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| <td class="back"><a id="top"></a><a href="s2.1.2.d.html" rel="prev"><< §2.1.2.(d) Base imports</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</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="s2.1.html" rel="section">§2.1 playedBy relation</a> > <a class="nav" href="s2.1.2.html" rel="section">§2.1.2 Legal base classes</a></div> |
| <div class="subsect depth4" id="s2.1.2.e"> |
| <h4 class="subsect">§2.1.2.(e) <span class="title">No free type parameters</span><a class="img" href="s2.1.2.e.html" |
| title="PermaLink to (e) No free type parameters"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p> |
| Neither the role class nor the base class in a playedBy binding must have any <em>free type parameters</em>. |
| If both classes are specified with a type parameter of the same name, both parameters are identified |
| and are not considered as <em>free</em>. |
| |
| </p> |
| <p> |
| From this follows that a role class cannot have more type parameters than its base. |
| Conversely, only one situation exists where a base class can have more type parameters than a role class |
| bound to it: if the role class has no type parameters a generic base class can be bound using |
| the base class's raw type, i.e., without specifying type arguments. |
| |
| </p> |
| <div class="note"> |
| <h5>Note:</h5> |
| The information from the <code>playedBy</code> declaration is used at run-time |
| to associate role instances to base instances. |
| Specifying a base class with free type parameters would imply that only such base instances |
| are decorated by a role whose type is conform to the specified parameterized class. |
| However, type arguments are not available at run-time, thus the run-time environment |
| is not able to decide which base instances should have a role and which should not. |
| This is due to the design of generics in Java which are realized by erasure. |
| |
| </div> |
| <p>The following example shows how generics can be used in various positions. Note, that some of the concepts used in the example |
| will be explained in later sections. |
| </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> ValueTrafo<em><T></em> {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>public</b> <em>T</em> transform(<em>T</em> val) <b>throws</b> Exception { /* ... */ }</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>team</b> <b>class</b> TransformTeam {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> <b>protected</b> <b>class</b> SafeTrafo<em><U></em> <b>playedBy</b> ValueTrafo<em><U></em> {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> <em>U</em> transform(<em>U</em> v) <b>-></b> <em>U</em> transform(<em>U</em> val); </pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> <b>protected</b> <em>U</em> safeTransform(<em>U</em> v) {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> <b>try</b> {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre> <b>return</b> transform(v);</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">10</td> |
| <td><pre> } <b>catch</b> (Exception e) {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">11</td> |
| <td><pre> <b>return</b> v;</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> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">14</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">15</td> |
| <td><pre> <em><V> V</em> perform(ValueTrafo<em><V></em> <b>as</b> SafeTrafo<em><V></em> trafo, <em>V</em> value) {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">16</td> |
| <td><pre> <b>return</b> trafo.safeTransform(value);</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">17</td> |
| <td><pre> } </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>...</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">20</td> |
| <td><pre>ValueTrafo<em><String></em> trafo = <b>new</b> ValueTrafo<em><String></em>();</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">21</td> |
| <td><pre>TransformTeam safeTrafo = <b>new</b> TransformTeam();</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">22</td> |
| <td><pre>String s = safeTrafo.perform(trafo, "Testing");</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">23</td> |
| <td><pre></pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Explanation</h5> |
| <ul> |
| <li>Line 5 shows a role with type parameter <code>U</code> where the type parameter is identified with the |
| corresponding type parameter of the role's base class (which is originally declared as <code>T</code> in line 1. |
| </li> |
| <li>Line 6 shows a callout binding (<a href="s3.html" title="§3 Callout Binding" class="sect">§3</a>) which mappes a base method to a corresponding role method |
| while maintaining the flexible typing. |
| </li> |
| <li>The regular method in lines 7-13 just passes values of type <code>U</code> around. |
| </li> |
| <li>The generic method in line 15 ff. uses declared lifting (<a href="s2.3.2.html" title="§2.3.2 Declared lifting" class="sect">§2.3.2</a>) to obtain a role for a given base object. |
| The method has no knowledge about the concrete type arguments of either role nor base, but works under the guarantee |
| that both type arguments will be the same for any single invocation. |
| </li> |
| <li>Lines 20 ff. finally create instances of base and team and invoke the behavior thereby instantiating type parameters to <code>String</code>. |
| </li> |
| </ul> |
| </div> |
| </div> |
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| <td class="back"><a href="s2.1.2.d.html" rel="prev"><< §2.1.2.(d) Base imports</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="s2.1.html" rel="section">§2.1 playedBy relation</a> > <a class="nav" href="s2.1.2.html" rel="section">§2.1.2 Legal base classes</a></div> |
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