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<h4 class="subsect">(d) <span class="title">Exceptions</span><a class="img" href="s6.2.d.html" title="PermaLink to (d) Exceptions"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"

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<p>The following <code>Exceptions</code> can be thrown during the execution of an ObjectTeam/Java program:

</p>

<dl>

<dt><code>ResultNotProvidedException</code></dt>

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<dd>Thrown if a replace callin without a base call does not provide the necessary (primitive type) base result

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(see <a href="s4.3.e.html" title="§4.3.(e) Fragile callin binding"

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class="sect">§4.3.(e)</a>).

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</dd>

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<dt><code>LiftingFailedException</code></dt>

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<dd>Thrown if an actual ambiguity occurs during lifting (see <a href="s2.3.4.c.html" title="§2.3.4.(c) Actual ambiguity"

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class="sect">§2.3.4.(c)</a>)

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or if lifting would need to instantiate an abstract role class (see <a href="s2.5.b.html" title="§2.5.(b) Relevant roles" class="sect">§2.5.(b)</a>).

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This is a checked exception.

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See <a href="s2.3.5.html"

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title="§2.3.5 Consequences of lifting problems"

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class="sect">§2.3.5</a> for more information.

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</dd>

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<dt><code>WrongRoleException</code></dt>

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<dd>Thrown during lifting if the base object has, with respect to the same team instance, previously been lifted

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to a role type that is not conform to the currently requested type

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(see <a href="s2.3.4.d.html" title="§2.3.4.(d) Mismatching role"

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class="sect">§2.3.4.(d)</a> and <a href="s2.4.3.html"

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title="§2.4.3 Role creation in the presence of smart lifting"

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class="sect">§2.4.3</a>).

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</dd>

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<dt><code>DuplicateRoleException</code></dt>

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<dd>Thrown during explicit role creation, if a new role is created for a base object, which already has a role

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of the required type in the given team (see <a href="s2.4.1.c.html"

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title="§2.4.1.(c) Duplicate role runtime check"

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class="sect">§2.4.1.(c)</a>).

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</dd>

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<dt><code>RoleCastException</code></dt>

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<dd>Thrown during cast of an externalized role, if the casted expression is anchored to a different team instance

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than the cast type (see <a href="s1.2.4.b.html" title="§1.2.4.(b) Casting" class="sect">§1.2.4.(b)</a>).

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</dd>

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<dt><code>LiftingVetoException</code></dt>

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<dd>This exception is used internally to abort the process of lifting when a relevant guard predicate (<a href="s5.4.html" title="§5.4 Guard predicates" class="sect">§5.4</a>) evaluated to false.

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Such exceptions thrown from generated code will never appear in client code, so there is usually no need to catch a <code>LiftingVetoException</code>.

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However, in some situations it is useful to explicitly <em>throw</em> a <code>LiftingVetoException</code> from a lifting constructor (<a href="s2.3.1.b.html"

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title="§2.3.1.(b) Default lifting constructor"

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class="sect">§2.3.1.(b)</a>) of a role.

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This style allows to abort lifting even after the lifting constructor has started to work and also for method parameters

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requiring lifting.

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If lifting was triggered due to a callin method binding, this binding will simply not trigger if a <code>LiftingVetoException</code> is thrown

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while preparing the call to the role method.

</dd>

</dl>

</div>

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<h4 class="subsect">(e) <span class="title">Role migration</span><a class="img" href="s6.2.e.html" title="PermaLink to (e) Role migration"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"

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Stephan Herrmann

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<p>The following interfaces can be used to enable role migration:</p>

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<dl>

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<dt><code>IBaseMigratable</code></dt>

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<dd>This interface declares a method

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<code><B> <b>void</b> migrateToBase(B otherBase)</code> </pre></div>

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and instructs the compiler to generate an implementation of this method

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for any bound role declaring <code>IBaseMigratable</code> as its super-interface.<br />

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The effect of calling <code>migrateToBase</code> on a role instance is to re-bind this role to a new base instance.

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The base instance must be compatible to the role's base class (in order to avoid problems during lifting the

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compiler may require the base to be of the exact type of the role's base class).

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Passing <code>null</code> to this method causes an <code>NullPointerException</code> to be thrown.

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</dd>

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<dt><code>ITeamMigratable</code></dt>

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<dd>This interface declares a method

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<code><R> R<@otherTeam> migrateToTeam(<b>final</b> ITeam otherTeam)</code></pre></div>

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and instructs the compiler to

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generate an implementation of this method for any role declaring <code>ITeamMigratable</code> as its super-interface.<br />

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The effect of calling <code>migrateToTeam</code> on a role instance is to re-bind this role to become a contained part of a new team instance.

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The team instance must be of the exact type of the role's enclosing team.

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Passing <code>null</code> to this method causes a <code>NullPointerException</code> to be thrown.<br /><div class="note">

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<h5>Caveat:</h5>

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This method intentionally breaks the rules of family polymorphism: any reference <code>R<@previousTeam> r</code>

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which was established before migration will incorrectly imply that the role's enclosing team still is <code>previousTeam</code>,

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which is no longer true after migration.

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While this does not effect any method lookup (which is still safe), further assumptions based on a role's dependent type

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are invalidated by team migration. The same holds for references from the migrating role to any sibling role instances.<br />

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If the rules of family polymorphism should be maintained one should just refrain from declaring <code>ITeamMigratable</code>

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as a role's super-interface.

</div>

</dd>

</dl>

<p>For both methods the signature declared in the interface is over-generalized, yet the compiler performs the necessary checks

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to

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ensure that role, base and team instances are indeed compatible and additionally the return type of <code>migrateToTeam</code>

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is checked as a self-type, i.e., it reflects the exact type of the call target.

</p>

</div>

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