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| <td class="back"><a id="top"></a><a href="s3.3.html" rel="prev"><< §3.3 Lifting and lowering</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s3.5.html" rel="next">§3.5 Callout to field >></a></td> |
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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="s3.4"> |
| <h2 class="sect">§3.4 Overriding access restrictions<a class="img" href="s3.4.html" |
| title="PermaLink to §3.4 Overriding access restrictions"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
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| <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="s2.1.2.c.html" title="§2.1.2.(c) Base class decapsulation" |
| class="sect">§2.1.2.(c)</a>) |
| </li> |
| <li>base constructor call (see <a href="s2.4.2.b.html" 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.html" title="§3.5.(e) Access control" class="sect">§3.5.(e)</a>) |
| </li> |
| <li>base call within a callin method (see <a href="s4.6.html" 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="s3.4.a.html" |
| 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="s3.4.b.html" |
| 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="s3.4.c.html" 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="s2.1.2.c.html" 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="s3.4.d.html" |
| 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.html" |
| 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> |
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| <td class="back"><a href="s3.3.html" rel="prev"><< §3.3 Lifting and lowering</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s3.5.html" rel="next">§3.5 Callout to field >></a></td> |
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| <div class="breadcrumb"><a class="nav" href="s3.html" rel="section">§3 Callout Binding</a></div> |
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