put("Alice", 90);
scores.put("Bob", 85);
System.out.println(scores.get("Alice")); // 90
đź’ˇ Tip: Always override both equals() and hashCode() when using custom objects as keys!
🧩 Example: Custom Key
class Employee {
int id;
String name;
Employee(int id, String name) {
this.id = id;
this.name = name;
}
@Override
public int hashCode() {
return id;
}
@Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (!(obj instanceof Employee)) return false;
Employee e = (Employee) obj;
return this.id == e.id;
}
}
public class Main {
public static void main(String[] args) {
HashMap<Employee, Double> salaries = new HashMap<>();
salaries.put(new Employee(1, "Ali"), 3000.0);
salaries.put(new Employee(2, "Sara"), 3500.0);
System.out.println(salaries.get(new Employee(1, "Ali"))); // 3000.0
}
}
âś… Even though we created a new Employee object, it works because of correctly overridden equals() and hashCode().
⚡ Performance
Average complexity for put(), get(), remove() → O(1)
Worst case (lots of hash collisions) → O(n)
Java 8 improved this by converting long linked lists into balanced trees when collisions become too large.
🧱 When to Use HashMap
| Use Case | HashMap? |
|---|---|
| Need to map keys to values | âś… Yes |
| Need fast lookup by key | âś… Yes |
| Need order preserved | ❌ No |
| Need thread-safety | ❌ No (Use ConcurrentHashMap) |
| Need sorting by key | ❌ No (Use TreeMap) |
🌍 Where HashMap Is Used in Real-World Projects
Caching Systems
Store recently used data for quick access.
HashMap<String, String> cache = new HashMap<>();
cache.put("user:101", "Ali");
Database Result Mapping
When reading from a database, mapping IDs to entities.
Configurations & Settings
Store app settings as key-value pairs.
Counting Frequency of Words or Items
HashMap<String, Integer> wordCount = new HashMap<>();
for (String word : sentence.split(" ")) {
wordCount.put(word, wordCount.getOrDefault(word, 0) + 1);
}
Session Management in Web Apps
Track active user sessions using userId as a key.
🧰 Common Methods
put(K key, V value)
get(Object key)
remove(Object key)
containsKey(Object key)
containsValue(Object value)
keySet()
values()
entrySet()
clear()
size()
Example:
for (Map.Entry<Integer, String> entry : users.entrySet()) {
System.out.println(entry.getKey() + " -> " + entry.getValue());
}
🧩 Pro Tips
Use LinkedHashMap if you need predictable iteration order.
Use ConcurrentHashMap for thread-safe operations.
Avoid mutable keys — changing them breaks the map’s structure.
Load Factor (default 0.75) controls when to resize the hash table.
đź’ˇ Summary
HashMap is the backbone of many Java systems — caches, databases, configurations, and beyond.
It’s fast, flexible, and incredibly powerful — but only when you understand how hashing really works.
This content originally appeared on DEV Community and was authored by Mohammed mhanna
Mohammed mhanna | Sciencx (2025-10-30T18:40:28+00:00) 🔥 Mastering Data Structures in Java – Part 7: HashMap. Retrieved from https://www.scien.cx/2025/10/30/%f0%9f%94%a5-mastering-data-structures-in-java-part-7-hashmap/
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