Extending an Abstract Class

Here is the abstract Roster class for your reference:

public abstract class Roster {
  protected Student[] students;
  protected int numStudents;

  public Roster(int size) {
    students = new Student[size];
    numStudents = 0;
  }

  public abstract void add(Student s);

  public abstract void remove(Student s);

  public abstract Student find(String email);
}

Here is how MoocRoster extends Roster:

public class MoocRoster extends Roster {

  public MoocRoster(int size) {
    super(size);
  }

  @Override
  public void add(Student s) {
    // Implementation omitted to save space
  }

  @Override
  public void remove(Student s) {
    // Implementation omitted to save space
  }

  @Override
  public Student find(String email) {
    // Implementation omitted to save space
  }
}

Notice:

• There is no abstract keyword in the declaration of MoocRoster.

• There is no abstract keyword in the method declarations within the MoocRoster class; instead, there is @Override annotation.

• MoocRoster has access to students and numStudents in Roster since those fields were defined as protected.

• MoocRoster's constructor invokes the constructor of Roster (using super(size)), which constructs the students array and initializes the numStudents attribute.