ObjectTeams/Java Language Definition

ObjectTeams/Java Language Definition - §7.

§6. Object Teams API

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Contents

§8. Join point queries

§7. Role encapsulation

Concepts of encapsulation


Protected roles	A role with visibility `protected` cannot be externalized, which means its type cannot be used outside the declaring team (§1.2.3)

Confined roles	Confined roles are encapsulated even stricter than protected roles: the compiler will ensure that by no means any object outside the enclosing team will ever have a reference to a confined role.

Opaque roles	Opaque roles build on the guarantees of confined roles but allow to be shared in a limited way such that no information is exposed.

§7.1. Opaque roles

The purpose of the two IConfined interfaces (see §6.2(a)) is to define opaque roles: Any role implementing IConfined can be externalized using this type, such that external clients cannot access any features of the role. The type IConfined exposes no features and references of this type cannot be widened to any type not even to java.lang.Object.
If the actual role type is furthermore invisible outside the team (by not declaring it public), it is perfectly safe to externalize such roles using type IConfined (which is a public interface) and pass them back to the owning team. The encapsulation of the team is in no way breached by externalizing opaque roles, which can only be used as a handle into internal state of the team.

The difference between the two mentioned interfaces is that Team.IConfined requires to use this type or any subtype as externalized role. Such a reference contains the information of the enclosing team. Even stricter control can be imposed using the regular interface IConfined. Here not even team membership is visible to clients using a reference of this type.

§7.2. Confined roles

Subclassing Team.Confined with a protected class yields a role class to which no object outside the team will ever have a reference. The point here is that a role class with a regular super class will widen the this reference to the super class when executing a method from this class. Within such a method a danger of leaking the reference exists, because the inherited method could pass the this reference to another object. Such leaking of the this reference would break encapsulation of a role object that should only be accessible within the enclosing team.
Subclasses of Team.Confined do not inherit any methods that have the danger of leaking this.

(a) Inhibition of overriding

The types Team.IConfined and Team.Confined cannot be overridden (cf. §1.3.1(c)).

Upcoming: Only by widening to a non-role super-type, a role instance can be accessed from outside the team. In the future this can be inhibited by restricted inheritance.

 public team class Company {
   private HashMap<String,Employee> employees;
   ...
   protected class Employee implements IConfined {
     void pay(int amount) { ... }
     ...
   }
   public IConfined getEmployee(String ID) {
     return employees.get(ID); // implicit widening to IConfined
   }
   public void payBonus(IConfined emp, int amount) {
     ((Employee)emp).pay(amount); // explicit narrowing
   }
 }

 public class Main {
   public static void main(String[] args) {
     final Company comp = new Company();
     IConfined<comp> emp = comp.getEmployee("emp1");
     //System.out.println(emp); <-- forbidden!
     comp.payBonus(emp, 100);
   }
 }

Effects:

The protected role Employee implements the above described interface IConfined and therefore becomes opaque (line 4).
Methods sharing such an opaque role with the outside of the enclosing team have to use the type IConfined (line 8, line 11).
It is possible to obtain an instance of such a role by using the type IConfined (line 18).
Trying to access any feature of this instance, for example toString(), will cause a compilation error (line 19).
Passing the opaque role reference back into the team works well (line 20).
Inside the team some conversions between the types IConfined and the intrinsic role type Employee may be necessary (line 9 and 12).

§6. Object Teams API

Table of

Contents

§8. Join point queries

OT/J Version 1.0 — Last modified: Fri Mar 30 2007