LinkedHashMap

The LinkedHashMap class in Java is a part of the Java Collections Framework and extends HashMap while maintaining the insertion order of elements. It is useful when you need a key-value mapping with predictable iteration order.

In this tutorial, we will cover:

1. Understanding LinkedHashMap

Key Features:

✔ Maintains insertion order of elements
✔ Allows one null key and multiple null values
✔ Not synchronized (use Collections.synchronizedMap() for thread safety)
✔ Provides constant-time performance for insertions and lookups

Class Declaration:

public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>

2. Creating and Initializing a LinkedHashMap

You can create a LinkedHashMap using different constructors.

Example: Creating a Simple LinkedHashMap

import java.util.LinkedHashMap;
import java.util.Map;

public class LinkedHashMapExample {
    public static void main(String[] args) {
        // Creating a LinkedHashMap with default capacity
        LinkedHashMap<Integer, String> map = new LinkedHashMap<>();

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

        System.out.println("LinkedHashMap: " + map);
    }
}

Output:

LinkedHashMap: {1=Apple, 2=Banana, 3=Cherry}

Example: Creating a LinkedHashMap with Initial Capacity

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

• 16: Initial capacity
• 0.75f: Load factor (default: 0.75)

3. Accessing and Modifying Elements

You can access, update, and modify elements using standard Map methods.

Example: Get, Replace, and Modify Elements

import java.util.LinkedHashMap;

public class LinkedHashMapDemo {
    public static void main(String[] args) {
        LinkedHashMap<String, Integer> scores = new LinkedHashMap<>();
        
        // Adding elements
        scores.put("Alice", 85);
        scores.put("Bob", 92);
        scores.put("Charlie", 78);

        // Accessing a value
        System.out.println("Bob's Score: " + scores.get("Bob"));

        // Updating a value
        scores.put("Alice", 90);
        System.out.println("Updated Scores: " + scores);
    }
}

Output:

Bob's Score: 92
Updated Scores: {Alice=90, Bob=92, Charlie=78}

4. Iterating Over a LinkedHashMap

You can iterate over keys, values, or entries using different approaches.

Example: Iterating Using a forEach Loop

import java.util.LinkedHashMap;
import java.util.Map;

public class IterateLinkedHashMap {
    public static void main(String[] args) {
        LinkedHashMap<Integer, String> map = new LinkedHashMap<>();
        map.put(101, "Java");
        map.put(102, "Python");
        map.put(103, "JavaScript");

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

Output:

101 -> Java
102 -> Python
103 -> JavaScript

Example: Iterating Using Java 8 forEach

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

5. Maintaining Access Order for Caching

By default, LinkedHashMap maintains insertion order. However, you can configure it to maintain access order by setting the accessOrder flag to true.

Example: Creating an LRU Cache Using LinkedHashMap

import java.util.LinkedHashMap;
import java.util.Map;

public class LRUCache<K, V> extends LinkedHashMap<K, V> {
    private final int maxSize;

    public LRUCache(int maxSize) {
        super(maxSize, 0.75f, true); // Access-order enabled
        this.maxSize = maxSize;
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
        return size() > maxSize; // Remove oldest entry when size exceeds maxSize
    }

    public static void main(String[] args) {
        LRUCache<Integer, String> cache = new LRUCache<>(3);

        cache.put(1, "A");
        cache.put(2, "B");
        cache.put(3, "C");

        // Access key 1 (moves it to the end)
        cache.get(1);

        // Insert a new element (removes least recently used entry, which is key 2)
        cache.put(4, "D");

        System.out.println("LRU Cache: " + cache);
    }
}

Output:

LRU Cache: {3=C, 1=A, 4=D}

In this example, the least recently used element is automatically removed when a new entry is added beyond capacity.

6. Removing Elements

Example: Removing Entries

map.remove(102); // Removes entry with key 102
map.clear(); // Removes all entries

7. Checking for Keys and Values

Example: Check Existence of Keys and Values

System.out.println(map.containsKey(101)); // true
System.out.println(map.containsValue("Python")); // false

8. Using LinkedHashMap with Generics

You can use LinkedHashMap with custom objects as keys and values.

Example: Using Custom Class as Key

import java.util.LinkedHashMap;
import java.util.Objects;

class Student {
    String name;
    int id;

    public Student(int id, String name) {
        this.id = id;
        this.name = name;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Student)) return false;
        Student student = (Student) o;
        return id == student.id && Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, name);
    }
}

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

        studentScores.put(new Student(1, "Alice"), 85);
        studentScores.put(new Student(2, "Bob"), 90);

        studentScores.forEach((student, score) ->
                System.out.println(student.name + "'s Score: " + score));
    }
}

9. Comparing LinkedHashMap vs HashMap vs TreeMap

Conclusion

• LinkedHashMap is useful when ordering of keys matters.
• It provides O(1) time complexity for get() and put() operations.
• It can be used for caching (LRU implementations).
• It maintains either insertion order or access order (if configured).

By leveraging LinkedHashMap, you can efficiently manage ordered key-value mappings while ensuring predictable iteration order.