Exploring Java LinkedHashMap: An In-Depth Guide

Java LinkedHashMap is a versatile data structure that combines the features of both HashMap and LinkedList. It maintains a linked list of the entries in the map, allowing iteration over the elements in the order they were inserted. In this guide, we'll delve into the details of Java LinkedHashMap, its features, usage, and constructor options.

Overview

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A LinkedHashMap is a subclass of HashMap that maintains the insertion order of its elements. It provides constant-time performance for basic operations such as get, put, and remove, similar to HashMap. Additionally, it allows iteration over the elements in the order they were inserted or in the order of their last access.

Features of LinkedHashMap

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  • Order Preservation : LinkedHashMap maintains the order of elements based on their insertion sequence.
  • Iterability : It provides methods to iterate over the elements in insertion order or access order.
  • Efficient Operations : Basic operations such as get, put, and remove have constant-time complexity.
  • Concurrency : LinkedHashMap is not thread-safe by default. You can use Collections.synchronizedMap() to make it thread-safe.

Usage

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Creating a LinkedHashMap

You can create a LinkedHashMap using various constructors:

  1. Default constructor: 

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

  2. Constructor with initial capacity: 

    LinkedHashMap<Integer, String> linkedHashMap = new LinkedHashMap<>(16);

  3. Constructor with initial capacity and load factor: 

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

  4. Constructor with initial capacity, load factor, and access order flag: 

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

Inserting Elements 

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

Accessing Elements 

String value = linkedHashMap.get(2); // Returns "Two"

Iterating Over Elements 

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

Removing Elements 

linkedHashMap.remove(3);

Iterating in Insertion Order 

Iterator<Map.Entry<Integer, String>> iterator = linkedHashMap.entrySet().iterator(); 
while (iterator.hasNext()) { 
    Map.Entry<Integer, String> entry = iterator.next(); 
    System.out.println(entry.getKey() + ": " + entry.getValue()); 
}

Performance

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LinkedHashMap provides constant-time performance for basic operations such as get, put, and remove. However, it has a higher memory overhead compared to HashMap due to the additional linked list. The iteration performance is also slightly slower compared to HashMap, especially for large datasets.

Conclusion

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Java LinkedHashMap is a versatile data structure that combines the features of a hash table and a linked list. It maintains the insertion order of elements while providing efficient lookup, insertion, and removal operations. Understanding its features and constructor options can help you leverage it effectively in your Java applications.