Mixing polymorphism with abstract classes???

Question

Hi,
Can you have polymorhism with abstract superclasses in Java?
For example, say I have a superclass called Pet:
abstract class Pet {
     private String name;
     public String getName() {
          return name;
    public void setName(String petName) {
         name = petName;
    public String speak() {
         return "I am a Pet";
}Now I have a Cat subclass:
public class Cat extends Pet{
    public String speak {
         return "i am a cat";
public class Dog extends Pet {
public String fetch() {
     return "fetching a stick";
}In my test method I have:
public class TestPet {
     public static void main(String[] args) {
          Pet p = new Pet();
          System.out.println(p.speak());
          Pet p = new Dog();
          System.out.println(((Dog)p).fetch());
          Pet p = new Cat();
          System.out.println(p.speak() +" is now a cat");
}So it appears to me that you cannot have polymorphic methods with an abstract superclass - is this right?
Cheers.

Answer 1

cotton.m wrote:
List<Pet> pets = new ArrayList<Pet>();
I was working (more or less) along the same lines...
package forums;
import java.util.List;
import java.util.ArrayList;
abstract class Pet
  private String name;
  public String getName() { return this.name; }
  public void setName(String name) { this.name = name; }
  public String speak() {
    return "I am a Pet";
  public abstract String fetch(); // <<<<< the new bit.
  public String getClassName() {
    final String name = this.getClass().getName();
    return name.substring(name.lastIndexOf('.')+1).toLowerCase();
class Cat extends Pet
  public Cat(String name) {
    setName(name);
  public String speak() {
    return "I am a cat";
  public String fetch() {
    return "No. I am a cat!";
  public String rollOver() {
    return "No. I am a cat!";
class Dog extends Pet
  public Dog(String name) {
    setName(name);
  public String fetch() {
    return "Look boss, I'm fetching the stick.";
  public String sit() {
    return "Look boss, I'm sitting!";
public class PetTest
  public static void main(String[] args) {
    List<Pet> pets = new ArrayList<Pet>();
    pets.add(new Cat("Fungus"));
    pets.add(new Dog("Spike"));
    for ( Pet pet : pets ) {
      System.out.println("\""+pet.speak()+"\" said "+pet.getName()+" the "+pet.getClassName()+".");
      System.out.println("\"Go fetch\" said the master.");
      System.out.println("\""+pet.fetch()+"\" said "+pet.getName()+".");
      System.out.println();
    System.out.println("Therefore we conclude that dogs are much better pets.");
}Both these examples demonstrate that polymorphism is very useful when we have a list of different-type-if-things which have a common interface... i.e. different classes which have (a) a common ancestor; or (b) a common interface... allowing us to treat those different types-of-things as if they where one-type-of-thing, except the-different-types actually do different stuff internally, which is appropriate-to-there-specific-type.
That's the essence of polymorphism... and, of course, this seperation of "type" and "implementation" isn't limited to lists-of-stuff... it can be very useful all over the show.
Cheers. Keith.
Edited by: corlettk on 10/05/2009 11:01 ~~ That's a bit clearer.