Understanding Java's LinkedHashMap

What is LinkedHashMap?

LinkedHashMap is a part of Java's Collections Framework. It extends HashMap and maintains a linked list of entries, preserving the order of insertion.

Key Features

• Order of Elements: Unlike HashMap, which does not guarantee an order, LinkedHashMap maintains the order of elements as they are added.
• Null Values: It allows one null key and multiple null values.
• Performance: It offers a performance similar to HashMap (O(1) for basic operations), though there is slight overhead due to the linked list.

Main Uses

LinkedHashMap is particularly useful when a predictable iteration order is required, such as in the display of map contents.

Key Concepts

• Insertion Order: Elements are iterated in the order they are added to the map.
• Access Order: It can be configured to maintain access order instead of insertion order, which is beneficial for implementing caches.

Example Usage

import java.util.LinkedHashMap;

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

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

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

Output

1 : Apple
2 : Banana
3 : Cherry

Conclusion

LinkedHashMap is a powerful data structure that allows you to maintain the order of entries while benefiting from the speed of hash-based lookups. It is ideal for scenarios where insertion order is crucial.