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| <title>OT/J Language Definition v1.3.1</title> |
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| <td class="back"><a id="top"></a><a href="s2.1.html" rel="prev"><< §2.1 playedBy relation</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s2.3.html" rel="next">§2.3 Lifting >></a></td> |
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| <div class="breadcrumb"><a class="nav" href="s2.html" rel="section">§2 Role Binding</a></div> |
| <div class="sect depth2" id="s2.2"> |
| <h2 class="sect">§2.2 Lowering<a class="img" href="s2.2.html" title="PermaLink to §2.2 Lowering"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h2> |
| <p>Each instance of a bound role class internally stores a reference to its |
| base object. The reference is guaranteed to exist for each bound role |
| instance, and cannot be changed during its lifetime. |
| |
| </p> |
| <div class="subsect depth3" id="s2.2.a"> |
| <h4 class="subsect">(a) <span class="title">Definition of lowering</span><a class="img" href="s2.2.a.html" |
| title="PermaLink to (a) Definition of lowering"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Retrieving the base object from a role object is called <strong>lowering</strong>. |
| No other means exists for accessing the base reference. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s2.2.b"> |
| <h4 class="subsect">(b) <span class="title">Places of lowering</span><a class="img" href="s2.2.b.html" |
| title="PermaLink to (b) Places of lowering"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The lowering translation is not meant to be invoked |
| by client code, but <strong>implicit translations</strong> are inserted by |
| the compiler at all places where a role type is provided while the |
| corresponding base type (or a super type) was expected.<br /> |
| In other words: lowering translations are inserted by the compiler at |
| all places in a program which would otherwise not be type correct |
| and which using lowering are statically type correct. |
| This may concern: |
| |
| </p> |
| <ul> |
| <li>the right hand side of an assignment wrt. the static type of the left hand side,</li> |
| <li>the argument values of a method or constructor call wrt. the static type of the corresponding formal parameter,</li> |
| <li>the return value of a method compared to the declared return type of the method.</li> |
| <li>a role parameter in a callout binding (<a href="s3.3.d.html" title="§3.3.(d) Typing rules" class="sect">§3.3.(d)</a>) |
| </li> |
| <li>or the return value in a callin binding (<a href="s4.5.d.html" title="§4.5.(d) Typing rules" class="sect">§4.5.(d)</a>) |
| </li> |
| </ul> |
| <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> MyTeamA {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>public</b> <b>class</b> <em>MyRole <b>playedBy</b> MyBase</em> { ... }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>void</b> useMyBase(<em>MyBase</em> myb) {...}</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> <em>MyRole</em> returnMyRole() {...}</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> <b>public</b> <b>void</b> doSomething() {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> <em>MyRole r</em> = <b>new</b> MyRole(<b>new</b> MyBase());</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">7</td> |
| <td><pre> <em>MyBase b</em> = <em>r</em>;</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">8</td> |
| <td><pre> useMyBase(<em>r</em>);</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">9</td> |
| <td><pre> <em>MyBase b2</em> = returnMyRole();</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>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| <div class="codecomment"> |
| <h5>Effects:</h5> |
| <p>An instance of type <code>MyRole</code> is lowered to type <code>MyBase</code> when |
| </p> |
| <ul> |
| <li>assigning it to <code>b</code> (line 7) |
| </li> |
| <li>passing it as argument to a method with formal parameter of type <code>MyBase</code> (line 8) |
| </li> |
| <li>assigning the return value to a variable of type <code>MyBase</code> (line 9) |
| </li> |
| </ul> |
| <p><em>Note</em>: The constructor call in line 6 uses the <em>lifting constructor</em> as defined in <a href="s2.4.1.html" |
| title="§2.4.1 Role creation via a lifting constructor" |
| class="sect">§2.4.1</a></p> |
| </div> |
| <p>Lowering translations are <span class="underline">not</span> inserted for |
| |
| </p> |
| <ul> |
| <li>reference comparison (using <code>==</code> or <code>!=</code>) |
| </li> |
| <li><code>instanceof</code> checks |
| </li> |
| <li>cast expressions</li> |
| <li>return values in callout bindings <a href="s3.3.d.html" title="§3.3.(d) Typing rules" class="sect">§3.3.(d)</a>) |
| </li> |
| <li>parameters in callin bindings (<a href="s4.5.d.html" title="§4.5.(d) Typing rules" class="sect">§4.5.(d)</a>) |
| </li> |
| </ul> |
| <p>For cases where lowering shall be <em>forced</em> see <a href="#s2.2.d" title="§2.2.(d) Explicit lowering" class="sect">§2.2.(d)</a> below. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s2.2.c"> |
| <h4 class="subsect">(c) <span class="title">Typing</span><a class="img" href="s2.2.c.html" title="PermaLink to (c) Typing"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>The static type of an implicit lowering translation is the base class |
| declared using <code>playedBy</code> in the respective role class. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s2.2.d"> |
| <h4 class="subsect">(d) <span class="title">Explicit lowering</span><a class="img" href="s2.2.d.html" |
| title="PermaLink to (d) Explicit lowering"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>If a base type is also the super type of its role, |
| which frequently happens, if a base reference is known only by |
| the type <code>Object</code>, lowering cannot be deduced automatically, |
| since a type could be interpreted both as a role type and a base type. |
| These cases may need <strong>explicit lowering</strong>. |
| For this purpose the role class must declare to implement the interface |
| <strong><code>ILowerable</code></strong> (from <code>org.objectteams.ITeam</code>). |
| This will cause the compiler to generate a method |
| </p> |
| <div class="listing plain"><pre><b>public</b> Object lower()</pre></div> |
| <p>for the given role class. Client code may use this method to |
| explicitly request the base object of a given role object. |
| </p> |
| <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> MyTeamA {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">2</td> |
| <td><pre> <b>public</b> <b>class</b> MyRole <em><b>implements</b> ILowerable</em> <b>playedBy</b> MyBase { ... }</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">3</td> |
| <td><pre> <b>public</b> <b>void</b> doSomething() {</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">4</td> |
| <td><pre> MyRole r = <b>new</b> MyRole(<b>new</b> MyBase());</pre></td> |
| </tr> |
| <tr class="line odd"> |
| <td class="ln">5</td> |
| <td><pre> Object oMyRole = r;</pre></td> |
| </tr> |
| <tr class="line even"> |
| <td class="ln">6</td> |
| <td><pre> Object oMyBase = r.<em>lower()</em>;</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>}</pre></td> |
| </tr> |
| </table> |
| </div> |
| </div> |
| <div class="subsect depth3" id="s2.2.e"> |
| <h4 class="subsect">(e) <span class="title">Lowering of arrays</span><a class="img" href="s2.2.e.html" |
| title="PermaLink to (e) Lowering of arrays"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>Lowering also works for arrays of role objects. |
| In order to lower an array of role objects, |
| a new array is created and filled with base objects, one for each |
| role object in the original array. The array may have any number |
| of dimensions at any shape. The lowered array will have exactly the |
| same shape.<br /> |
| Note, that each lowering translation will create a new array. |
| |
| </p> |
| </div> |
| <div class="subsect depth3" id="s2.2.f"> |
| <h4 class="subsect">(f) <span class="title">Ambiguous lowering</span><a class="img" href="s2.2.f.html" |
| title="PermaLink to (f) Ambiguous lowering"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png" |
| alt="" /></a></h4> |
| <p>When assigning a value of a bound role type to a variable or argument of type <code>java.lang.Object</code> |
| this situation is considered as ambiguous lowering because the assignment could apply either (a) a direct upcast to <code>Object</code> |
| or (b) lowering and then upcasting. |
| In such situations the compiler will <em>not</em> insert a lowering translation, but a configurable warning will be issued. |
| |
| </p> |
| </div> |
| </div> |
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| <td class="back"><a href="s2.1.html" rel="prev"><< §2.1 playedBy relation</a></td> |
| <td class="top"><a href="index.html" rel="contents">↑ Table of Contents ↑</a></td> |
| <td class="next"><a href="s2.3.html" rel="next">§2.3 Lifting >></a></td> |
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