Java and XML serialization

I'm kind of new to java and have trouble manipulating arrays, but especially arrays of objects.

I'm taking a new XML class and I have not learned the advanced Java concepts yet.

here is the problem that I am having

The file below contains the main method.

import java.io.FileWriter;
import java.io.IOException;


public class SistersTest
{
  public static void main (String[] args)
    throws IOException
  {
    FileWriter fw;
    SisterCities sc;

    sc = buildSisterCities ();
    fw = new FileWriter ("output-01.xml");
    fw.write (toXML (sc));
    fw.close ();  
  }


  public static String toXML (SisterCities sc)
  {
      //    throw new RuntimeException ("Not implemented yet.");
  }


  public static SisterCities buildSisterCities ()
  {
    City chicago = new City ("Chicago", "USA");
    City accra = new City ("Accra", "Ghana");
    City amman = new City ("Amman", "Jordan");
    City athens = new City ("Athens", "Greece");
    City belgrade = new City ("Belgrade", "Serbia and Montenegro");
    City losAngeles = new City("Los Angeles", "USA");

    City [] chicagoSisters = {accra, amman, athens, belgrade};
    City [] athensSisters = {chicago, losAngeles};

    SisterCities sc = new SisterCities ();
    chicago.setSisters(chicagoSisters);
    sc.addCity(chicago);
    athens.setSisters(athensSisters);
    sc.addCity(athens);

    return sc;
  }
}


I need help building the methods in the two skeleton classes (see below) so that they will work with the implementation given above.
P.S
Please put in comments so that that I will understand how this works (or not).

here are the  2 classes that I need to develope.

(1)
public class City
{
      
    public City (String name, String country)
    {
       //throw new RuntimeException ("Not implemented yet.");
        
    }

    public City (String name, String country, City [] sisters)
    {
       //throw new RuntimeException ("Not implemented yet.");
    }

    public void setSisters (City [] sisters)
    {
      // throw new RuntimeException ("Not implemented yet.");
            
    }

    public String getName ()
    {
      // throw new RuntimeException ("Not implemented yet.");
    }

    public String getCountry ()
    {
      // throw new RuntimeException ("Not implemented yet.");
    }

    public City [] getSisters ()
    {
      // throw new RuntimeException ("Not implemented yet.");
    }
}

(2)

package hw01;

public class SisterCities
{
    public SisterCities ()
    {
       throw new RuntimeException ("Not implemented yet.");
    }


    public void addCity (City city)
    {
       throw new RuntimeException ("Not implemented yet.");
    }


    public int getNumCities ()
    {
      // throw new RuntimeException ("Not implemented yet.");
    }


    public City getCity (int i)
    {
      // throw new RuntimeException ("Not implemented yet.");
    }
}


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