Type Substitution

Assume Student and GradStudent were not linked through inheritance (i.e., they were two independent data types). To allow both grad and non-grad students to take the same course, we must update the Roster class as follows, essentially duplicating every field and method to accommodate two types of students.

public class Roster {
  private Student[] students;
  private int numStudents;
  private GradStudent[] gradStudents;
  private int numGradStudents;

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

  public void add(Student s) { ... }
  public void remove(Student s) { ... }
  public Student find(String email) { ... }
  public void addGrad(GradStudent s) { ... }
  public void removeGrad(GradStudent s) { ... }
  public GradStudent findGrad(String email) { ... }
}

Suppose GradStudent is declared as a subtype of Student. In that case, the (original) Roster class (as defined in the previous chapter) needs no changes at all. You will be able to add/remove grad students by leveraging type substitution.

So, since we can substitute GradStudent for Student, we can

• have one student array of type Student and store both objects of Student and GradStudent in it;
• have one pair of add/remove methods that take a parameter of type Student and invoke it by passing either an object of Student or GradStudent;
• have one find method that returns an object of type Student, and use it to search both grad and non-grad students.

Exercise Can we redefine Roster and use GradStudent in place of Student (in declaration of fields/parameters data types) to achieve the same effect?