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| <td class="back"><a id="top"></a><a href="s2.4.2.html" rel="prev"><< §2.4.2 Role creation via a regular constructor</a></td> |
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| <div class="breadcrumb"><a class="nav" href="s2.html" rel="section">§2 Role Binding</a> > <a class="nav" href="s2.4.html" rel="section">§2.4 Explicit role creation</a></div> |
| <div class="sect depth3" id="s2.4.3"> |
| <h3 class="sect">§2.4.3 Role creation in the presence of smart lifting<a class="img" href="s2.4.3.html" |
| title="PermaLink to §2.4.3 Role creation in the presence of smart lifting"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h3> |
| <p>Explicitly instantiating a role <code>R1</code> bound to a base <code>B</code> where smart lifting of <code>B</code> to <code>R1</code> would actually |
| provide a subrole <code>R2</code> is dangerous: Instantiation enters the <code>R1</code> into the team's internal cache. If at any time later lifting |
| this <code>B</code> to <code>R2</code> is requested, which is a legal request, the runtime system will answer by throwing a <code>org.objectteams.WrongRoleException</code> |
| because it finds the <code>R1</code> instead of the required <code>R2</code>. |
| For this reason, in this specific situation the explicit instantiation <code>new R1(..)</code> will be flagged by a warning. |
| The problem can be avoided by using <code>R2</code> in the instantiation expression. |
| |
| </p> |
| <h5 class="listing">Example code (WrongRoleException):</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> B { <b>void</b> bm() {} }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre><b>public</b> <b>team</b> <b>class</b> T {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>protected</b> <b>class</b> R1 <b>playedBy</b> B {...}</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> <b>protected</b> <b>class</b> R2 <b>extends</b> R1 { <span class="comment">// inherits the binding to B</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> <b>void</b> rm() { <span class="comment">/* body omitted */</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> <b>public</b> B getDecoratedB() {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> <b>return</b> <em><b>new</b> R1</em>(<b>new</b> B()); <span class="comment">// <span class="error">compile-time warning!</span></span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">10</td> |
| <td><pre> <b>public</b> <b>void</b> requestLifting(B <b>as</b> R2 r) {}</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><span class="comment">// plus these calls:</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">13</td> |
| <td><pre>T t = <b>new</b> T();</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">14</td> |
| <td><pre>B b = t.getDecoratedB(); <span class="comment">// creates an R1 for b</span></pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">15</td> |
| <td><pre>t.requestLifting(b); <span class="comment">// => <span class="error"><code>org.objectteams.WrongRoleException!</code></span></span></pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <ul> |
| <li>A note on line 8: this line passes a fresh instance of <code>B</code> to the lifting constructor of <code>R1</code> |
| (see <a href="s2.4.1.b.html" title="§2.4.1.(b) Fresh base object" |
| class="sect">§2.4.1.(b)</a>). In order to return this <code>B</code> instance lowering is implicitly used for the return statement. |
| </li> |
| <li>When line 15 is executed, a lifting of <code>b</code> to <code>R2</code> is requested but due to line 8 an <code>R1</code> is found in the internal cache. |
| </li> |
| </ul> |
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| <div class="breadcrumb"><a class="nav" href="s2.html" rel="section">§2 Role Binding</a> > <a class="nav" href="s2.4.html" rel="section">§2.4 Explicit role creation</a></div> |
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