Prompt Title: AP System Modification.

To implement the Accounts Payable System Modification, we need to modify the Employee class to implement the Payable interface and add a getPaymentAmount method to invoke the earnings method for the subclasses. Here's the modified Employee class:

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public abstract class Employee implements Payable {
   private final String firstName;
   private final String lastName;
   private final String socialSecurityNumber;

   public Employee(String firstName, String lastName, String socialSecurityNumber) {
      this.firstName = firstName;
      this.lastName = lastName;
      this.socialSecurityNumber = socialSecurityNumber;
   }

   public String getFirstName() {
      return firstName;
   }

   public String getLastName() {
      return lastName;
   }

   public String getSocialSecurityNumber() {
      return socialSecurityNumber;
   }

   @Override
   public double getPaymentAmount() {
      return earnings();
   }

   public abstract double earnings();

   @Override
   public String toString() {
      return String.format("%s %s%nsocial security number: %s",
         getFirstName(), getLastName(), getSocialSecurityNumber());
   }
}

Now we can use this modified Employee class to implement the Accounts Payable System. Here's the modified version of Exercise 10.15:

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import java.util.ArrayList;

public class AccountsPayableSystemModification {
   public static void main(String[] args) {
      ArrayList<Payable> payableObjects = new ArrayList<>();

      // add some random Payable objects to the list
      payableObjects.add(new Invoice("01234", "seat", 2, 375.00));
      payableObjects.add(new Invoice("56789", "tire", 4, 79.95));
      payableObjects.add(new SalariedEmployee("John", "Smith", "111-11-1111", 800.00));
      payableObjects.add(new HourlyEmployee("Karen", "Price", "222-22-2222", 16.75, 40));
      payableObjects.add(new CommissionEmployee("Sue", "Jones", "333-33-3333", 10000, .06));
      payableObjects.add(new BasePlusCommissionEmployee("Bob", "Lewis", "444-44-4444", 5000, .04, 300));

      System.out.println("Invoices and Employees processed polymorphically:\n");

      for (Payable payable : payableObjects) {
         System.out.printf("%n%s %n%s: $%,.2f%n",
            payable.toString(),
            "payment due", payable.getPaymentAmount());
      }
   }
}

As you can see, we create an ArrayList of Payable objects and add some random objects to it. Then we use a for-each loop to iterate over the list and invoke the toString and getPaymentAmount methods on each object. Since the Employee class now implements the Payable interface and has a getPaymentAmount method that invokes the earnings method for the appropriate subclass, the correct payment amount is calculated and displayed for each object.