In this lab, we will use the well-known song 'Old MacDonald Had a Farm' to learn about Inheritance and Polymorphism. Old MacDonald had a farm and several types of animals. Every animal shared certain characteristics. They each had a type (such as cow, chick or pig) and each made a sound (moo, cluck or oink). An Interface defines those things required to be an animal on the farm.
Once we know what it takes to be an Animal, we can define new classes for the cow, chick and pig that implement the Animal interface. Here is a Cow class meeting the minimum requirements to be an Animal.
class Cow implements Animal{
private String myType;
private String mySound;
Cow(){
myType = "cow";
mySound = "moo";
}
public String getSound(){
return mySound;
}
public String getType(){
return myType;
}
}
Implement classes for the chick and the pig. Also complete the test program below to verify your work so far:
public class FarmDriver{
public static void main(String[] args){
Cow c = new Cow();
System.out.println( c.getType() + " goes " + c.getSound() );
// < your code here >
}
}
Create a complete farm to test all your animals. Here is the Farm.java source code.
class Farm{
private ArrayList <Animal> myFarm = new ArrayList <Animal>();
public Farm() {
myFarm.add(new Cow());
myFarm.add(new Chick());
myFarm.add(new Pig());
}
public void animalSounds() {
Animal temp;
for (int i = 0; i < myFarm.size(); i++) {
temp = myFarm.get(i);
System.out.println(temp.getType() + " goes " + temp.getSound());
}
}
}
You will need to change your FarmDriver.java code to create an object of type Farm and then to invoke its animalSounds method.
As it turns out, the chick seems a little confused. Sometimes it makes one sound, when it is feeling childish, and another when it is feeling more grown up. Its two sounds are "cheep" and "cluck". Modify the Chick.java code to add a second constructor that sets a flag for the chick to indicate whether the chick makes one or two sounds. The getSound() method returns either sound, with equal probability, if there are two sounds available. You will also have to modify your Farm.java code to construct the Chick with two possible sounds.
Finally, it also came to pass that the cows received personal names, such as Elsie. Create a new class, NamedCow, which extends the Cow class, adding a constructor, a field for the Cow's name, and a new method: getName. Shown below is the final Farm.java code to exercise all your modifications:
class Farm
{
private ArrayList <Animal> myFarm;
public Farm() {
myFarm = new ArrayList <Animal>();
myFarm.add(new Cow());
myFarm.add(new Chick());
myFarm.add(new Pig());
myFarm.add(new NamedCow("Elsie"));
}
public void animalSounds(){
Animal temp;
for(int i = 0; i < myFarm.size(); i++){
temp = myFarm.get(i);
System.out.println(temp.getType() + " goes " + temp.getSound());
}
NamedCow named = (NamedCow)myFarm.get(3);
System.out.println(named.getName());
}
}
Make sure that you understand what you've just accomplished. Having an array of Animal objects and then having the getSound() method dynamically decide what sound to make is known as polymorphism. This is also known as late binding because it wasn't known until run-time that a[1], for example, really was a Chick object.
You started with an Interface for an Animal and then used the keyword implements in making the three types of animals. Then you created a specialized version of the Cow, a NamedCow, using the keyword extends. This illustrates the concept of inheritance. The NamedCow had all the attributes and methods of the Cow and then added some: a new field and a new method to access the cow's name.
Tino APCS