JavaLinkedHashMap

Java LinkedHashMap

ALinkedHashMap stores keys and values, and keeps them in the same order you put them in.

It is part of thejava.util package and implements theMap interface.

Create a LinkedHashMap

Create aLinkedHashMap object calledcapitalCities that will storeString keys andString values:

import java.util.LinkedHashMap; // Import the LinkedHashMap classLinkedHashMap<String, String> capitalCities = new LinkedHashMap<>();

Now you can use methods likeput() to add key/value pairs,get() to retrieve a value by key, andremove() to delete an entry - all while maintaining insertion order.

Add Items

Use theput() method to add items to theLinkedHashMap:

// Import the LinkedHashMap classimport java.util.LinkedHashMap;public class Main {  public static void main(String[] args) {    LinkedHashMap<String, String> capitalCities = new LinkedHashMap<>();    capitalCities.put("England", "London");    capitalCities.put("India", "New Dehli");    capitalCities.put("Austria", "Wien");    capitalCities.put("Norway", "Oslo");    capitalCities.put("Norway", "Oslo"); // Duplicate    capitalCities.put("USA", "Washington DC");    System.out.println(capitalCities);  }}

Access an Item

Useget() with a key to get its associated value:

capitalCities.get("England");

Remove an Item

Useremove() to remove an item by key:

capitalCities.remove("England");

Useclear() to remove all items:

capitalCities.clear();

LinkedHashMap Size

Usesize() to check how many key/value pairs are in the map:

capitalCities.size();

Note: The size only counts unique keys. If a key is added more than once, only the latest value is kept.

Loop Through a LinkedHashMap

You can loop through aLinkedHashMap using a for-each loop. Use:

• keySet() to get all keys
• values() to get all values

// Print keysfor (String key : capitalCities.keySet()) {  System.out.println(key);}

// Print valuesfor (String value : capitalCities.values()) {  System.out.println(value);}

// Print keys and valuesfor (String key : capitalCities.keySet()) {  System.out.println("Key: " + key + ", Value: " + capitalCities.get(key));}

HashMap vs LinkedHashMap

The var Keyword

From Java 10, you can use thevar keyword to declare aLinkedHashMap variable without writing the type twice. The compiler figures out the type from the value you assign.

This makes code shorter,but many developers still use the full type for clarity. Sincevar is valid Java, you may see it in other code, so it's good to know that it exists:

// Without varLinkedHashMap<String, String> capitalCities = new LinkedHashMap<String, String>();// With varvar capitalCities = new LinkedHashMap<String, String>();

The Map Interface

Note: Sometimes you will see bothMap andLinkedHashMap in Java code, like this:

import java.util.Map;import java.util.LinkedHashMap;Map<String, String> capitalCities = new LinkedHashMap<>();

This means the variable (capitalCities) is declared as aMap (the interface), but it stores aLinkedHashMap object (the actual map). SinceLinkedHashMap implements theMap interface, this is possible.

It works the same way, but some developers prefer this style because it gives them more flexibility to change the type later.