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| <td class="back"><a id="top"></a><a href="s2.1.1.html" rel="prev"><< §2.1.1 Binding interfaces</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></div> |
| <div class="sect depth3" id="s2.1.2"> |
| <h3 class="sect">§2.1.2 Legal base classes<a class="img" href="s2.1.2.html" |
| title="PermaLink to §2.1.2 Legal base classes"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h3> |
| <p>Generally, the base class mentioned after <code>playedBy</code> must be |
| visible in the enclosing scope (see <a href="#s2.1.2.c" title="§2.1.2.(c) Base class decapsulation" |
| class="sect">below (§2.1.2.(c))</a> for an exception). |
| Normally, this scope is defined just by the imports of the enclosing team. |
| For role files (<a href="s1.2.5.b.html" title="§1.2.5.(b) Role files" class="sect">§1.2.5.(b)</a>) |
| also additional imports in the role file are considered. |
| <br /><a href="#s2.1.2.d" title="§2.1.2.(d) Base imports" class="sect">§2.1.2.(d)</a> below defines how imports can be constrained so that certain types |
| can be used as base types, only. |
| |
| </p> |
| <div class="subsect depth4" id="s2.1.2.a"> |
| <h4 class="subsect">(a) <span class="title">No role of the same team</span><a class="img" href="s2.1.2.a.html" |
| title="PermaLink to (a) No role of the same team"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The base class of any role class must not be a role of the same team. |
| <br /> |
| It is also not allowed to declare a role class of the same name |
| as a base class bound to this or another role of the enclosing team, |
| if that base class is given with its simple name and resolved using a regular import. |
| Put differently, a base class mentioned after <code>playedBy</code> |
| may not be <em>shadowed</em> by any role class of the enclosing team. |
| <br /><em>Base imports</em> as defined below (<a href="#s2.1.2.d" title="§2.1.2.(d) Base imports" class="sect">§2.1.2.(d)</a>) relax this rule by |
| allowing to import a class as a base class only. In that case no shadowing occurs since the scopes for |
| base classes and roles are disjoint. |
| |
| </p> |
| </div> |
| <div class="subsect depth4" id="s2.1.2.b"> |
| <h4 class="subsect">(b) <span class="title">No cycles</span><a class="img" href="s2.1.2.b.html" title="PermaLink to (b) No cycles"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The base class mentioned after <code>playedBy</code> may not be |
| an enclosing type (at any depth) of the role class being defined. |
| <br /> |
| This rule prohibits the creation of cycles where the base instance of |
| a given role <code>R</code> contains roles of the same type <code>R</code>. |
| <br /> |
| More generally any sequence of classes <code>C<sub>1</sub>, C<sub>2</sub>, .. C<sub>n</sub></code> |
| were each <code>C<sub>i+1</sub></code> is either a member or the base class of |
| <code>C<sub>i</sub></code> and <code>C<sub>n</sub> = C<sub>1</sub></code> is forbidden. |
| <br /> |
| Conversely, it is also prohibited to bind a role class to its own inner class. |
| |
| </p> |
| </div> |
| <div class="subsect depth4" id="s2.1.2.c"> |
| <h4 class="subsect">(c) <span class="title">Base class decapsulation</span><a class="img" href="s2.1.2.c.html" |
| title="PermaLink to (c) Base class decapsulation"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If a base class referenced after <code>playedBy</code> exists but is not visible under normal visibility rules of Java, |
| this restriction may be overridden. This concept is called <strong>decapsulation</strong>, i.e., the opposite of encapsulation |
| (see also <a href="s3.4.html" title="§3.4 Overriding access restrictions" |
| class="sect">§3.4</a>). A compiler should signal any occurrence of base class 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. |
| |
| </p> |
| <p> |
| Binding to a <code>final</code> base class is also considered as decapsulation, since a <code>playedBy</code> relationship has |
| powers similar to an <code>extends</code> relationship, which is prohibited by marking a class as <code>final</code>. |
| |
| </p> |
| <p> |
| Decapsulation is not allowed if the base class is a confined role (see <a href="s7.2.html" title="§7.2 Confined roles" class="sect">§7.2</a>). |
| |
| </p> |
| <p> |
| Within the current role a decapsulated base class can be mentioned in the right-hand-side of any method binding |
| (<a href="s3.html" title="§3 Callout Binding" class="sect">callout (§3)</a> or <a href="s4.html" title="§4 Callin Binding" class="sect">callin (§4)</a>). Also arguments in these positions are allowed to mention the decapsulated base class: |
| |
| </p> |
| <ul> |
| <li>the first argument of one of the role's constructors (see <a href="s2.4.1.html" |
| title="§2.4.1 Role creation via a lifting constructor" |
| class="sect">lifting constructor (§2.4.1)</a>). |
| </li> |
| <li>the base side of an argument with declared lifting (see <a href="s2.3.2.html" title="§2.3.2 Declared lifting" class="sect">declared lifting (§2.3.2)</a>). |
| </li> |
| </ul> |
| </div> |
| <div class="subsect depth4" id="s2.1.2.d"> |
| <h4 class="subsect">(d) <span class="title">Base imports</span><a class="img" href="s2.1.2.d.html" title="PermaLink to (d) Base imports"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If the main type in a file denotes a team, the modifier <code>base</code> can be applied to an import in order to specify that this type |
| should be imported for application as a base type only. Example: |
| |
| </p> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><em><b>import</b> base</em> some.pack.MyBase;</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre><b>public</b> <b>team</b> <b>class</b> MyTeam {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <span class="comment">// simple name resolves to imported class:</span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> <b>protected</b> <b>class</b> MyRole <em><b>playedBy</b> MyBase</em> { } </pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> <span class="error"><em>MyBase</em> illegalDeclaration;</span> <span class="comment">// base import does not apply for this position</span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <p>Types imported by a base import can only be used in the same positions where also base class decapsulation (<a href="#s2.1.2.c" title="§2.1.2.(c) Base class decapsulation" |
| class="sect">§2.1.2.(c)</a>) |
| is applicable.<br /> |
| It is recommended that a type mentioned after the keyword <code>playedBy</code> is always imported with the <code>base</code> modifier, otherwise the compiler |
| will give a warning.<br /> |
| Base imports create a scope that is disjoint from the normal scope. Thus, names that are imported as base will never clash |
| with normally visible names |
| (in contrast to <a href="s1.4.html" title="§1.4 Name clashes" class="sect">§1.4</a>). More specifically, it is not a problem to use a base class's name also for its role if a base import is used. |
| |
| </p> |
| </div> |
| <div class="subsect depth4" id="s2.1.2.e"> |
| <h4 class="subsect">(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) throws 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> |
| </div> |
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| <td class="back"><a href="s2.1.1.html" rel="prev"><< §2.1.1 Binding interfaces</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></div> |
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| OT/J version 1.3 — last modified: 2010-06-08 |
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