public synchronized AnimalAddedListener registerAnimalAddedListener (AnimalAddedListener listener) { /*...*/ } 
public synchronized void unregisterAnimalAddedListener (AnimalAddedListener listener) { /*...*/ } 
public synchronized void notifyAnimalAddedListeners (Animal animal) { /*...*/ } 

public class ThreadSafeZoo { 
private final ReadWriteLock readWriteLock = new ReentrantReadWriteLock();
protected final Lock readLock = readWriteLock.readLock();
protected final Lock writeLock = readWriteLock.writeLock();
private List<Animal> animals = new ArrayList<>();
private List<AnimalAddedListener> listeners = new ArrayList<>();
public void addAnimal (Animal animal) {
// Add the animal to the list of animals
this.animals.add(animal);
// Notify the list of registered listeners
this.notifyAnimalAddedListeners(animal);
}
public AnimalAddedListener registerAnimalAddedListener (AnimalAddedListener listener) {
// Lock the list of listeners for writing
this.writeLock.lock();
try {
// Add the listener to the list of registered listeners
this.listeners.add(listener);
}
finally {
// Unlock the writer lock
this.writeLock.unlock();
}
return listener;
}
public void unregisterAnimalAddedListener (AnimalAddedListener listener) {
// Lock the list of listeners for writing
this.writeLock.lock();
try {
// Remove the listener from the list of the registered listeners
this.listeners.remove(listener);
}
finally {
// Unlock the writer lock
this.writeLock.unlock();
}
}
public void notifyAnimalAddedListeners (Animal animal) {
// Lock the list of listeners for reading
this.readLock.lock();
try {
// Notify each of the listeners in the list of registered listeners
this.listeners.forEach(listener -> listener.updateAnimalAdded(animal));
}
finally {
// Unlock the reader lock
this.readLock.unlock();
}
}
}

public class ThreadSafeCountingAnimalAddedListener implements AnimalAddedListener { 
private static AtomicLong animalsAddedCount = new AtomicLong(0);
@Override
public void updateAnimalAdded (Animal animal) {
// Increment the number of animals
animalsAddedCount.incrementAndGet();
// Print the number of animals
System.out.println("Total animals added: " + animalsAddedCount);
}
}

public class CountingAnimalAddedListener implements AnimalAddedListener { 
private static int animalsAddedCount = 0;
@Override
public synchronized void updateAnimalAdded (Animal animal) {
// Increment the number of animals
animalsAddedCount++;
// Print the number of animals
System.out.println("Total animals added: " + animalsAddedCount);
}
}

public class OrderedThreadSafeZoo { 
private final ReadWriteLock readWriteLock = new ReentrantReadWriteLock(true);
protected final Lock readLock = readWriteLock.readLock();
protected final Lock writeLock = readWriteLock.writeLock();
private List<Animal> animals = new ArrayList<>();
private List<AnimalAddedListener> listeners = new ArrayList<>();
public void addAnimal (Animal animal) {
// Add the animal to the list of animals
this.animals.add(animal);
// Notify the list of registered listeners
this.notifyAnimalAddedListeners(animal);
}
public AnimalAddedListener registerAnimalAddedListener (AnimalAddedListener listener) {
// Lock the list of listeners for writing
this.writeLock.lock();
try {
// Add the listener to the list of registered listeners
this.listeners.add(listener);
}
finally {
// Unlock the writer lock
this.writeLock.unlock();
}
return listener;
}
public void unregisterAnimalAddedListener (AnimalAddedListener listener) {
// Lock the list of listeners for writing
this.writeLock.lock();
try {
// Remove the listener from the list of the registered listeners
this.listeners.remove(listener);
}
finally {
// Unlock the writer lock
this.writeLock.unlock();
}
}
public void notifyAnimalAddedListeners (Animal animal) {
// Lock the list of listeners for reading
this.readLock.lock();
try {
// Notify each of the listeners in the list of registered listeners
this.listeners.forEach(listener -> listener.updateAnimalAdded(animal));
}
finally {
// Unlock the reader lock
this.readLock.unlock();
}
}
}

public abstract class ObservableSubjectMixin<ListenerType> { 
private List<ListenerType> listeners = new ArrayList<>();
public ListenerType registerListener (ListenerType listener) {
// Add the listener to the list of registered listeners
this.listeners.add(listener);
return listener;
}
public void unregisterAnimalAddedListener (ListenerType listener) {
// Remove the listener from the list of the registered listeners
this.listeners.remove(listener);
}
public void notifyListeners (Consumer<? super ListenerType> algorithm) {
// Execute some function on each of the listeners
this.listeners.forEach(algorithm);
}
}

public class ZooUsingMixin extends ObservableSubjectMixin<AnimalAddedListener> { 
private List<Animal> animals = new ArrayList<>();
public void addAnimal (Animal animal) {
// Add the animal to the list of animals
this.animals.add(animal);
// Notify the list of registered listeners
this.notifyListeners((listener) -> listener.updateAnimalAdded(animal));
}
}

public interface ZooListener { 
public void onAnimalAdded (Animal animal);
public void onAnimalRemoved (Animal animal);
}

public class ZooUsingMixin extends ObservableSubjectMixin<ZooListener> { 
private List<Animal> animals = new ArrayList<>();
public void addAnimal (Animal animal) {
// Add the animal to the list of animals
this.animals.add(animal);
// Notify the list of registered listeners
this.notifyListeners((listener) -> listener.onAnimalAdded(animal));
}
public void removeAnimal (Animal animal) {
// Remove the animal from the list of animals
this.animals.remove(animal);
// Notify the list of registered listeners
this.notifyListeners((listener) -> listener.onAnimalRemoved(animal));
}
}

public class ZooAdapter implements ZooListener { 
@Override
public void onAnimalAdded (Animal animal) {}
@Override
public void onAnimalRemoved (Animal animal) {}
}

public class NamePrinterZooAdapter extends ZooAdapter { 
@Override
public void onAnimalAdded (Animal animal) {
// Print the name of the animal that was added
System.out.println("Added animal named " + animal.getName());
}
}

public interface ZooListener { 
default public void onAnimalAdded (Animal animal) {}
default public void onAnimalRemoved (Animal animal) {}
}

public class ZooListener { 
public void onAnimalAdded (Animal animal) {}
public void onAnimalRemoved (Animal animal) {}
}

public class AnimalAddedFunctor { 
private final AnimalAddedListener listener;
private final Animal parameter;
public AnimalAddedFunctor (AnimalAddedListener listener, Animal parameter) {
this.listener = listener;
this.parameter = parameter;
}
public void execute () {
// Execute the listener with the parameter provided during creation
this.listener.updateAnimalAdded(this.parameter);
}
}