Urgent - Association Issue - Driving me nuts!!!

I am trying to modify some code which lists the vehicles held by a Hire Company. The Cars are stored in an array and I am trying to include a minibus which holds the same details as the Car (Make, Colour etc) but also no. of seats (Minibus Inherits Car Details).
How do I add to & extract the Car & Minibus details together from the Array?
Do I need a sepearte Array list for minibuses? Do I need to create a separate Association for each?

import javax.swing.JOptionPane;
import java.util.*;
public class VehicleHireCompany{

//instance variables
private String companyname;
private ArrayList owns = new ArrayList();

// Constructor
public VehicleHireCompany(String companyname){
      setCompanyName(companyname);
      }

// Create Text for Output
public String toString() {
String output;
output = "        " + this.companyname + "\n";
output = output + "     Own The Following Vehicles\n";
return output;
}

// Create Association
public void ownsCar(Car owns) {
this.owns.add(owns);
}

public void printDetails(){
String output;
output = toString();

// Extract cars one by one and get details for each
Iterator allCars = this.owns.iterator();
while (allCars.hasNext()) {
      output = output + "\n" + ((Car)(allCars.next())).toString();
}

// Display Message
JOptionPane.showMessageDialog(null,output,"Vehicle List",JOptionPane.INFORMATION_MESSAGE);
}


// Set methods for properties

  public void setCompanyName(String aCompanyName){
    this.companyname = aCompanyName;
  }


} //end class


import javax.swing.JOptionPane;
public class Car{

//instance variables
private String make, colour, registration, motDue;


// Constructor
public Car(String make, String colour, String registration, String motDue){
      setMake(make);
      setColour(colour);
      setRegistration(registration);
      setMot(motDue);
}

// Create Text for Output
public String toString() {
String output;
output = "Make:                             " + make + "\n";
output = output + "Colour:                           " + colour + "\n";
output = output + "Registration:                " + registration + "\n";
output = output + "MOT Due:                      " + motDue + "\n";

return output;
}


public void printDetails(){
String output;

output = toString();
JOptionPane.showMessageDialog(null,output,"Details",JOptionPane.INFORMATION_MESSAGE);
}

//Set methods for properties

  public void setMake(String aMake){
    make = aMake;
  }

  public void setColour(String aColour){
    colour = aColour;
  }

  public void setRegistration(String aRegistration){
    registration = aRegistration;
  }

  public void setMot(String aMot){
    motDue = aMot;
  }

  //Get methods for properties
  public String getMake(){
    return make;
  }

  public String getColour(){
    return colour;
  }

  public String getRegistration(){
    return registration;
  }

  public String getMot(){
       return motDue;
  }

} //end class


import javax.swing.JOptionPane;

public class Minibus extends Car{

//instance variables
private String make, colour, registration, motDue;
private int seats;

// Constructor
public Minibus(String make, String colour, String registration, String motDue, int seats){
     super(make, colour, registration, motDue);
     setSeats(seats);

}


// Create Text for Output
public String toString() {
String output;


output = "Make:                             " + make + "\n";
output = output + "Colour:                           " + colour + "\n";
output = output + "Registration:                " + registration + "\n";
output = output + "MOT Due:                      " + motDue + "\n";
output = output + "No. of Seats:                " + seats + "\n";

return output;
}


public void printDetails(){
String output;

output = toString();
JOptionPane.showMessageDialog(null,output,"Details",JOptionPane.INFORMATION_MESSAGE);
}

  //Set methods for properties

  public void setMake(String aMake){
    make = aMake;
  }

  public void setColour(String aColour){
    colour = aColour;
  }

  public void setRegistration(String aRegistration){
    registration = aRegistration;
  }

  public void setMot(String aMot){
    motDue = aMot;
  }
  public void setSeats(int aSeats){
    seats = aSeats;
  }

   //Get methods for properties

  public String getMake(){
    return make;
  }

  public String getColour(){
    return colour;
  }

  public String getRegistration(){
    return registration;
  }

  public String getMot(){
       return motDue;
  }

  public int getSeats(){
       return seats;
  }
} //end class