Fixing a class to ensure that it adds objects to its list, only if it implements a certain interface - Java

Fixing a class to ensure that it adds objects to its list, only if it
implements a certain interface - Java

I've been given a class - GoodPetStoreClient - that makes use of another
class - NoisyPetStore - in order to create a list of objects - Cat, Dog
and Cow - which implement an interface - MakesSound - and have been asked
to modify NoisyPetStore to ensure that GoodPetStoreClient will compile
properly. I've been trying to work out what I'm missing, but without luck
so far, and would appreciate some more experienced insight.
Thanks!
Here's the code public class GoodPetStoreClient {
public static void main(String[] args) {
NoisyPetStore petStore = new NoisyPetStore();
petStore.addPet(new Cat());
petStore.addPet(new Cow());
System.out.println("I bought an animal, and it goes: " +
petStore.buyNewestPet().makeNoise()); //moo...
System.out.println("The rest of the pet store goes: " +
petStore.makeHugeNoise()); //meow
System.out.println("I bought another animal, and it goes: " +
petStore.buyNewestPet().makeNoise()); //meow
petStore.addPet(new Dog());
System.out.println("The pet store now goes: " +
petStore.buyNewestPet().makeNoise());
}
private static class CollisionInSpace {
// makes no sound at all
}
}
import java.util.ArrayList; import java.util.List;
public class NoisyPetStore
{
//Stores pets
private List list;
public NoisyPetStore()
{
list = new ArrayList();
}
/* add a pet to the pet store after checking
* whether or not the object implements
* <MakesSound>
* @param o takes in a object of an unspecified
* class
**/
public void addPet(Object o )
{
//check if the instance implements the MakesSound interface
if(o instanceof MakesSound)
{
list.add(o);
}
}
/* get the last pet from the store by accessing
* the last item in the list, and hence the one
* which has been added most recently
**/
public Object buyNewestPet()
{
Object ans = null;
if (list.size() > 0)
{
ans = list.remove(list.size() - 1);
}
return ((MakesSound)ans);
}
/* creates a string representation of all of the noises
* made by pets which have been added to the list using
* a <StringBuilder>
* @return returns a String representation of all the noises
* made by the pets in the list
**/
public String makeHugeNoise() {
StringBuilder ans = new StringBuilder();
for (int i = 0; i < list.size(); i++) {
ans.append(((MakesSound) list.get(i)).makeNoise());
}
return ans.toString();
}
}
public class Cat implements MakesSound
{
String sound;
/* Constructor for Cat object
* takes in no parameters and instantiates
* the instance variable sound
**/
public Cat()
{
sound = "Meeow";
}
/* Overrides the <makeNoise> method defined by the
* <MakeSounds> interface
* takes in no parameters and returns the sound made
* by a cat, represented as a <String> {@link String}
**/
@Override
public String makeNoise()
{
String s = sound;
return s;
}
}
//dog and cow have identical codes to cat, with the exception that they
produce the sounds "Woof!" and "Moo!" respectively.