Design Question: Parent/Child Distribution

Question

Hi,
I have what I think is a straightforward design question. I tried
searching this forum for the answer, but struggled with what keywords
to use in the searches. (I also struggled with what to put on the
subject line for this post!)
Anyhow, here is my question: Given:
public class Pet {...}
public class Dog extends Pet {...}
public class Cat extends Pet {...}
public class Pig extends Pet {...}
Which is the better design?
DESIGN A:
public abstract class PetOwner {
private Pet pet;
PetOwner(Pet pet) {
this.pet = pet;
public Pet getPet() { return this.pet; }
public class DogOwner extends PetOwner {
public DogOwner(Dog dog) {
super(dog);
public class CatOwner extends PetOwner {
public CatOwner(Cat cat) {
super(cat);
public class PigOwner extends PetOwner {
public PigOwner(Pig pig) {
super(pig);
===============================================
or DESIGN B:
public abstract class PetOwner {
PetOwner() {
public class DogOwner extends PetOwner {
private Dog dog;
public DogOwner(Dog dog) {
super();
this.dog = dog;
public class CatOwner extends PetOwner {
private Cat cat;
public CatOwner(Cat cat) {
super();
this.cat = cat;
public class PigOwner extends PetOwner {
private Pig pig;
public PigOwner(Pig pig) {
super();
this.pig = pig;
This is a somewhat contrived example, so don't take it too literally.
My question is where should I put the field that holds a Pet object?
All pet owners own a pet, so should I put it in the parent class
(Design A)? Note the constructors for the child classes (e.g.
DogOwner) are designed so that the correct type of pet is checked
for. Or should each child class hold its own type of pet (Design B)?
If the answer is "it depends" (as I suspect it is), then what are
the questions I should be asking myself in order to decide?
Thanks very much, in advance!!
Chris

Answer 1

eg,
import java.util.*;
class Pet {
    private String name = null;
    public Pet( String n ) {
     this.name = n;
    public String toString() {
     return name;
class Dog extends Pet {
    public Dog( String n ) {
     super( n );
class Pig extends Pet {
    public Pig( String n ) {
     super( n );
class Cat extends Pet {
    public Cat( String n ) {
     super( n );
class PetOwner {
    private Vector pets = new Vector();
    private int dogs = 0;
    private int cats = 0;
    private int pigs = 0;
    public void add( Dog d ) {
     pets.add( d );
     dogs++;
    public void add( Cat c ) {
     pets.add( c );
     cats++;
    public void add( Pig p ) {
     pets.add( p );
     pigs++;
    public boolean isCatOwner() {
     return cats > 0;
    public boolean isDogOwner() {
     return dogs > 0;
    public boolean isPigOwner() {
     return pigs > 0;
    public int numberOfPets() {
     return pets.size();
    public String toString() {
     String s = "";
     Enumeration en = pets.elements();
     while( en.hasMoreElements() ) {
         Pet pp = (Pet) en.nextElement();
         s += pp.toString() + "\n";
     return s;
public class Pets {
    private PetOwner po = new PetOwner();
    public Pets() {
     po.add( new Dog( "doggy" ) );
     po.add( new Cat( "catty" ) );
    public String toString() {
     String s = "";
     s += po.toString();
     s += po.isDogOwner() + "\n";
     s += po.isPigOwner() + "\n";
     s += po.isCatOwner() + "\n";
     s += po.numberOfPets();
     return s;
    public static void main( String[] args ) {
     Pets p = new Pets();
     System.out.println( p );
}