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2026-04-025 min read

Java LinkedHashMap

Learn Java LinkedHashMap step by step with clear examples and exercises.

Why This Matters

In this full guide on Java's LinkedHashMap, we aim to provide a deep understanding of what LinkedHashMap is, its key features, and how to effectively use it in your Java projects. We will cover various aspects such as the importance of maintaining element order, common mistakes, practice questions, and frequently asked questions to help you master the art of using LinkedHashMaps.

Why This Matters

Java developers often encounter situations where they need to maintain the insertion order of elements in a Map data structure. While HashMap does not preserve the order, LinkedHashMap comes to the rescue. Understanding and utilizing LinkedHashMap can help you create more efficient and user-friendly applications by ensuring that your data is processed in the correct sequence.

Prerequisites

To fully grasp this tutorial, you should have a good understanding of:

  1. Java programming basics
  2. Data structures and algorithms
  3. Interfaces Map, HashMap, and Collection
  4. Concepts like key-value pairs, iterators, and collections
  5. Understanding the difference between HashMap and LinkedHashMap
  6. Familiarity with Java's collection framework and its various implementations

Core Concept

A LinkedHashMap is an implementation of the Map interface in Java that maintains the insertion order of its entries. It extends the AbstractLinkedHashMap class, which provides additional functionality for implementing ordered maps.

Key Features

  1. Insertion Order: Unlike HashMap, LinkedHashMap preserves the order in which elements are inserted. This makes it useful when you need to iterate through the map and process elements sequentially.
  2. Access Order: By setting the accessOrder parameter to true, LinkedHashMap will maintain the order in which elements were last accessed.
  3. Size Estimation: LinkedHashMap provides a more accurate estimation of its size compared to HashMap, as it takes into account the memory used by both nodes and keys.
  4. Removal Policies: LinkedHashMap offers two removal policies: accessOrder (remove least recently accessed entries) and insertionOrder (remove least recently inserted entries).
  5. Doubly-Linked List Implementation: LinkedHashMap uses a doubly-linked list to maintain the order of its elements. Each node in the list contains an entry, key, value, and pointers to the previous and next nodes. This allows for efficient insertion, removal, and traversal operations.
  6. Custom Removal Policy: Developers can provide a removeEldestEntry method to customize the removal policy based on specific application requirements.
  7. Access Order vs Insertion Order: When creating a LinkedHashMap instance, you can choose between maintaining the order of insertion or the order of last accessed elements by setting the accessOrder parameter accordingly.

Worked Example

Let's create a simple LinkedHashMap example that demonstrates its insertion order feature:

import java.util.*;

public class Main {
public static void main(String[] args) {
LinkedHashMap<Integer, String> linkedMap = new LinkedHashMap<>(5, 0.75f, true); // Create a LinkedHashMap with an initial capacity of 5, load factor of 0.75, and access order enabled

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

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

// Accessing an element to demonstrate access order
System.out.println("\nAccess Order after accessing an element:");
linkedMap.get(1); // Access the value associated with key 1

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

Output:

Insertion Order:
1: One
2: Two
3: Three

Access Order after accessing an element:
2: Two
3: Three
1: One

Common Mistakes

  1. Forgotten import statements: Always ensure you have the necessary import statements for LinkedHashMap and other related classes.
  2. Incorrect usage of LinkedHashMap methods: Be mindful when using methods like putIfAbsent(), removeEldestEntry(), and accessOrder.
  3. Assuming HashMap behavior: Remember that LinkedHashMap has different performance characteristics than HashMap, especially regarding insertion order and memory usage.
  4. Ignoring access order: If you need to maintain the order of last accessed elements, set the accessOrder parameter to true when creating a new LinkedHashMap instance.
  5. Misunderstanding removal policies: Understand that the default removal policy for LinkedHashMap is LRU (Least Recently Used) when accessOrder is true and LFU (Least Frequently Used) when accessOrder is false.
  6. Incorrect capacity and load factor settings: Properly set the initial capacity and load factor of a LinkedHashMap to optimize its performance based on your specific use case.
  7. Overlooking custom removal policy: If you require a custom removal policy, provide an appropriate implementation of the removeEldestEntry method in your LinkedHashMap subclass.

Practice Questions

  1. How does LinkedHashMap differ from HashMap in terms of maintaining element order?
  2. What is the purpose of the removeEldestEntry() method in LinkedHashMap, and how can it be used to implement a specific removal policy?
  3. How can you create a LinkedHashMap that maintains the order of last accessed elements while using a custom removal policy?
  4. Explain the memory usage differences between LinkedHashMap and HashMap, and provide an example demonstrating their performance impact.
  5. Write a program that implements a custom removal policy for LinkedHashMap based on the frequency of element access.
  6. Compare the time complexity of common operations in LinkedHashMap and HashMap.
  7. How can you use LinkedHashMap as a Stack or Queue, and what are the trade-offs involved?
  8. What is the impact of setting different values for accessOrder, initialCapacity, and loadFactor on the performance of a LinkedHashMap instance?

FAQ

  1. Why should I use LinkedHashMap over HashMap?
  • If you need to maintain the insertion or access order of elements, LinkedHashMap is a better choice than HashMap.
  1. How does LinkedHashMap handle collisions during insertion?
  • Like HashMap, LinkedHashMap uses hash codes and separate chaining to handle collisions during insertion.
  1. Can I use LinkedHashMap as a Stack or Queue?
  • Yes, you can create a LinkedHashMap that behaves like a Stack (Last-In-First-Out) by setting the accessOrder parameter to false and using the last entry as the top of the stack. Similarly, you can create a Queue (First-In-First-Out) by iterating through the map in order.
  1. What is the time complexity of LinkedHashMap operations?
  • The time complexity for common operations like get(), put(), and remove() is O(1) for constant-time average performance, but may degrade to O(n) in the worst case (when a collision chain is long).
  1. What is the impact of setting different values for accessOrder, initialCapacity, and loadFactor on the performance of a LinkedHashMap instance?
  • Setting accessOrder to true increases the time complexity of common operations, as the map needs to maintain the order of last accessed elements. Increasing the initial capacity or load factor can improve the performance of the map by reducing the number of rehashes and collisions.
Java LinkedHashMap | Java | XQA Learn