ZCW-PhoneBook/PhoneBook.java

import java.util.Map;
import java.util.*;
public class PhoneBook {
    Map<String,List<String>> phoneBook = new TreeMap<String,List<String>>();

    public PhoneBook(){

    }

    public void addEntrant(String name, String...phoneNumber){
        String checkName = lookup(name);
        if(checkName == null){
            phoneBook.put(name, Arrays.asList(phoneNumber));
        } else {
            System.out.println("Error, user already exists");
        }
    }

    public void removeEntrant(String name){
        phoneBook.remove(name);   
    }

    public String lookup(String name){
        if (phoneBook.get(name) == null){
            return null;
        }
        StringBuilder numbersList = new StringBuilder();
        int counter = 0;
        for(String number: phoneBook.get(name)){
            numbersList.append(number);
            counter++;
            if(counter != phoneBook.get(name).size()){
                numbersList.append(", ");
            }
        }

        return numbersList.toString();
    }

    public String reverseLookup(String phoneNumber){
        for(Map.Entry<String, List<String>> element: phoneBook.entrySet()){
            String key = element.getKey();
            List<String> value = element.getValue();
            if(value.equals(phoneNumber)){
                return key;
            }
        }
        return "This number does not belong to anyone";
    }

    public void display(){
        for(Map.Entry<String, List<String>> element: phoneBook.entrySet()){
            String key = element.getKey();
            List<String> value = element.getValue();
            System.out.println( key + ":" + value);
        }
    }
}