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.

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:

👁 LinkedHashMap Node Representation in Java

• 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.

Constructors of LinkedHashMap Class

LinkedHashMap class provides various constructors for different use cases:

1. LinkedHashMap()

Creates an empty LinkedHashMap with default initial capacity (16) and load factor (0.75)

LinkedHashMap<K, V> lhm = new LinkedHashMap<>();

2. LinkedHashMap(int initialCapacity, float loadFactor)

Creates a LinkedHashMap with specified capacity and load factor.

LinkedHashMap<K, V> lhm = new LinkedHashMap<>(20, 0.75f);

3. LinkedHashMap(Map<? extends K,​? extends V> map)

Creates a LinkedHashMap containing all elements from the specified map, maintaining their insertion order.

LinkedHashMap<K, V> lhm = new LinkedHashMap<>(existingMap);

Performing Various Operations on LinkedHashMap

Let’s see how to perform a few frequently used operations on the LinkedHashMap class instance.

1. Adding Elements in LinkedHashMap

We can add elements to a LinkedHashMap using the put() method, which retains the insertion order unlike HashMap.

{3=Geeks, 2=For, 1=Geeks}

2. Updating Elements in LinkedHashMap

To update an element in a LinkedHashMap, use the put() method again with the same key and a new value , it replaces the old value while keeping the insertion order.

{3=Geeks, 2=Geeks, 1=Geeks}
Updated Map: {3=Geeks, 2=For, 1=Geeks}

3. Removing Element in LinkedHashMap

We can remove elements from a LinkedHashMap using the remove() method by specifying the key of the element to be deleted.

{3=Geeks, 2=Geeks, 1=Geeks, 4=For}
{3=Geeks, 2=Geeks, 1=Geeks}

4. Iterating through the LinkedHashMap

We can iterate through a LinkedHashMap using a for-each loop over map.entrySet(), accessing each key and value with getKey() and getValue() methods.

3 : Geeks
2 : For
1 : Geeks

Methods of LinkedHashMap

Below are some commonly used methods of the LinkedHashMap class: