Java LinkedHashMap

Java LinkedHashMap

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

It is part of the java.util package and implements the Map interface.

Create a LinkedHashMap

Create a LinkedHashMap object called capitalCities that will store String keys and String values:

import java.util.LinkedHashMap; // Import the LinkedHashMap class

LinkedHashMap<String, String> capitalCities = new LinkedHashMap<>();

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

Add Items

Use the put() method to add items to the LinkedHashMap:

// Import the LinkedHashMap class
import 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

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

capitalCities.get("England");

Remove an Item

Use remove() to remove an item by key:

capitalCities.remove("England");

Use clear() to remove all items:

capitalCities.clear();

LinkedHashMap Size

Use size() 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 a LinkedHashMap using a for-each loop. Use:

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

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

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

// Print keys and values
for (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 the var keyword to declare a LinkedHashMap 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. Since var is valid Java, you may see it in other code, so it's good to know that it exists:

// Without var
LinkedHashMap<String, String> capitalCities = new LinkedHashMap<String, String>();

// With var
var capitalCities = new LinkedHashMap<String, String>();

The Map Interface

Note: Sometimes you will see both Map and LinkedHashMap 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 a Map (the interface), but it stores a LinkedHashMap object (the actual map). Since LinkedHashMap implements the Map 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.