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<td class="back"><a id="top"></a><a href="s4.2.html" rel="prev">&lt;&lt;&nbsp;&sect;4.2&nbsp;Callin modifiers (before, after, replace)</a></td>
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<div class="breadcrumb"><a class="nav" href="s4.html" rel="section">&sect;4&nbsp;Callin Binding</a></div>
<div class="sect depth2" id="s4.3">
<h2 class="sect">&sect;4.3&nbsp;Base calls<a class="img" href="s4.3.html" title="PermaLink to &sect;4.3&nbsp;Base calls"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h2>
<div class="syntaxlink"><a href="sA.html#sA.5.3" title="&sect;A.5.3&nbsp;BaseCall" class="syntax">&rarr;&nbsp;Syntax&nbsp;&sect;A.5.3</a></div>
<p>Role methods with a <code>callin</code> modifier should contain
a <em>base call</em> which uses the special name <code>base</code>
in order to invoke the original base method (original means: before replacement).
</p>
<div class="subsect depth3" id="s4.3.a">
<h4 class="subsect">(a)&nbsp;<span class="title">Syntax</span><a class="img" href="s4.3.a.html" title="PermaLink to (a)&nbsp;Syntax"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>The syntax for base calls is <code>base.m()</code>,
which is in analogy to super calls.
A <code>base.m()</code> call must use the same name and signature
as the enclosing method. This again follows the rule, that roles
should never explicitly use base names, except in binding declarations.
</p>
</div>
<div class="subsect depth3" id="s4.3.b">
<h4 class="subsect">(b)&nbsp;<span class="title">Missing base call</span><a class="img" href="s4.3.b.html"
title="PermaLink to (b)&nbsp;Missing base call"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>For each callin method, the compiler uses some flow analysis to check whether
a base call will be invoked on each path of execution (analysis is very similar
to the analysis for definite assignment regarding final variables -
<a href="http://java.sun.com/docs/books/jls/third_edition/html/defAssign.html"
class="ext">JLS &sect;16</a>).
The compiler will issue a warning if a base call is missing either
on each path (definitely missing) or on some paths (potentially missing).
Instead of directly invoking a base call, a callin method may also call
its explicit or implicit super version using <code>super.m()</code> or <code>tsuper.m()</code> (see <a href="s1.3.1.f.html" title="&sect;1.3.1.(f)&nbsp;tsuper" class="sect">&sect;1.3.1.(f)</a>).
In this case the flow analysis will transitively include the called super/tsuper version.
</p>
</div>
<div class="subsect depth3" id="s4.3.c">
<h4 class="subsect">(c)&nbsp;<span class="title">Duplicate base call</span><a class="img" href="s4.3.c.html"
title="PermaLink to (c)&nbsp;Duplicate base call"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>If a callin method contains several base calls, the compiler gives
a warning if this will result in duplicate base call invocations on all
paths (definitely duplicate) or on some paths (potentially duplicate).
Again super/tsuper calls are included in the flow analysis (see 4.3(b)).
</p>
</div>
<div class="subsect depth3" id="s4.3.d">
<h4 class="subsect">(d)&nbsp;<span class="title">Parameter tunneling</span><a class="img" href="s4.3.d.html"
title="PermaLink to (d)&nbsp;Parameter tunneling"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>If a base method has more parameters
than a callin method to which it is composed, additional
parameters are implicitly passed unchanged from the original
call to the base call (original means: before interception).
I.e., a call <code>base.m()</code> may invisibly pass additional
parameters that were provided by the caller, but are hidden from the
role method.
</p>
</div>
<div class="subsect depth3" id="s4.3.e">
<h4 class="subsect">(e)&nbsp;<span class="title">Fragile callin binding</span><a class="img" href="s4.3.e.html"
title="PermaLink to (e)&nbsp;Fragile callin binding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>If a role method returns void, but the bound base method declares a
non-void result, this is reported as a <em>fragile callin binding</em>:
The result can still be provided by the base call, but omitting the
base call may cause problems depending on the return type:
</p>
<ul>
<li>For reference return types <code>null</code> will be returned
in this case.
</li>
<li>In the case of primitive return types this will cause a
<code>ResultNotProvidedException</code> at run-time.
</li>
</ul>
<p>It is an error if a callin method involved in a fragile callin binding
has definitely no base call.
</p>
</div>
<div class="subsect depth3" id="s4.3.f">
<h4 class="subsect">(f)&nbsp;<span class="title">Base super calls</span><a class="img" href="s4.3.f.html" title="PermaLink to (f)&nbsp;Base super calls"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>If a callin method <code>rm</code> is bound to a base method <code>B1.m</code>
that in turn overrides an inherited method <code>B0.m</code>
(<code>B0</code> is a super class of <code>B1</code>),
the callin method may use a special form of a base call denoted as
</p>
<div class="listing plain"><pre><strong>base.super</strong>.rm();</pre></div>
<p>Such base super call invokes the super method of the bound base method,
here <code>B0.m</code>. This invocation is not affected by any further callin binding.
</p>
<p>A base super call bypasses both the original method <code>B1.m</code> and
also other callin bindings that would be triggered by a regular base call.
For this reason any application of this construct is flagged by a decapsulation warning
(see <a href="s3.4.html" title="&sect;3.4&nbsp;Overriding access restrictions"
class="sect">&sect;3.4</a>).
</p>
</div>
<div class="note">
<h5>Comment:</h5>
Base calls can occur in callin methods that are not
yet bound. These methods have no idea of the names of base methods that
a sub-role will bind to them. Also multiple base methods may be bound
to the same callin method.
Hence the use of the role method's own name and signature.
The language implementation translates the method name and signature
back to the base method that has originally been invoked.
</div>
<h5 class="listing">Example code (Base Call):</h5>
<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> ValidatorRole <b>playedBy</b> Po<b>int</b> {</pre></td>
</tr>
<tr class="line even">
<td class="ln">2</td>
<td><pre> <em>callin</em> <b>void</b> checkCoordinate(<b>int</b> value) {</pre></td>
</tr>
<tr class="line odd">
<td class="ln">3</td>
<td><pre> <b>if</b> (value &lt; 0)</pre></td>
</tr>
<tr class="line even">
<td class="ln">4</td>
<td><pre> <em>base</em>.checkCoordinate(-value);</pre></td>
</tr>
<tr class="line odd">
<td class="ln">5</td>
<td><pre> else</pre></td>
</tr>
<tr class="line even">
<td class="ln">6</td>
<td><pre> <em>base</em>.checkCoordinate(value);</pre></td>
</tr>
<tr class="line odd">
<td class="ln">7</td>
<td><pre> }</pre></td>
</tr>
<tr class="line even">
<td class="ln">8</td>
<td><pre> checkCoordinate <b>&lt;-</b> <b>replace</b> setX, setY;</pre></td>
</tr>
<tr class="line odd">
<td class="ln">9</td>
<td><pre>}</pre></td>
</tr>
</table>
</div>
<div class="codecomment">
<h5>Effects:</h5>
<ul>
<li>Line 2 defines a callin method which is bound to two methods
of the base class <code>Point</code> (see line 8).
</li>
<li>The value passed to either setX or setY is checked if it is
positive (line 3).
</li>
<li>Lines 4 and 6 show calls of the original method (base calls).
While line 6 passes the original value, in the negative case (line 4)
the passed value is made positive.
</li>
</ul>
</div>
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OT/J version 1.3 &mdash; last modified: 2010-06-08
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