The class ApartmentOwner: realization of the methods

public class ApartmentOwner {
  private String   owner;
  private String[] apartments;

  public ApartmentOwner(String owner) {
    this.owner = owner;
    apartments = new String[10];
  }

  public String getOwner() {
    return owner;
  }

  public String getApartment(int slot) {
    return apartments[slot];
  }

  public void setApartment(String address, int slot) {
    apartments[slot] = address;
  }

  public int countApartments() {
    int num = 0;
    for (int i = 0; i < 10; i++)
      if (apartments[i] != null)
        num ++;
    return num;
  }

  public void reorganizeApartments() {
    int empty = -1;  // index of the first empty slot
    for (int i = 0; i < 10; i++) {
      if (apartments[i] == null && empty == -1)
        empty = i;
      if (apartments[i] != null && empty != -1) {
        apartments[empty] = apartments[i];
        apartments[i] = null;
        empty++;
      }
    }
  }

  public String toString() {
    String ris = "";
    ris += "Owner: " + owner;
    for (int i = 0; i < 10; i++)
      if (apartments[i] != null)
        ris += "\nApartment " + i + ": " + apartments[i];
    return ris;
  }
}

Example of usage:

public class TestApartmentOwner {

  public static void main (String[] args) {
    ApartmentOwner p = new ApartmentOwner("Mario Rossi");
    p.setApartment("Via della Camilluccia, 29", 0);
    p.setApartment("Largo di Torre Argentina, 42", 1);
    p.setApartment("P.zza Conca D'Oro, 9", 2);
    p.setApartment("Via del Corso, 30", 5);
    p.setApartment("Via Trionfale, 2500", 8);
    System.out.println(p);

    System.out.println();
    System.out.println(p.getOwner() + " has " +
                       p.countApartments() + " apartments");
    System.out.println("Apartment 2: " + p.getApartment(2));

    System.out.println();
    p.reorganizeApartments();
    System.out.println(p);
  }
}