Implementing Interfaces

Interfaces can be used in type hierarchy just like abstract classes. However, the syntax is slightly different: a class implements an interface whereas it extends another (regular or abstract) class.

For example, here MoocRoster implements the Roster interface:

public class MoocRoster implements Roster {
  private Student[] students;
  private int numStudents;

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

  @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:

• The use of keyword implements (instead of extends).

• MoocRoster declares its fields (since the Roster interface does not include fields) and initializes them in its constructor (no super keyword).