Java: HashMap

A HashMap however, store items in "key/value" pairs, and you can access them by an index of another type (e.g. a String).

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

import java.util.HashMap; // import the HashMap class

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

Add Items

import java.util.HashMap;

public class Main {
  public static void main(String[] args) {
    // Create a HashMap object called capitalCities
    HashMap<String, String> capitalCities = new HashMap<String, String>();

    // Add keys and values (Country, City)
    capitalCities.put("England", "London");
    capitalCities.put("Germany", "Berlin");
    capitalCities.put("Norway", "Oslo");
    capitalCities.put("USA", "Washington DC");
    System.out.println(capitalCities);
  }
}

// Outputs:
{USA=Washington DC, Norway=Oslo, England=London, Germany=Berlin}

Access an Item

capitalCities.get("England");

// Outputs: 
London

Remove an Item

capitalCities.remove("England");

To remove all items, use the clear() method:

capitalCities.clear();

HashMap Size

capitalCities.size();

Loop Through a HashMap

Use the keySet() method if you only want the keys, and use the values() method if you only want the values:

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

// Outputs:
USA
Norway
England
Germany

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

// Outputs:
Washington DC
Oslo
London
Berlin

// Print keys and values
for (String i : capitalCities.keySet()) {
  System.out.println("key: " + i + " value: " + capitalCities.get(i));
}

// Outputs:
key: USA value: Washington DC
key: Norway value: Oslo
key: England value: London
key: Germany value: Berlin