## §6.2 Other API Elements

#### (a) Interfaces for role encapsulation

A set of pre-defined types exist that do not extend `java.lang.Object` and have no features except the operators `==` and `!=`.

##### Note:

The JLS defines that each interface declares all methods defined in `java.lang.Object` (JLS §9.2) and also each object referenced by an interface type can be widened to `java.lang.Object`. Compilers commonly implement this by declaring `java.lang.Object` the super-type of all interfaces. Such implementation has no visible difference with respect to the more complex definition in the JLS.

These predefined types are

`org.objectteams.IConfined`
  regular interface
`org.objectteams.ITeam.IConfined`
  role interface
`org.objectteams.Team.Confined`
  role class

These types provide no new functionality but inheriting from these types influences the semantics with respect to encapsulation. The purpose and usage of these types is described in §7.

#### (b) Interface for explicit lowering

The following role interface exists for the purpose of allowing explicit lowering:

`org.objectteams.ITeam.ILowerable`
  role interface

This interface was introduced in detail in §2.2.(d).

#### (c) Team activation methods

Every team can be activated and deactivated by predefined methods of the interface `org.objectteams.ITeam`.

`activate()` and `activate(Thread th)`
  Methods for activation of a team
`deactivate()` and `deactivate(Thread th)`
  Methods for deactivation of a team

The usage of these Methods is described in §5.2.(b).

#### (d) Exceptions

The following `Exceptions` can be thrown during the execution of an ObjectTeam/Java program:

`ResultNotProvidedException`
  Thrown if a replace callin without a base call does not provide the necessary (primitive type) base result (see §4.3.(e)).
`LiftingFailedException`
  Thrown if an actual ambiguity occurs during lifting (see §2.3.4.(c)) or if lifting would need to instantiate an abstract role class (see §2.5.(b)). This is a checked exception. See §2.3.5 for more information.
`WrongRoleException`
  Thrown during lifting if the base object has, with respect to the same team instance, previously been lifted to a role type that is not conform to the currently requested type (see §2.3.4.(d) and §2.4.3).
`DuplicateRoleException`
  Thrown during explicit role creation, if a new role is created for a base object, which already has a role of the required type in the given team (see §2.4.1.(c)).
`RoleCastException`
  Thrown during cast of an externalized role, if the casted expression is anchored to a different team instance than the cast type (see §1.2.4.(b)).
`LiftingVetoException`
  This exception is used internally to abort the process of lifting when a relevant guard predicate (§5.4) evaluated to false. Such exceptions thrown from generated code will never appear in client code, so there is usually no need to catch a `LiftingVetoException`. However, in some situations it is useful to explicitly throw a `LiftingVetoException` from a lifting constructor (§2.3.1.(b)) of a role. This style allows to abort lifting even after the lifting constructor has started to work and also for method parameters requiring lifting. If lifting was triggered due to a callin method binding, this binding will simply not trigger if a `LiftingVetoException` is thrown while preparing the call to the role method.

#### (e) Role migration

The following interfaces can be used to enable role migration:

`IBaseMigratable`
  This interface declares a method

          <B> void migrateToBase(B otherBase)

  and instructs the compiler to generate an implementation of this method for any bound role declaring `IBaseMigratable` as its super-interface.
  The effect of calling `migrateToBase` on a role instance is to re-bind this role to a new base instance. The base instance must be compatible to the role's base class (in order to avoid problems during lifting the compiler may require the base to be of the exact type of the role's base class). Passing `null` to this method causes an `NullPointerException` to be thrown.
`ITeamMigratable`
  This interface declares a method

          <R> R<@otherTeam> migrateToTeam(final ITeam otherTeam)

  and instructs the compiler to generate an implementation of this method for any role declaring `ITeamMigratable` as its super-interface.
  The effect of calling `migrateToTeam` on a role instance is to re-bind this role to become a contained part of a new team instance. The team instance must be of the exact type of the role's enclosing team. Passing `null` to this method causes a `NullPointerException` to be thrown.

  ##### Caveat:

  This method intentionally breaks the rules of family polymorphism: any reference `R<@previousTeam> r` which was established before migration will incorrectly imply that the role's enclosing team still is `previousTeam`, which is no longer true after migration. While this does not effect any method lookup (which is still safe), further assumptions based on a role's dependent type are invalidated by team migration. The same holds for references from the migrating role to any sibling role instances.
  If the rules of family polymorphism should be maintained one should just refrain from declaring `ITeamMigratable` as a role's super-interface.

For both methods the signature declared in the interface is over-generalized, yet the compiler performs the necessary checks to ensure that role, base and team instances are indeed compatible and additionally the return type of `migrateToTeam` is checked as a self-type, i.e., it reflects the exact type of the call target.