What are the Differences Between LinkedHashMap and HashMap Implementations in Java?

What distinguishes the implementation of LinkedHashMap from HashMap in Java?

// Java code example demonstrating LinkedHashMap and HashMap
import java.util.*;

public class MapComparison {
    public static void main(String[] args) {
        // Using HashMap
        Map<Integer, String> hashMap = new HashMap<>();
        hashMap.put(1, "One");
        hashMap.put(2, "Two");

        // Using LinkedHashMap
        Map<Integer, String> linkedHashMap = new LinkedHashMap<>();
        linkedHashMap.put(1, "One");
        linkedHashMap.put(2, "Two");

        System.out.println("HashMap: " + hashMap);
        System.out.println("LinkedHashMap: " + linkedHashMap);
    }
}

LinkedHashMap and HashMap are both part of the Java Collections Framework and offer similar functionalities for storing key-value pairs. However, they differ in their implementation details, order retention, and performance characteristics under certain conditions.

// Example showing insertion order in LinkedHashMap:
LinkedHashMap<Integer, String> linkedHashMap = new LinkedHashMap<>();
linkedHashMap.put(3, "Three");
linkedHashMap.put(1, "One");
System.out.println(linkedHashMap.keySet()); // Output: [3, 1]

Causes

• LinkedHashMap maintains a doubly-linked list to preserve the order of insertion, while HashMap does not guarantee any order of its elements.
• The additional linked structure in LinkedHashMap incurs some overhead for memory and performance, effectively making operations slightly slower compared to HashMap.

Solutions

• Use HashMap when the order of storage is not important, as it is typically more efficient and has a smaller memory footprint.
• Choose LinkedHashMap when you need to maintain the insertion order of keys or require predictable iteration order.

Mistake: Choosing LinkedHashMap when performance is critical without needing order.

Solution: Opt for HashMap in scenarios where order is irrelevant, as it will likely yield better performance.

Mistake: Assuming the order of iteration in HashMap will be consistent across different runs.

Solution: Always use LinkedHashMap if you require reliable iteration order based on insertion.