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| <td class="back"><a id="top"></a><a href="s2.html" rel="prev"><< §2 Role Binding</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s4.html" rel="next">§4 Callin Binding >></a></td> |
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| </table> |
| <div class="chapter" id="s3"> |
| <div class="headl"> |
| <div class="headr"> |
| <h1>§3 Callout Binding</h1> |
| </div> |
| </div> |
| <div id="toc-box"> |
| <ul class="toc-box"> |
| <li><a href="s3.html">§3 Callout Binding</a></li> |
| <li><a href="#s3.1">§3.1 Callout method binding</a></li> |
| <li><a href="#s3.2">§3.2 Callout parameter mapping</a></li> |
| <li><a href="#s3.3">§3.3 Lifting and lowering</a></li> |
| <li><a href="#s3.4">§3.4 Overriding access restrictions</a></li> |
| <li><a href="#s3.5">§3.5 Callout to field</a></li> |
| </ul> |
| </div> |
| <div class="intro"> |
| <h3>Notion of callout binding</h3> |
| <div class="line"></div> |
| <div class="term">callout binding</div> |
| <div class="termdesc">A callout binding declares that a method call to a role |
| object may be <strong>forwarded</strong> to a base method of the associated |
| base object <em>(the role object "calls out" to the base)</em>. |
| </div> |
| <div class="line"></div> |
| <div class="term">declarative completeness</div> |
| <div class="termdesc"> Even if a role class does not implement all needed methods, |
| but forwards some to its base, also these methods must be declared |
| within the role. |
| Secondly, no forwarding occurs, unless explicitly declared by a callout binding. |
| </div> |
| <div class="line"></div> |
| <div class="term">expected/provided</div> |
| <div class="termdesc"> A callout binding binds an <strong>expected</strong> method of the role |
| class (needed but not implemented here) to a <strong>provided</strong> |
| method of the base class. |
| </div> |
| <div class="line"></div> |
| </div> |
| <div class="sect depth2" id="s3.1"> |
| <h2 class="sect">§3.1 Callout method binding<a class="img" href="s3.html#s3.1" |
| title="PermaLink to §3.1 Callout method binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §3</a></span></h2> |
| <div class="syntaxlink"><a href="sA.html#sA.3.2" title="§A.3.2 CalloutBinding" |
| class="syntax">→ Syntax §A.3.2</a></div> |
| <p>A role class may acquire the implementation for any of its |
| (expected) methods by declaring a <strong>callout</strong> binding. |
| |
| </p> |
| <div class="subsect depth3" id="s3.1.a"> |
| <h4 class="subsect">(a) <span class="title">Prerequisite: Class binding</span><a class="img" href="s3.html#s3.1.a" |
| title="PermaLink to (a) Prerequisite: Class binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A callout binding requires the enclosing class to be a role class |
| bound to a base class according to <a href="s2.html#s2.1" title="§2.1 playedBy relation" class="sect">§2.1</a>. However, callout bindings are not |
| allowed if the role is involved in base class circularity (see <a href="s2.html#s2.1.2.b" title="§2.1.2.(b) Cycles" class="sect">§2.1.2.(b)</a>). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.b"> |
| <h4 class="subsect">(b) <span class="title">Definition</span><a class="img" href="s3.html#s3.1.b" title="PermaLink to (b) Definition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A callout binding maps an abstract role method ("expected method") |
| to a concrete base method ("provided method"). |
| It may appear within the role class at any place where feature |
| declarations are allowed. It is denoted by |
| |
| </p> |
| <div class="listing plain"><pre><i>expected_method_designator</i> <b>-></b> <i>provided_method_designator;</i></pre></div> |
| <p>The effect is that any call to the role method will be forwarded to the |
| associated base object using the provided base method. |
| |
| </p> |
| <h5 class="listing">Example code (Callout):</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>team</b> <b>class</b> Company {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>public</b> <b>class</b> Employee <b>playedBy</b> Person {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>abstract</b> String getIdentification();</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> <span class="comment">// callout binding see below...</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| </div> |
| <div class="subsect depth3" id="s3.1.c"> |
| <h4 class="subsect">(c) <span class="title">Kinds of method designators</span><a class="img" href="s3.html#s3.1.c" |
| title="PermaLink to (c) Kinds of method designators"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A method designator may either be a method name |
| |
| </p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">4</td> |
| <td><pre>getIdentification <em>-></em> getName;</pre></td> |
| </tr> |
| </table> |
| </div> |
| <p><strong>or</strong> |
| a complete method signature including parameter declarations and |
| return type declaration, but excluding any modifiers and declared exceptions. |
| |
| </p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">4</td> |
| <td><pre>String getIdentification() <em>-></em> String getName();</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Effects:</h5> |
| <ul> |
| <li> Line 4 declares a callout binding for the role method <code>getIdentification()</code>, |
| providing an implementation for the abstract method defined in line 3. |
| </li> |
| <li> In combination with the role binding in line 2 this has the following effect:</li> |
| <li> Any call to <code>Employee.getIdentification</code> |
| is forwarded to the method <code>Person.getName</code>. |
| </li> |
| </ul> |
| </div> |
| <p>Both sides of a callout binding must use the same kind of |
| designators, i.e., designators with and without signature may not be mixed. |
| <br /> |
| Each method designator must uniquely select one method. |
| If a method designator contains a signature this signature must match exactly with the signature |
| of an existing method, i.e., no implicit conversions are applied for this matching. |
| If overloading is involved, signatures <em>must</em> be used to disambiguate. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.d"> |
| <h4 class="subsect">(d) <span class="title">Inheritance of role method declarations</span><a class="img" href="s3.html#s3.1.d" |
| title="PermaLink to (d) Inheritance of role method declarations"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The role method being bound by a callout may be declared in the same |
| class as the binding or it may be inherited from a super class or |
| super interface. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.e"> |
| <h4 class="subsect">(e) <span class="title">Callout override</span><a class="img" href="s3.html#s3.1.e" |
| title="PermaLink to (e) Callout override"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If an inherited role method is concrete, callout binding regarding this |
| method must use the token "<code>=></code>" instead of "<code>-></code>" |
| in order to declare that this binding overrides an existing implementation. |
| <br /> |
| |
| Using the "<code>=></code>" operator for an abstract method is an error. |
| <br /> |
| It is also an error (and not useful anyway) to callout-bind a method that is |
| implemented in the same class as the binding. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.f"> |
| <h4 class="subsect">(f) <span class="title">Inheritance of callout bindings</span><a class="img" href="s3.html#s3.1.f" |
| title="PermaLink to (f) Inheritance of callout bindings"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> Callout bindings are inherited along explicit and implicit inheritance. |
| Inherited callout bindings can be overridden using "<code>=></code>". |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.g"> |
| <h4 class="subsect">(g) <span class="title">Duplicate bindings</span><a class="img" href="s3.html#s3.1.g" |
| title="PermaLink to (g) Duplicate bindings"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>It is an error if a role class has multiple callout bindings for the |
| same role method. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.h"> |
| <h4 class="subsect">(h) <span class="title">Declared exceptions</span><a class="img" href="s3.html#s3.1.h" |
| title="PermaLink to (h) Declared exceptions"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>It is an error if a base method to be bound by <strong>callout</strong> |
| declares in its <code>throws</code> clause any exceptions that |
| are not declared by the corresponding role method. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.i"> |
| <h4 class="subsect">(i) <span class="title">Shorthand definition</span><a class="img" href="s3.html#s3.1.i" |
| title="PermaLink to (i) Shorthand definition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> A callout binding whose method designators specify |
| full method signatures does not require an existing role method. |
| If no role method is found matching the expected method of |
| such a callout binding, a new method is implicitly generated. |
| The new method is static iff the bound base method is static, |
| and it declares the same exceptions as the bound base method. |
| |
| </p> |
| <p> |
| A shorthand callout may optionally declare a <strong>visibility modifier</strong>, |
| otherwise the generated method inherits the visibility modifier of the bound base method. |
| No further modifiers are set. |
| If a callout overrides an inherited method or callout, |
| it must not reduce the visibility of the inherited method/callout. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.j"> |
| <h4 class="subsect">(j) <span class="title">Inferred callout</span><a class="img" href="s3.html#s3.1.j" |
| title="PermaLink to (j) Inferred callout"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> If a non-abstract role class inherits an abstract method the compiler |
| tries to infer a callout binding for implementing the abstract method. |
| Similarly, if a self-call in a role class cannot be resolved, the compiler |
| tries to infer a callout to resolve the self-call.<br /> |
| Inference searches for a method in the bound base class such that |
| |
| </p> |
| <ol> |
| <li>both methods have the same name</li> |
| <li>both methods have the same number of arguments</li> |
| <li>each argument of the abstract role method is compatible to the |
| corresponding argument of the base method directly, or using |
| boxing/unboxing or lowering. |
| </li> |
| </ol> |
| <p> |
| Callouts inferred from an interface have <code>public</code> visibility, |
| callouts inferred from a self-call have <code>private</code> visibility. |
| |
| </p> |
| <p> |
| Per default inferred callout bindings are disabled, i.e., a compiler |
| must report these as an error. However, a compiler should allow to |
| configure reporting to produce a warning only (which can be suppressed |
| using a <code>@SuppressWarnings("inferredcallout")</code> annotation), |
| or to completely ignore the diagnostic. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.1.k"> |
| <h4 class="subsect">(k) <span class="title">Callout to generic method</span><a class="img" href="s3.html#s3.1.k" |
| title="PermaLink to (k) Callout to generic method"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>When referring to a generic base method</p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><T> T bm(T a)</pre></td> |
| </tr> |
| </table> |
| </div> |
| <p>a callout binding may either propagate the method's genericity as in</p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">2</td> |
| <td><pre><T> T rm(T a) <b>-></b> T bm(T a);</pre></td> |
| </tr> |
| </table> |
| </div> |
| <p>or it may supply a valid substitution for the type parameter as in</p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">2</td> |
| <td><pre>String rm(String a) <b>-></b> String bm(String a);</pre></td> |
| </tr> |
| </table> |
| </div> |
| </div> |
| <p>A callout binding either attaches an implementation to a previously declared method |
| or adds (<a href="#s3.1.i" title="§3.1.(i) Shorthand definition" class="sect">§3.1.(i)</a> above) a forwarding method to a role class. |
| Apart from this implementation, callout-bound methods do not differ from regular methods. |
| |
| </p> |
| <p>When we say, a callout binding defines <strong>forwarding</strong> this means that |
| control is passed to the base object. In contrast, by a <strong>delegation</strong> |
| semantics control <em>would</em> remain at the role object, such that self-calls |
| would again be dispatched starting at the role. Callout bindings on |
| their own do not support delegation. However, in conjunction with method |
| overriding by means of callin bindings (see <a href="s4.html" title="§4 Callin Binding" class="sect">§4</a>) |
| the effect of delegation can easily be achieved. |
| |
| </p> |
| </div> |
| <div class="sect depth2" id="s3.2"> |
| <h2 class="sect">§3.2 Callout parameter mapping<a class="img" href="s3.html#s3.2" |
| title="PermaLink to §3.2 Callout parameter mapping"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §3</a></span></h2> |
| <div class="syntaxlink"><a href="sA.html#sA.4.1" title="§A.4.1 CalloutParameterMappings" |
| class="syntax">→ Syntax §A.4.1</a></div> |
| <div class="subsect depth3" id="s3.2.a"> |
| <h4 class="subsect">(a) <span class="title">with clause</span><a class="img" href="s3.html#s3.2.a" title="PermaLink to (a) with clause"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> If the method designators in a callout binding are signatures |
| (not just method names), parameters and return value may |
| be mapped by a <code>with{...}</code> sub-clause. Parameter mappings may only occur if the enclosing role is a class, not an interface. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.2.b"> |
| <h4 class="subsect">(b) <span class="title">Mapping one parameter</span><a class="img" href="s3.html#s3.2.b" |
| title="PermaLink to (b) Mapping one parameter"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> For each parameter of the provided base method, exactly one parameter |
| mapping defines, which value will actually be passed to |
| the base method. Callout parameter mappings have this form: |
| |
| </p> |
| <div class="listing plain"><pre><i>expression</i> <b>-></b> <i>base_method_parameter_name</i></pre></div> |
| </div> |
| <div class="subsect depth3" id="s3.2.c"> |
| <h4 class="subsect">(c) <span class="title">Result mapping</span><a class="img" href="s3.html#s3.2.c" |
| title="PermaLink to (c) Result mapping"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The return value of a callout method may be provided by a result mapping: |
| |
| </p> |
| <div class="listing plain"><pre>result <b><-</b> <i>expression</i></pre></div> |
| <p>The right hand side expression of a result mapping may use the special identifier |
| <code>result</code> to refer to the value returned by the base method. |
| <br /> |
| In a method binding with parameter mappings, it is an error to use |
| <code>result</code> as the name of a regular method argument. |
| |
| </p> |
| <h5 class="listing">Example code (Callout Parameter Mapping):</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre>Integer absoluteValue(Integer integer) <b>-></b> <b>int</b> abs(<b>int</b> i) <em><b>with</b> {</em></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> integer.intValue() <b>-></b> i,</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <em>result</em> <b><-</b> <b>new</b> Integer(<em>result</em>)</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre><em>}</em></pre></td> |
| </tr> |
| </table> |
| </div> |
| </div> |
| <div class="subsect depth3" id="s3.2.d"> |
| <h4 class="subsect">(d) <span class="title">Visible names</span><a class="img" href="s3.html#s3.2.d" title="PermaLink to (d) Visible names"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Each identifier that appears within the expressions of a parameter |
| mapping must be either: |
| |
| </p> |
| <ul> |
| <li>a feature visible in the scope of the role instance.</li> |
| <li>a parameter of the role method (for parameter mappings).</li> |
| <li>the special name <code>result</code> (for result mappings). |
| </li> |
| <li>in a result mapping also the special name <code>base</code> can be used |
| in order to refer to the bound base instance (provided the method being |
| bound is not static). |
| </li> |
| </ul> |
| <p>The names of base method arguments (i.e., names after mapping) are only |
| legal in the position given in <a href="#s3.2.b" title="§3.2.(b) Mapping one parameter" |
| class="sect">§3.2.(b)</a>. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.2.e"> |
| <h4 class="subsect">(e) <span class="title">Implicit parameter mappings</span><a class="img" href="s3.html#s3.2.e" |
| title="PermaLink to (e) Implicit parameter mappings"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If parameter mappings should be omitted the following conditions must hold: |
| |
| </p> |
| <ol> |
| <li>each method parameter of the role method must conform to the |
| corresponding parameter of the base method, <em>and</em></li> |
| <li>the result type of the base method must conform to the result type |
| of the role method. |
| </li> |
| </ol> |
| <p>Here conformance includes translation polymorphism (cf. <a href="#s3.3.d" title="§3.3.(d) Typing rules" class="sect">§3.3.(d)</a>). |
| <br /> |
| Parameter correspondence without parameter mapping is determined by declaration order not by names. |
| <br /> |
| Two adjustments can, however, be performed implicitly: |
| |
| </p> |
| <ul> |
| <li>If the role method has more parameters than the base method, |
| unused trailing parameters may be silently ignored. |
| </li> |
| <li>If the role method returns <code>void</code>, any result from |
| the base method may be silently ignored. |
| </li> |
| </ul> |
| </div> |
| <h5 class="listing">Example code (Callout with Parameter Mapping):</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>public</b> <b>team</b> <b>class</b> MyTeamA {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>public</b> <b>abstract</b> <b>class</b> Role1 {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>abstract</b> <b>void</b> payEuro(float euro);</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> <b>abstract</b> float earnEuro();</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> <b>void</b> idle(<b>int</b> seconds) { <span class="comment">/* do nothing */</span> };</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> Role1 boss, worker = <span class="comment">// initialization omitted</span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> <b>public</b> <b>void</b> transaction () {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre> boss.payEuro(worker.earnEuro());</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">10</td> |
| <td><pre> boss.idle(123);</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">11</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">12</td> |
| <td><pre>}</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">13</td> |
| <td><pre><b>public</b> <b>class</b> Staff { <span class="comment">// a base class </span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">14</td> |
| <td><pre> <b>public</b> <b>void</b> payDM (float dm) { … };</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">15</td> |
| <td><pre> <b>public</b> float earnDM () { … };</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">16</td> |
| <td><pre> <b>public</b> <b>int</b> doze() { … };</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">17</td> |
| <td><pre> <span class="comment">// other methods omitted</span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">18</td> |
| <td><pre>}</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">19</td> |
| <td><pre><b>public</b> <b>team</b> <b>class</b> MySubTeam <b>extends</b> MyTeamA {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">20</td> |
| <td><pre> <b>public</b> <b>class</b> Role1 <b>playedBy</b> Staff {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">21</td> |
| <td><pre> <b>void</b> payEuro(float euro) <b>-></b> <b>void</b> payDM(float dm) <b>with</b> {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">22</td> |
| <td><pre> euro * 1.95583f <b>-></b> dm</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">23</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">24</td> |
| <td><pre> float earnEuro() <b>-></b> float earnDM () <b>with</b> {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">25</td> |
| <td><pre> result <b><-</b> result / 1.95583f</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">26</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">27</td> |
| <td><pre> idle <b>=></b> doze; <span class="comment">// override existing implementation of idle()</span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">28</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">29</td> |
| <td><pre> <b>void</b> doit() {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">30</td> |
| <td><pre> transaction();</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">31</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">32</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Effects:</h5> |
| <ul> |
| <li>Class <code>MyTeamA</code> is declaratively complete and can be |
| type checked because it only uses methods that are visible or declared within this |
| context. <code>MyTeamA.Role1</code> can, however, not be instantiated, because it is |
| abstract. |
| </li> |
| <li>Line 30 has the normal effect of invoking <code>transaction</code>. |
| </li> |
| <li>When executing <code>transaction</code>, the call of <code>worker.earnEuro()</code> |
| is forwarded to the corresponding base object using method <code>earnDM()</code> |
| (binding declaration in line 24). |
| The result is converted by "<code>result / 1.95583f</code>" (line 25). |
| </li> |
| <li>Within the same execution of <code>transaction</code>, the call of |
| <code>boss.payEuro()</code> |
| is forwarded to the corresponding base object using method <code>payDM()</code> |
| (binding declaration in line 21). |
| The parameter <code>euro</code> is converted by "<code>euro * 1.95583f</code>" |
| (line 22). |
| </li> |
| <li>Method <code>idle</code> is forwarded to <code>doze</code> without any |
| parameter mapping. This requires <code>doze</code> to have a signature |
| that is conformable to the signature of <code>idle</code>. |
| In this case a role parameter and a base result are ignored.<br /> |
| Using the <code>=></code> operator, this binding overrides the existing |
| implementation of <code>idle</code>. |
| </li> |
| </ul> |
| </div> |
| </div> |
| <div class="sect depth2" id="s3.3"> |
| <h2 class="sect">§3.3 Lifting and lowering<a class="img" href="s3.html#s3.3" |
| title="PermaLink to §3.3 Lifting and lowering"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §3</a></span></h2> |
| <p> (For basic definitions see <a href="s2.html#s2.2" title="§2.2 Lowering" class="sect">§2.2</a> and <a href="s2.html#s2.3" title="§2.3 Lifting" class="sect">§2.3</a>) |
| |
| </p> |
| <div class="subsect depth3" id="s3.3.a"> |
| <h4 class="subsect">(a) <span class="title">Call target translation</span><a class="img" href="s3.html#s3.3.a" |
| title="PermaLink to (a) Call target translation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Invoking a base method due to a callout binding first |
| <strong>lowers</strong> the role object in order to obtain the effective call target. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.3.b"> |
| <h4 class="subsect">(b) <span class="title">Parameter translation</span><a class="img" href="s3.html#s3.3.b" |
| title="PermaLink to (b) Parameter translation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Passing a role object as parameter to a callout method implicitly <strong>lowers</strong> |
| this parameter, if the base method declares a corresponding base type parameter. |
| <br /> |
| Lifting of callout parameters is not possible. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.3.c"> |
| <h4 class="subsect">(c) <span class="title">Result translation</span><a class="img" href="s3.html#s3.3.c" |
| title="PermaLink to (c) Result translation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> When returning a base object from a callout method where the role |
| method declares the result to be of a role class, |
| this object is implicitly <strong>lifted</strong> to the appropriate role. |
| <br /> |
| Lowering the result of a callout binding is not possible. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.3.d"> |
| <h4 class="subsect">(d) <span class="title">Typing rules</span><a class="img" href="s3.html#s3.3.d" title="PermaLink to (d) Typing rules"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A parameter mapping (implicit by parameter position or explicit |
| by a <code>with</code> clause) is <strong>well typed</strong> if |
| the left hand side conforms to the right hand side, either by |
| |
| </p> |
| <ul> |
| <li>type equality</li> |
| <li>implicit primitive type conversion</li> |
| <li>subtype polymorphism</li> |
| <li>translation polymorphism, here: <em>lowering</em>, |
| </li> |
| <li><em>or</em> by a combination of the above. |
| </li> |
| </ul> |
| <p>A result mapping (implicit or explicit by a <code>with</code> clause) |
| is well typed, if the value at the right hand side conforms to the |
| left hand side according to the rules given above, except that |
| translation polymorphism here applies <em>lifting</em> instead of |
| lowering. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.3.e"> |
| <h4 class="subsect">(e) <span class="title">Role arrays</span><a class="img" href="s3.html#s3.3.e" title="PermaLink to (e) Role arrays"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>For arrays of roles as parameters <a href="s2.html#s2.2.e" title="§2.2.(e) Lowering of arrays" |
| class="sect">§2.2.(e)</a> applies accordingly. |
| For arrays as a return value <a href="s2.html#s2.3.d" title="§2.3.(d) Lifting of arrays" |
| class="sect">§2.3.(d)</a> applies. |
| |
| </p> |
| </div> |
| </div> |
| <div class="sect depth2" id="s3.4"> |
| <h2 class="sect">§3.4 Overriding access restrictions<a class="img" href="s3.html#s3.4" |
| title="PermaLink to §3.4 Overriding access restrictions"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §3</a></span></h2> |
| <p>In contrast to normal access restrictions, method bindings |
| may refer to hidden base methods. |
| This concept is the inverse of encapsulation, hence it is called |
| <strong>decapsulation</strong>. |
| <br /> |
| Decapsulation may occur in these positions: |
| |
| </p> |
| <ul> |
| <li><code>playedBy</code> declaration (see <a href="s2.html#s2.1.2.c" |
| title="§2.1.2.(c) Base class decapsulation" |
| class="sect">§2.1.2.(c)</a>) |
| </li> |
| <li>base constructor call (see <a href="s2.html#s2.4.2.b" title="§2.4.2.(b) Bound roles" |
| class="sect">§2.4.2.(b)</a>). |
| </li> |
| <li>callout bindings (see next)</li> |
| <li>callout to field (see <a href="#s3.5.e" title="§3.5.(e) Access control" class="sect">§3.5.(e)</a>) |
| </li> |
| <li>base call within a callin method (see <a href="s4.html#s4.6" title="§4.6 Overriding access restrictions" |
| class="sect">§4.6</a>) |
| </li> |
| </ul> |
| <div class="subsect depth3" id="s3.4.a"> |
| <h4 class="subsect">(a) <span class="title">Callout to inaccessible base method</span><a class="img" href="s3.html#s3.4.a" |
| title="PermaLink to (a) Callout to inaccessible base method"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>By means of <strong>callout</strong> bindings it is possible to access |
| methods of a base class regardless of their access modifiers. |
| Method bindings are the only place in a program which may mention otherwise |
| inaccessible methods. |
| Access to the callout method at the role side is controlled by regular mechanisms, |
| based on the declaration of the role method. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.4.b"> |
| <h4 class="subsect">(b) <span class="title">Sealing against decapsulation</span><a class="img" href="s3.html#s3.4.b" |
| title="PermaLink to (b) Sealing against decapsulation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A base package may be "sealed" which re-establishes the standard Java visibility rules. |
| <br /> |
| Sealing is achieved by the corresponding capability of Jar files. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.4.c"> |
| <h4 class="subsect">(c) <span class="title">Warning levels</span><a class="img" href="s3.html#s3.4.c" |
| title="PermaLink to (c) Warning levels"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A compiler should signal any occurrence of decapsulation. |
| If a compiler supports to configure warnings this may be used to let the user choose to |
| (a) ignore base class decapsulation, (b) treat it as a warning or even |
| (c) treat it as an error (cf. <a href="s2.html#s2.1.2.c" |
| title="§2.1.2.(c) Base class decapsulation" |
| class="sect">§2.1.2.(c)</a>). |
| <br /> |
| Optionally, a batch compiler may support three levels of verbosity with respect to decapsulation: |
| |
| </p> |
| <table border="1"> |
| <tr> |
| <td rowspan="1" colspan="1"><tt>-nodecapsulation</tt></td> |
| <td rowspan="1" colspan="1">No warnings.</td> |
| </tr> |
| <tr> |
| <td rowspan="1" colspan="1"><em>default</em></td> |
| <td rowspan="1" colspan="1">Warn only if/that access restrictions are overridden.</td> |
| </tr> |
| <tr> |
| <td rowspan="1" colspan="1"><tt>-decapsulation</tt></td> |
| <td rowspan="1" colspan="1">Detailed messages containing the binding and the hidden base method.</td> |
| </tr> |
| </table> |
| </div> |
| <div class="subsect depth3" id="s3.4.d"> |
| <h4 class="subsect">(d) <span class="title">Private methods from super classes</span><a class="img" href="s3.html#s3.4.d" |
| title="PermaLink to (d) Private methods from super classes"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If a callout binding shall bind to a private base method, that method |
| must be defined in the exact base class to which the current role |
| class is bound using <code>playedBy</code>. I.e., for private methods |
| <a href="#s3.1.d" |
| title="§3.1.(d) Inheritance of role method declarations" |
| class="sect">§3.1.(d)</a> does not hold. |
| <br /> |
| The same holds for private base fields (see below). |
| <br /> |
| If a private base feature must indeed be callout-bound, a role class |
| must be defined that is played by the exact base class defining the |
| private feature. Another role bound to a sub-base-class can then |
| be defined as a sub class of the first role. It will inherit the |
| callout binding and through this it can access the desired feature. |
| |
| </p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>public</b> <b>class</b> SuperBase {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <em><b>private</b> <b>int</b> secret;</em></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre>}</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre><b>public</b> <b>class</b> SubBase <b>extends</b> SuperBase { <span class="comment">/* details omitted */</span> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre><b>public</b> <b>team</b> <b>class</b> MyTeam {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> <b>protected</b> <b>class</b> SuperRole <b>playedBy</b> SuperBase {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> <b>int</b> steal() <b>-></b> <b>get</b> <b>int</b> <em>secret</em>; <span class="comment">// <span class="green"><strong>OK</strong></span></span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre> <b>protected</b> <b>class</b> SubRole <b>extends</b> SuperRole <b>playedBy</b> SubBase {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">10</td> |
| <td><pre> <b>int</b> steal() <b>-></b> <b>get</b> <b>int</b> <em>secret</em>; <span class="comment">// <span class="error"><strong>illegal!</strong></span></span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">11</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">12</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| </div> |
| </div> |
| <div class="sect depth2" id="s3.5"> |
| <h2 class="sect">§3.5 Callout to field<a class="img" href="s3.html#s3.5" |
| title="PermaLink to §3.5 Callout to field"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a><span class="toplink"><a href="#top">↑ §3</a></span></h2> |
| <p>Also fields of a base class can be made accessible using a callout binding. |
| |
| </p> |
| <div class="subsect depth3" id="s3.5.a"> |
| <h4 class="subsect">(a) <span class="title">Syntax</span><a class="img" href="s3.html#s3.5.a" title="PermaLink to (a) Syntax"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Using one of the callout modifiers <code>get</code> or <code>set</code> a role method |
| can be bound to a field of the role's base class: |
| |
| </p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre>getValue <b>-></b> <em>get</em> value;</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre>setValue <b>-></b> <em>set</em> value;</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre><b>int</b> getValue() <b>-></b> <em>get</em> <b>int</b> value;</pre></td> |
| </tr> |
| </table> |
| </div> |
| <p>where <code>getValue</code>, <code>setValue</code> are abstract role methods of |
| appropriate signatures and <code>value</code> is a field of the bound base class. |
| <br /> |
| A longer syntax is available, too (see line 3 above), which uses complete signatures. |
| For the left hand side <a href="#s3.1.c" title="§3.1.(c) Kinds of method designators" |
| class="sect">§3.1.(c)</a> applies, for the |
| right hand side, this longer version prepends the field type to the field name. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.5.b"> |
| <h4 class="subsect">(b) <span class="title">Compatibility</span><a class="img" href="s3.html#s3.5.b" title="PermaLink to (b) Compatibility"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>A role method bound with the modifier <code>get</code> |
| should have no arguments |
| (it <em>may</em> have arbitrary arguments, which are silently ignored) |
| and should have a return type to which the base field is compatible. |
| A role method returning void will ignore the given value and thus has |
| no effect at all, which will be signaled by a compiler warning. |
| <br /> |
| A role method bound with the modifier <code>set</code> must have a first argument |
| that is compatible to the base field's type (additional arguments - if present - |
| are silently ignored) and must not declare a return type. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.5.c"> |
| <h4 class="subsect">(c) <span class="title">Value mapping</span><a class="img" href="s3.html#s3.5.c" title="PermaLink to (c) Value mapping"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Values can be mapped similar to parameter mappings in pure method bindings |
| (<a href="#s3.2" title="§3.2 Callout parameter mapping" class="sect">§3.2</a>). Such mappings can be used to establish compatibility as required above. |
| <br /> |
| In both <code>get</code> and <code>set</code> bindings, the base side value is denoted |
| by the field's name (lines 2 and 4 below). |
| |
| </p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre>Integer getValue() -> <em>get</em> <b>int</b> val</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <em>with</em> { result <- <b>new</b> Integer(val) }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre><b>void</b> setValue(Integer i) -> <em>set</em> <b>int</b> val</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> <em>with</em> { i.intValue() -> val }</pre></td> |
| </tr> |
| </table> |
| </div> |
| </div> |
| <div class="subsect depth3" id="s3.5.d"> |
| <h4 class="subsect">(d) <span class="title">Effect</span><a class="img" href="s3.html#s3.5.d" title="PermaLink to (d) Effect"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Callout-binding a role method to a base field generates an implementation for |
| this role method, by which it acts as a getter or setter for the given |
| field of the associated base object. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.5.e"> |
| <h4 class="subsect">(e) <span class="title">Access control</span><a class="img" href="s3.html#s3.5.e" |
| title="PermaLink to (e) Access control"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>For accessing an otherwise invisible field, the rules for |
| <a href="#s3.4" title="§3.4 Overriding access restrictions" |
| class="sect">decapsulation (§3.4)</a> apply accordingly. |
| <br /> |
| Recall, that according to |
| <a href="http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#40898" |
| class="ext">JLS §8.3</a> |
| fields may be hidden in sub-classes of a given base class. Therefore, it is relevant to know that a callout to |
| a field will always access the field that is visible in the exact base |
| class of the role class defining the callout. This is especially relevant |
| for accessing private fields. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.5.f"> |
| <h4 class="subsect">(f) <span class="title">Shorthand definition</span><a class="img" href="s3.html#s3.5.f" |
| title="PermaLink to (f) Shorthand definition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Just as in <a href="#s3.1.i" title="§3.1.(i) Shorthand definition" class="sect">§3.1.(i)</a> a shorthand definition allows to introduce |
| a callout field access method without prior abstract declaration. |
| This requires the callout field binding to specify types as in line 3 of <a href="#s3.5.a" title="§3.5.(a) Syntax" class="sect">§3.5.(a)</a> above. |
| The generated access method is static iff the bound base field is static. |
| |
| </p> |
| <p> |
| A shorthand callout to field may optionally declare a <strong>visibility modifier</strong>, |
| otherwise the generated method inherits the visibility modifier of the bound base field. |
| No further modifiers are set. |
| If a callout to field overrides an inherited method or callout, |
| it must not reduce the visibility of the inherited method/callout. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.5.g"> |
| <h4 class="subsect">(g) <span class="title">Callout override</span><a class="img" href="s3.html#s3.5.g" |
| title="PermaLink to (g) Callout override"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Similar to method callouts a callout to field may override an existing role |
| method if and only if the token <code>=></code> is used instead of <code>-></code> |
| (see <a href="#s3.1.e" title="§3.1.(e) Callout override" class="sect">§3.1.(e)</a> and <a href="#s3.1.f" title="§3.1.(f) Inheritance of callout bindings" |
| class="sect">§3.1.(f)</a>). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s3.5.h"> |
| <h4 class="subsect">(h) <span class="title">Inferred callout</span><a class="img" href="s3.html#s3.5.h" |
| title="PermaLink to (h) Inferred callout"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> If a statement or expression within the body of a bound role class |
| uses a simple name or a name qualified by <code>this</code> |
| which can not be resolved using normal rules, |
| the compiler may infer to use a callout to field instead, |
| given that a field of the required name can be found in the role's declared baseclass. |
| |
| </p> |
| <p> If a callout to field has explicitly been declared it is used for the otherwise |
| unresolved name, if and only if: |
| |
| </p> |
| <ul> |
| <li>the callout declares a role method name the is constructed from the token "set" for |
| a setter or "get" for a getter plus the field name with capital first letter, |
| </li> |
| <li>the base field referenced by the callout has exactly the required name, and</li> |
| <li>the callout kind (set/get) matches the application of the unresolved name as either |
| the left-hand side of an assignment (set) or as an expression (get). |
| </li> |
| </ul> |
| <p> |
| If a matching callout to field is not found, the compiler generates one automatically, |
| which has <code>private</code> visibility. |
| |
| </p> |
| <p> |
| If a callout to field has been inferred it is an error to directly invoke the |
| implicitly generated callout accessor that is formed according to the above rules. |
| |
| </p> |
| <p> |
| Per default inferred callout bindings are disabled, i.e., a compiler |
| must report these as an error. However, a compiler should allow to |
| configure reporting to produce a warning only (which can be suppressed |
| using a <code>@SuppressWarnings("inferredcallout")</code> annotation), |
| or to completely ignore the diagnostic. See also <a href="#s3.1.j" title="§3.1.(j) Inferred callout" class="sect">§3.1.(j)</a>. |
| |
| </p> |
| </div> |
| </div> |
| </div> |
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| <tr> |
| <td class="back"><a href="s2.html" rel="prev"><< §2 Role Binding</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s4.html" rel="next">§4 Callin Binding >></a></td> |
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