🗺️ Java HashMap – Complete Guide with Syntax, Examples & Best Practices

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🧲 Introduction – Why HashMap Is Essential in Java

Imagine storing and retrieving student records by roll number, product prices by ID, or configuration settings by name — all in constant time. That’s what Java HashMap is built for.

A HashMap lets you associate a key with a value and provides fast lookups, inserts, and deletions. It’s one of the most powerful and frequently used classes in the Java Collections Framework.

By the end of this article, you’ll understand:

✅ What HashMap is and how it works internally
✅ How to create, insert, retrieve, update, and delete entries
✅ How to iterate through a HashMap
✅ Best practices, performance tips, and real-world examples

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🔑 What Is HashMap in Java?

🗺️ A HashMap is a key-value based, unordered, and non-synchronized map implementation in Java. It allows one null key and multiple null values.

📦 Located in java.util package
🔧 Implements Map<K, V> interface
📌 Uses hashing for fast access (O(1) average time)

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🧪 Basic Syntax – Creating a HashMap

import java.util.HashMap;

public class Example {
    public static void main(String[] args) {
        HashMap<Integer, String> map = new HashMap<>();

        map.put(1, "Apple");
        map.put(2, "Banana");
        map.put(3, "Cherry");

        System.out.println(map); // Output: {1=Apple, 2=Banana, 3=Cherry}
    }
}

✅ put(K key, V value) stores a value associated with the key
✅ Keys must be unique — duplicate keys overwrite existing entries

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📦 Common HashMap Methods

Method	Description	Example
put(K, V)	Add or update a key-value pair	map.put(1, "Java")
get(K)	Retrieve value by key	map.get(1)
containsKey(K)	Checks if a key exists	map.containsKey(1)
containsValue(V)	Checks if a value exists	map.containsValue("Java")
remove(K)	Remove by key	map.remove(1)
clear()	Remove all entries	map.clear()
isEmpty()	Checks if map is empty	map.isEmpty()
size()	Number of key-value pairs	map.size()

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🔁 Iterating Through a HashMap

🧩 Using entrySet()

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

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🧩 Using forEach() (Java 8+)

map.forEach((key, value) -> System.out.println(key + ": " + value));

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🔍 Real-World Example – Product Price Map

HashMap<String, Double> prices = new HashMap<>();
prices.put("Laptop", 999.99);
prices.put("Phone", 499.49);
prices.put("Tablet", 299.99);

System.out.println("Laptop price: $" + prices.get("Laptop"));

✅ Easily manage key-value relationships like product catalogs, configuration settings, etc.

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⚙️ Java HashMap Internal Working (Simplified)

1. HashMap uses hashCode() of the key to calculate a bucket index
2. If two keys hash to the same index, it uses a linked list (or tree in Java 8+)
3. Load factor (default 0.75) determines when to resize
4. Java 8+: switches to balanced tree if a bucket gets too many elements

📘 Override equals() and hashCode() in custom keys to avoid collisions.

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⚠️ Key Notes & Limitations

• ❗ Unordered — no guarantee of key/value order
• ❗ Not thread-safe — use Collections.synchronizedMap() or ConcurrentHashMap for concurrency
• ✅ Allows null key (only one) and multiple null values
• ⚠️ Poor hash function = more collisions = degraded performance

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📊 HashMap vs TreeMap vs LinkedHashMap

Feature	HashMap	TreeMap	LinkedHashMap
Order	❌ No	✅ Sorted by keys	✅ Insertion order
Performance	✅ Fastest	❌ Slower (O(log n))	✅ Good (slightly slower)
Null keys	✅ 1 allowed	❌ Not allowed	✅ 1 allowed
Thread-safe	❌ No	❌ No	❌ No

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💡 Best Practices for Using HashMap

✅ Use generics to ensure type safety
✅ Always override hashCode() and equals() when using custom keys
✅ Initialize with proper capacity if size is known
✅ Use computeIfAbsent() (Java 8+) for smart initialization
✅ Prefer ConcurrentHashMap in multi-threaded applications

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✅ Summary

• HashMap is a key-value store with constant-time operations
• Allows null key, fast lookups, and dynamic resizing
• Ideal for caches, configuration maps, lookup tables, etc.
• Use with care in multi-threaded environments

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❓ FAQs – Java HashMap

❓ Can I have duplicate keys in HashMap?

No. Keys must be unique. If you put an existing key again, it will overwrite the previous value.

❓ Can HashMap store null values?

Yes. It allows one null key and multiple null values.

❓ Is HashMap ordered?

No. The order of keys and values is not guaranteed. Use LinkedHashMap to maintain insertion order.

❓ Is HashMap thread-safe?

No. For thread safety, use ConcurrentHashMap or wrap it with Collections.synchronizedMap().

❓ How to sort a HashMap?

You can sort it using a TreeMap, or sort entries using stream().sorted().

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