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          <div class="breadcrumb"><a class="nav" href="s3.html" rel="section">&sect;3&nbsp;Callout Binding</a></div>
          <div class="sect depth2" id="s3.1">
             <h2 class="sect">&sect;3.1&nbsp;Callout method binding<a class="img" href="s3.1.html"
                   title="PermaLink to &sect;3.1&nbsp;Callout method binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
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             <div class="syntaxlink"><a href="sA.html#sA.3.2" title="&sect;A.3.2&nbsp;CalloutBinding"
                   class="syntax">&rarr;&nbsp;Syntax&nbsp;&sect;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)&nbsp;<span class="title">Prerequisite: Class binding</span><a class="img" href="s3.1.a.html"
                      title="PermaLink to (a)&nbsp;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="s2.1.html" title="&sect;2.1&nbsp;playedBy relation" class="sect">&sect;2.1</a>. However, callout bindings are not
                   	    allowed if the role is involved in base class circularity (see <a href="s2.1.2.b.html" title="&sect;2.1.2.(b)&nbsp;Cycles" class="sect">&sect;2.1.2.(b)</a>).

                </p>
             </div>
             <div class="subsect depth3" id="s3.1.b">
                <h4 class="subsect">(b)&nbsp;<span class="title">Definition</span><a class="img" href="s3.1.b.html" title="PermaLink to (b)&nbsp;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>-&gt;</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)&nbsp;<span class="title">Kinds of method designators</span><a class="img" href="s3.1.c.html"
                      title="PermaLink to (c)&nbsp;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>-&gt;</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>-&gt;</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)&nbsp;<span class="title">Inheritance of role method declarations</span><a class="img" href="s3.1.d.html"
                      title="PermaLink to (d)&nbsp;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)&nbsp;<span class="title">Callout override</span><a class="img" href="s3.1.e.html" title="PermaLink to (e)&nbsp;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>=&gt;</code>" instead of "<code>-&gt;</code>"
                   	    in order to declare that this binding overrides an existing implementation.
                   	    <br />

                   		Using the "<code>=&gt;</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)&nbsp;<span class="title">Inheritance of callout bindings</span><a class="img" href="s3.1.f.html"
                      title="PermaLink to (f)&nbsp;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>=&gt;</code>".

                </p>
             </div>
             <div class="subsect depth3" id="s3.1.g">
                <h4 class="subsect">(g)&nbsp;<span class="title">Duplicate bindings</span><a class="img" href="s3.1.g.html"
                      title="PermaLink to (g)&nbsp;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)&nbsp;<span class="title">Declared exceptions</span><a class="img" href="s3.1.h.html"
                      title="PermaLink to (h)&nbsp;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)&nbsp;<span class="title">Shorthand definition</span><a class="img" href="s3.1.i.html"
                      title="PermaLink to (i)&nbsp;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)&nbsp;<span class="title">Inferred callout</span><a class="img" href="s3.1.j.html" title="PermaLink to (j)&nbsp;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)&nbsp;<span class="title">Callout to generic method</span><a class="img" href="s3.1.k.html"
                      title="PermaLink to (k)&nbsp;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>&lt;T&gt; 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>&lt;T&gt; T rm(T a) <b>-&gt;</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>-&gt;</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="&sect;3.1.(i)&nbsp;Shorthand definition" class="sect">&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="&sect;4&nbsp;Callin Binding" class="sect">&sect;4</a>)
                		the effect of delegation can easily be achieved.

             </p>
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	<div class="breadcrumb"><a class="nav" href="s3.html" rel="section">§3 Callout Binding</a></div>
	<div class="sect depth2" id="s3.1">
	<h2 class="sect">§3.1 Callout method binding<a class="img" href="s3.1.html"
	title="PermaLink to §3.1 Callout method binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
	alt="" /></a></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="s3.1.a.html"
	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="s2.1.html" 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="s2.1.2.b.html" 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="s3.1.b.html" 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="s3.1.c.html"
	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="s3.1.d.html"
	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="s3.1.e.html" 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="s3.1.f.html"
	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="s3.1.g.html"
	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="s3.1.h.html"
	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="s3.1.i.html"
	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="s3.1.j.html" 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="s3.1.k.html"
	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>
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