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| <td class="back"><a id="top"></a><a href="s7.1.html" rel="prev"><< §7.1 Opaque roles</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="s7.html" rel="section">§7 Role Encapsulation</a></div> |
| <div class="sect depth2" id="s7.2"> |
| <h2 class="sect">§7.2 Confined roles<a class="img" href="s7.2.html" |
| title="PermaLink to §7.2 Confined roles"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h2> |
| <p> |
| Subclassing <code>Team.Confined</code> with a protected class |
| yields a role class to which no object outside the team will |
| ever have a reference. |
| The point here is that instances of a role class with a regular super class |
| can be widened to this super class. |
| Widening can occur either in an assignment or when invoking a method which the role |
| inherits from the regular super class, where the <code>this</code> reference is widened. |
| In both cases the widened reference is no longer protected by the team and can leak out. |
| This would break encapsulation of a role object that should only be accessible within the enclosing team.<br /> |
| Subclasses of <code>Team.Confined</code> are not compatible to any class outside their enclosing team (including <code>java.lang.Object</code>) and do not inherit any methods |
| that have the danger of leaking <code>this</code>. |
| |
| </p> |
| <div class="subsect depth3" id="s7.2.a"> |
| <h4 class="subsect">(a) <span class="title">Inhibition of overriding</span><a class="img" href="s7.2.a.html" |
| title="PermaLink to (a) Inhibition of overriding"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The types <code>ITeam.IConfined</code> and <code>Team.Confined</code> |
| cannot be overridden (cf. <a href="s1.3.1.c.html" |
| title="§1.3.1.(c) Overriding and implicit inheritance" |
| class="sect">§1.3.1.(c)</a>). |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s7.2.b"> |
| <h4 class="subsect">(b) <span class="title">Arrays of Confined</span><a class="img" href="s7.2.b.html" |
| title="PermaLink to (b) Arrays of Confined"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>For any confined type <code>C</code>, i.e., a type which is not compatible to <code>Object</code>, an array of <code>C</code> is not compatible to an array of <code>Object</code> nor to <code>Object</code> itself. This rule ensures that confinement cannot be bypassed by a sequence of compatible assignments and casts. |
| |
| </p> |
| <div class="note"> |
| <h5>Upcoming:</h5> |
| Only by widening to a non-role super-type, a role instance can |
| be accessed from outside the team. In the future this can be inhibited by |
| restricted inheritance. |
| |
| </div> |
| </div> |
| <h5 class="listing">Example code (Role Encapsulation):</h5> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">1</td> |
| <td><pre><b>public</b> <b>team</b> <b>class</b> Company {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>private</b> HashMap<String,Employee> employees;</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>protected</b> <b>class</b> Employee <em><b>implements</b> IConfined</em> {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> <b>void</b> pay(<b>int</b> amount) { ... }</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> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> <b>public</b> <em>IConfined</em> getEmployee(String ID) {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre> <b>return</b> employees.get(ID); <span class="comment">// implicit widening to IConfined</span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">10</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">11</td> |
| <td><pre> <b>public</b> <b>void</b> payBonus(<em>IConfined</em> emp, <b>int</b> amount) {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">12</td> |
| <td><pre> ((Employee)emp).pay(amount); <span class="comment">// explicit narrowing</span></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> |
| </table> |
| </div> |
| <div class="listing example frame"> |
| <table class="listing"> |
| <tr class="line odd"> |
| <td class="ln">15</td> |
| <td><pre><b>public</b> <b>class</b> Ma<b>in</b> {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">16</td> |
| <td><pre> <b>public</b> <b>static</b> <b>void</b> main(String[] args) {</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">17</td> |
| <td><pre> <b>final</b> Company comp = <b>new</b> Company();</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">18</td> |
| <td><pre> <em>IConfined</em><@comp> emp = comp.getEmployee("emp1");</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">19</td> |
| <td><pre> <span class="comment">// System.out.println(emp); <– <span class="error"><strong>forbidden!</strong></span></span></pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">20</td> |
| <td><pre> comp.payBonus(emp, 100);</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">21</td> |
| <td><pre> }</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">22</td> |
| <td><pre>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Effects:</h5> |
| <ul> |
| <li>The <code>protected</code> role <code>Employee</code> implements |
| the above described interface <code>IConfined</code> and therefore |
| becomes <strong>opaque</strong> (line 4). |
| </li> |
| <li>Methods sharing such an opaque role with the outside of the enclosing team |
| have to use the type <code>IConfined</code> (line 8, line 11). |
| </li> |
| <li>It is possible to obtain an instance of such a role by using the type <code>IConfined</code> (line 18). |
| </li> |
| <li>Trying to access any feature of this instance, for example <code>toString()</code>, |
| will cause a compilation error (line 19). |
| </li> |
| <li>Passing the opaque role reference back into the team works well (line 20).</li> |
| <li>Inside the team some conversions between the types <code>IConfined</code> |
| and the intrinsic role type <code>Employee</code> may be necessary (line 9 and 12). |
| </li> |
| </ul> |
| </div> |
| </div> |
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| <td class="back"><a href="s7.1.html" rel="prev"><< §7.1 Opaque roles</a></td> |
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