• To add an item in a map use the method: .put() and inside the brackets put your Key and Value (line 8);
• To get a Key and a Value you are using the method: .get() and inside the brackets put the Key (NOT the Value - otherwise a null will be shown in the console) (line 15)
• To remove Key + Value use the method: .remove() and inside the brackets put the Key (line 18)

See the code and read the comments to clear things up:

import java.util.HashMap;
import java.util.Map;
 
public class HashMapCode {
    public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();//CREATING A MAP
 
        map.put("Germany", "Berlin");//ADDING AN ITEM TO A MAP<Key, Value>
        map.put("Spain", "Madrid");
        map.put("Greece", "Athens");
        map.put("Turkey", "Ankara");
 
        System.out.println(map);
 
        String capital = map.get("Germany");//GETTING THE Value OF THE Key: Germany
        System.out.println("The capital of Germany is: " + capital);
 
        map.remove("Spain");//REMOVING A Key+Value
 
        System.out.println(map);
    }
}