IS-A and HAS-A: association, aggregation and composition

// Association: uses another object; no ownership
class Doctor {
    Prescription treat(Patient patient) { return new Prescription(patient.id(), "Rest"); }
}

// Aggregation: holds parts that exist independently (created elsewhere, can move on)
class Department {
    private final List<Employee> employees = new ArrayList<>();
    void hire(Employee e) { employees.add(e); }          // the employee existed before
    void letGo(Employee e) { employees.remove(e); }        // and keeps existing after
}

// Composition: creates and owns its parts; they die with it
class Order {
    private final List<OrderLine> lines = new ArrayList<>();
    void addItem(String sku, int qty) { lines.add(new OrderLine(sku, qty)); }   // created inside
    List<OrderLine> lines() { return List.copyOf(lines); }                      // never hand out the internals
}
record OrderLine(String sku, int qty) {}

@Entity
class Order {
    // Composition: lines are saved and deleted with the order
    @OneToMany(mappedBy = "order", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<OrderLine> lines = new ArrayList<>();
}

@Entity
class Department {
    // Aggregation: employees outlive the department, so no cascading delete
    @OneToMany(mappedBy = "department")
    private List<Employee> employees = new ArrayList<>();
}