LinkedHashMap

About

LinkedHashMap is a subclass of HashMap introduced in Java 1.4. It maintains the order of elements, unlike HashMap, which is unordered. This class is part of the java.util package and implements the Map interface. The order of elements can be insertion order or access order, depending on the configuration.

  • Insertion Order: The order in which entries are inserted is preserved.

  • Access Order: The order is updated based on recent access if configured.

Features

  1. Preserves Order: Maintains insertion or access order.

  2. Backed by HashMap: Internally uses a HashMap for storage.

  3. Doubly Linked List: Maintains a linked list of entries for ordering.

  4. Allows Null Keys and Values: Similar to HashMap, it allows one null key and multiple null values.

  5. Not Thread-Safe: By default, it is not synchronized. Use Collections.synchronizedMap() for thread-safe operations.

  6. Performance: Slightly slower than HashMap due to linked list maintenance but faster than TreeMap for ordered operations.

Internal Working

The LinkedHashMap combines the functionality of HashMap with a doubly linked list to maintain insertion or access order.

Data Structure

LinkedHashMap relies on two main data structures:

  1. HashMap:

    • Stores the key-value pairs in an array of buckets.

    • Each bucket contains a singly linked list (or a tree for large buckets in modern Java versions).

    • Hashing is used to distribute keys uniformly across buckets.

  2. Doubly Linked List:

    • Maintains the order of entries.

    • Each entry in LinkedHashMap is linked to the previous and next entries using two additional pointers (beforeand after).

Components

  • Entry Class: Each entry in LinkedHashMap is represented by an inner class LinkedHashMap.Entry<K, V>, which extends HashMap.Node<K, V>. It has two extra fields:

    • before: Points to the previous entry in the linked list.

    • after: Points to the next entry in the linked list.

static class Entry<K, V> extends HashMap.Node<K, V> {
    Entry<K, V> before, after;
    Entry(int hash, K key, V value, Node<K, V> next) {
        super(hash, key, value, next);
    }
}

  • Head and Tail Pointers:

    • The doubly linked list is anchored by a head and tail.

    • The head points to the first inserted entry.

    • The tail points to the most recently inserted (or accessed, in access-order mode) entry.

How Order Is Maintained

  1. Insertion Order:

    • When a new entry is added, it is appended to the end of the doubly linked list.

    • This preserves the order of insertion.

  2. Access Order:

    • If accessOrder is set to true, the linked list is updated every time an entry is accessed (via get, put, or putIfAbsent methods).

    • The accessed entry is moved to the end of the list.

Rehashing

  1. Triggered when the size exceeds the load factor threshold.

  2. The HashMap resizes its bucket array, and entries are rehashed to new buckets.

  3. During rehashing:

    • The linked list structure is preserved.

    • The before and after pointers of all entries remain intact.

Operations

put(K key, V value)

  1. Calculates the hash for the key.

  2. Determines the bucket index based on the hash.

  3. Checks if the key already exists:

    • If yes, updates the value.

    • If no, creates a new Entry.

  4. Inserts the entry in the HashMap.

  5. Updates the doubly linked list:

    • Links the new entry to the current tail.

    • Updates the tail pointer.

get(Object key)

  1. Uses the hash to locate the bucket.

  2. Searches for the key in the bucket’s chain.

  3. If the key is found:

    • Returns the value.

    • Updates the order in the doubly linked list if accessOrder is true.

remove(Object key)

  1. Locates the entry in the HashMap using the hash.

  2. Removes the entry from the bucket's chain.

  3. Updates the doubly linked list:

    • Unlinks the entry from its before and after neighbors.

    • Adjusts the head or tail pointers if necessary.

Key Methods

Method

Description

put(K key, V value)

Adds a key-value pair. If the key exists, updates the value.

get(Object key)

Retrieves the value associated with the key. Updates order if accessOrder is true.

remove(Object key)

Removes the key-value pair for the specified key.

containsKey(Object key)

Checks if the map contains the specified key.

containsValue(Object value)

Checks if the map contains the specified value.

keySet()

Returns a Set of keys in order.

values()

Returns a Collection of values in order.

entrySet()

Returns a Set of key-value mappings in order.

clear()

Removes all mappings.

removeEldestEntry(Map.Entry<K,V>)

Determines whether to remove the eldest entry when a new entry is added.

Big-O for Operations

Operation

Average Case

Worst Case

Put

O(1)

O(n)

Get

O(1)

O(n)

Remove

O(1)

O(n)

Iteration

O(n)

O(n)

Limitations

  1. Memory Overhead: The doubly linked list adds memory overhead compared to HashMap.

  2. Not Thread-Safe: Requires external synchronization for concurrent use.

  3. Slightly Slower Than HashMap: Maintaining order introduces additional overhead.

  4. Unpredictable Behavior with Null Keys: While it allows null keys, unexpected behaviors may occur if null is frequently accessed in access-order mode.

Real-World Usage

  1. Caches: Used to implement Least Recently Used (LRU) caches by overriding removeEldestEntry().

  2. Maintaining Order: Useful in applications where insertion or access order is critical.

  3. Data Stores: Ideal for implementing ordered data stores, like log files or access history.

Examples

1. Basic Operations

import java.util.LinkedHashMap;

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

        // Adding elements
        map.put(1, "Alice");
        map.put(2, "Bob");
        map.put(3, "Charlie");
        System.out.println(map); // Output: {1=Alice, 2=Bob, 3=Charlie}

        // Accessing elements
        System.out.println("Value for key 2: " + map.get(2)); // Output: Value for key 2: Bob

        // Removing an element
        map.remove(1);
        System.out.println(map); // Output: {2=Bob, 3=Charlie}

        // Iterating through entries
        for (var entry : map.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
            // Output:
            // 2: Bob
            // 3: Charlie
        }
    }
}

2. Access Order

import java.util.LinkedHashMap;

public class AccessOrderExample {
    public static void main(String[] args) {
        LinkedHashMap<Integer, String> map = new LinkedHashMap<>(16, 0.75f, true);

        map.put(1, "Alice");
        map.put(2, "Bob");
        map.put(3, "Charlie");

        // Accessing elements
        map.get(1);
        map.get(3);

        System.out.println(map); // Output: {2=Bob, 1=Alice, 3=Charlie}
    }
}

3. Implementing LRU Cache

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

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

    public LRUCache(int capacity) {
        super(capacity, 0.75f, true);
        this.capacity = capacity;
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
        return size() > capacity;
    }
}

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

        cache.put(1, "Alice");
        cache.put(2, "Bob");
        cache.put(3, "Charlie");
        System.out.println(cache); // Output: {1=Alice, 2=Bob, 3=Charlie}

        cache.get(1); // Access key 1
        cache.put(4, "Diana"); // Add a new entry
        System.out.println(cache); // Output: {2=Bob, 3=Charlie, 1=Alice}
    }
}
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