LinkedHashMap in Java

LinkedHashMap in Java implements the Map interface of the Collections Framework. It stores key-value pairs while maintaining the insertion order of the entries. It maintains the order in which elements are added.

• Stores unique key-value pairs.
• Maintains insertion order.
• Allows one null key and multiple null values.
• It is not thread-safe; to synchronize it, use Collections.synchronizedMap().

Declaration of LinkedHashMap

public class LinkedHashMap<K,​V> extends HashMap<K,​V> implements Map<K,​V>

Here, K is the key Object type and V is the value Object type

• K: The type of keys in the map.
• V: The type of values mapped in the map.

import java.util.LinkedHashMap;

public class Geeks{
    
    public static void main(String[] args){
        
        // Create a LinkedHashMap of Strings (keys) and Integers (values)
        LinkedHashMap<String, Integer> lhm = new LinkedHashMap<>();
        
        // Adding key-value pairs
        lhm.put("Apple", 50);
        lhm.put("Banana", 30);
        lhm.put("Mango", 70);
        lhm.put("Orange", 40);
        
        // Displaying the LinkedHashMap
        System.out.println("LinkedHashMap: " + lhm);
    }
}

LinkedHashMap: {Apple=50, Banana=30, Mango=70, Orange=40}

Internal Working of LinkedHashMap

LinkedHashMap extends HashMap and implements the Map interface:

public class LinkedHashMap extends HashMap implements Map

It stores data as nodes similar to a doubly-linked list, maintaining insertion order. Each node contains:

• Key: Since this class extends HashMap, the data is stored in the form of a key-value pair. Therefore, this parameter is the key to the data.
• Value: For every key, there is a value associated with it. This parameter stores the value of the keys. Due to generics, this value can be of any form.
• Next: Since the LinkedHashMap stores the insertion order, this contains the address to the next node of the LinkedHashMap.
• Previous: This parameter contains the address to the previous node of the LinkedHashMap.

Hierarchy of LinkedHashMap

It extends HashMap and maintains a doubly-linked list to preserve the insertion order of elements.