Week 3: Objects, files and exceptions/21. Phone book 3: Listing numbers - Extract HASHMAP KEYS, then SORT

q21
Write the method

void listBook(HashMap<String,String> numbers

which prints the users in the book and their numbers below them on the screen. 
The names should be printed in alphabetical order.


An example of how to call the method:
public static void main(String[] args){
    HashMap<String,String> numbers = new HashMap<>();
    numbers.put("Tina", "12345");
    numbers.put("Keith", "54321");
    numbers.put("Jack", "55555");

    listBook(numbers);
}

Program outputs:
Name: Jack, number: 55555
Name: Keith, number: 54321
Name: Tina, number: 12345







import java.util.Random;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Collections;
import java.util.ArrayList;
import java.util.Scanner;



public class Test{
    public static void main(String[] args){
        final Random r = new Random();
        
        // <hide>
        
        HashMap<String, String> numbers = new HashMap<>();
        numbers.put("Pete", "" + r.nextInt(1000000));
        numbers.put("Lena", "" + r.nextInt(1000000));
        numbers.put("Kim", "" + r.nextInt(1000000));
        numbers.put("Laura", "" + r.nextInt(1000000));
        numbers.put("Lisa", "" + r.nextInt(1000000));
        numbers.put("Zorro", "" + r.nextInt(1000000));
        numbers.put("Tarzan", "" + r.nextInt(1000000));

        listBook(numbers);
    }
    


    //q21
    public static void listBook(HashMap<String,String> numbers) {
        // https://stackoverflow.com/questions/9047090/how-to-sort-hashmap-keys
        // https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html#keySet--
        // sort the HashMap by key
        // String[] arrKeys = numbers.keySet().toArray();              //incorrect

        // new String[0] is an empty array of type String -> ie we want String[] array, even if empty array
        String[] arrKeys = numbers.keySet().toArray(new String[0]);    //correct
        // sort inplace
        Arrays.sort(arrKeys);



        // correct option 2 - extract keySet as ArrayList, then use Collection to sort
        // Convert keySet to a List and sort it
        // ArrayList<String> arrKeys = new ArrayList<>(numbers.keySet());
        // Collections.sort(arrKeys);


        

        // print the sorted HashMap
        for (String key : arrKeys) {
            System.out.println("Name: " + key + ", number: " + numbers.get(key));
        }
    }




}





Name: Kim, number: 701584
Name: Laura, number: 417806
Name: Lena, number: 627033
Name: Lisa, number: 506742
Name: Pete, number: 292799
Name: Tarzan, number: 297377
Name: Zorro, number: 538240