Composite Data Types

A composite data type is a type that is made up of other types. For example, here is a Roster class that holds an array of Student objects:

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

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

  public void add(Student s) {
    // stub
  }

  public void remove(Student s) {
    // stub
  }

  // Assumption: students' emails are unique.
  public Student find(String email) {
    return null; // stub
  }
}

Roster is a class, so it too is an abstraction, and a data type. However, it is different from the Student class; the latter is to model a student in our problem domain, the former represents a collection of students. Sometimes this distinction is interesting so to highlight it, we call a class like Roster a container class, or an aggregate class.