LinkedHashMap in Java

🔄 LinkedHashMap in Java

A LinkedHashMap is a HashMap that maintains the insertion order (or access order if configured). It is part of the java.util package and combines hash table and linked list features.

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📌 Key Features

Feature	Description
Maintains Order	Keeps the insertion order (or access order)
Allows null keys/values	✅ Yes (only one null key, multiple null values)
Thread-safe	❌ No (use Collections.synchronizedMap() for safety)
Performance	Slightly slower than HashMap due to order maintenance
Internal Structure	Hash table + doubly linked list

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✅ Syntax

import java.util.LinkedHashMap;

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

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🧠 How It Works

• Uses a hash table for storage.
• Uses a doubly linked list to maintain order.
• Each entry maintains a before and after reference.

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🔍 Constructors

LinkedHashMap()                         // Default constructor
LinkedHashMap(int initialCapacity)      // Set initial size
LinkedHashMap(int initialCapacity, float loadFactor)
LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)

• accessOrder = false: Keeps insertion order (default)
• accessOrder = true: Reorders entries on access (used in LRU cache)

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📄 Example: Insertion Order

import java.util.LinkedHashMap;

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

        map.put(3, "Three");
        map.put(1, "One");
        map.put(2, "Two");

        System.out.println(map); // Output: {3=Three, 1=One, 2=Two}
    }
}

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🕹️ Example: Access Order (LRU Cache)

LinkedHashMap<Integer, String> lruCache = new LinkedHashMap<>(16, 0.75f, true);

lruCache.put(1, "One");
lruCache.put(2, "Two");
lruCache.get(1);  // Accesses key 1

System.out.println(lruCache); // Output: {2=Two, 1=One}

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🚨 Removing Eldest Entry (e.g., LRU Cache)

LinkedHashMap<Integer, String> cache = new LinkedHashMap<>(16, 0.75f, true) {
    protected boolean removeEldestEntry(Map.Entry<Integer, String> eldest) {
        return size() > 3; // Keep max 3 items
    }
};

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📊 Comparison with HashMap and TreeMap

Feature	HashMap	LinkedHashMap	TreeMap
Order Maintained	❌ No	✅ Yes (Insertion/Access)	✅ Yes (Sorted)
Performance	⚡ Fast	⚡ Slightly slower	🐢 Slower (sorted)
Underlying Structure	Hash Table	Hash Table + Linked List	Red-Black Tree

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✅ When to Use LinkedHashMap

• You need a Map with predictable insertion order
• You want to implement LRU caching
• You want fast access but also ordered output

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