Using Object-Oriented Patterns - Beginning Java Programming: The Object-Oriented Approach - page 601

In order to make your program future‐proof, go for the most flexible route. Start with defining an
AccountListener interface:
public interface AccountListener {
public void notifyFundsChanged(double newAmount);
}
Next up, modify your model to accept AccountListeners :
import java.util.ArrayList;
import java.util.List;
public class Account {
private List<AccountListener> listeners;
private String name;
private double amount;
public Account(String name) {
this(name, 0);
}
public Account(String name, double startAmount) {
this.name = name;
this.amount = startAmount;
this.listeners = new ArrayList<>();
}
public boolean isOverdrawn() {
return this.amount < 0;
}
public void addFunds(double amount) {
this.amount += amount;
for (AccountListener listener : listeners)
listener.notifyFundsChanged(getAmount());
}
public String getName() {
return name;
}
public double getAmount() {
return amount;
}
public void addAccountListener(AccountListener listener) {
listeners.add(listener);
}
public void removeAccountListener(AccountListener listener) {
listeners.remove(listener);
}
public void removeAllAccountListeners() {