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<td class="top"><a href="index.html" rel="contents">&uarr;&nbsp;Table of Contents&nbsp;&uarr;</a></td>
<td class="next"><a href="s1.3.2.html" rel="next">&sect;1.3.2&nbsp;Regular role inheritance&nbsp;&gt;&gt;</a></td>
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<div class="breadcrumb"><a class="nav" href="s1.html" rel="section">&sect;1&nbsp;Teams and Roles</a>&nbsp;&gt;&nbsp;<a class="nav" href="s1.3.html" rel="section">&sect;1.3&nbsp;Acquisition and implicit inheritance of role classes</a></div>
<div class="sect depth3" id="s1.3.1">
<h3 class="sect">&sect;1.3.1&nbsp;Acquisition and implicit inheritance of role classes<a class="img" href="s1.3.1.html"
title="PermaLink to &sect;1.3.1&nbsp;Acquisition and implicit inheritance of role classes"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h3>
<p>A team acquires all roles from its super-team. This relation is
similar to inheritance of inner classes, but with a few decisive
differences as defined next. Two implementation options are mentioned <a href="s1.html#aux1.1" class="int">below</a>,
which can be used to realize the special semantics of role
acquisition (virtual classes and copy inheritance).
</p>
<h5 class="listing">Implicit role inheritance</h5>
<div class="listing example frame" id="l1.3.1-1">
<table class="listing">
<tr class="line odd">
<td class="ln">1</td>
<td><pre><b>public</b> <b>team</b> <b>class</b> S {</pre></td>
</tr>
<tr class="line even">
<td class="ln">2</td>
<td><pre> <b>protected</b> <b>class</b> R0 {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">3</td>
<td><pre> <b>protected</b> <b>class</b> R1 <em><b>extends</b> R0</em> {</pre></td>
</tr>
<tr class="line even">
<td class="ln">4</td>
<td><pre> <b>boolean</b> ok;</pre></td>
</tr>
<tr class="line odd">
<td class="ln">5</td>
<td><pre> R2 m() {...}</pre></td>
</tr>
<tr class="line even">
<td class="ln">6</td>
<td><pre> <b>void</b> n(<em>R2</em> r) {...}</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>protected</b> <b>class</b> R2 {...}</pre></td>
</tr>
<tr class="line odd">
<td class="ln">9</td>
<td><pre>}</pre></td>
</tr>
</table>
</div>
<div class="listing example frame" id="l1.3.1-2">
<table class="listing">
<tr class="line odd">
<td class="ln">10</td>
<td><pre><b>public</b> <b>team</b> <b>class</b> T <em><b>extends</b> S</em> {</pre></td>
</tr>
<tr class="line even">
<td class="ln">11</td>
<td><pre> @Override <b>protected</b> <em><b>class</b> R1</em> {</pre></td>
</tr>
<tr class="line odd">
<td class="ln">12</td>
<td><pre> <strong>R2</strong> m() {</pre></td>
</tr>
<tr class="line even">
<td class="ln">13</td>
<td><pre> if(<em>ok</em>) { <b>return</b> <em>tsuper</em>.m(); }</pre></td>
</tr>
<tr class="line odd">
<td class="ln">14</td>
<td><pre> <b>else</b> { <b>return</b> null; }</pre></td>
</tr>
<tr class="line even">
<td class="ln">15</td>
<td><pre> }</pre></td>
</tr>
<tr class="line odd">
<td class="ln">16</td>
<td><pre> <b>void</b> doIt() {</pre></td>
</tr>
<tr class="line even">
<td class="ln">17</td>
<td><pre> n(m());</pre></td>
</tr>
<tr class="line odd">
<td class="ln">18</td>
<td><pre> }</pre></td>
</tr>
<tr class="line even">
<td class="ln">19</td>
<td><pre> }</pre></td>
</tr>
<tr class="line odd">
<td class="ln">20</td>
<td><pre>}</pre></td>
</tr>
</table>
</div>
<div class="subsect depth4" id="s1.3.1.a">
<h4 class="subsect">(a)&nbsp;<span class="title">Role class acquisition</span><a class="img" href="s1.3.1.a.html"
title="PermaLink to (a)&nbsp;Role class acquisition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>A team <code>T</code> which extends a super-team <code>S</code>
has one role class <code>T.R</code> corresponding to each role <code>S.R</code>
of the super-team.
The new type <code>T.R</code>&nbsp;<strong>overrides</strong>&nbsp;<code>R</code> for the
context of <code>T</code> and its roles.
Acquisition of role classes can either be direct (see (b) below), or
it may involve overriding and implicit inheritance ((c) below).
</p>
<div class="codecomment">In the above example (<a href="#l1.3.1-1" class="listing">Listing 1.3.1-1</a>) the team <code>S</code> operates
on types <code>S.R0</code>, <code>S.R1</code> and <code>S.R2</code>,
while <code>T</code> operates on types <code>T.R0</code>, <code>T.R1</code>
and <code>T.R2</code>.<br /><em>(Type references like "<code>S.R0</code>" are actually illegal in source code
(<a href="s1.2.3.b.html" title="&sect;1.2.3.(b)&nbsp;Qualified role types"
class="sect">&sect;1.2.3.(b)</a>). Here they are used for explanatory purposes only)</em></div>
</div>
<div class="subsect depth4" id="s1.3.1.b">
<h4 class="subsect">(b)&nbsp;<span class="title">Direct role acquisition</span><a class="img" href="s1.3.1.b.html"
title="PermaLink to (b)&nbsp;Direct role acquisition"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>Within a sub-team <code>T</code> each role <code>S.R</code> of its
super-team <code>S</code> is available by the simple name <code>R</code>
without further declaration.
</p>
<div class="codecomment">The role <code>R2</code> in <a href="#l1.3.1-1" class="listing">Listing 1.3.1-1</a> can be used in the sub-team
<code>T</code> (line 12), because this role type is defined in the super class of the enclosing team.
</div>
</div>
<div class="subsect depth4" id="s1.3.1.c">
<h4 class="subsect">(c)&nbsp;<span class="title">Overriding and implicit inheritance</span><a class="img" href="s1.3.1.c.html"
title="PermaLink to (c)&nbsp;Overriding and implicit inheritance"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>If a team contains a role class definition by the same name as
a role defined in its super-team,
the new role class overrides the corresponding role from the super-team
and <strong>implicitly inherits</strong> all of its features.
Such relation is established only by name correspondence.
</p>
<p>A role that overrides an inherited role should be marked with an <code>@Override</code> annotation.
A compiler should optionally flag a missing <code>@Override</code> annotation with a warning.
Conversely, it is an error if a role is marked with an <code>@Override</code> annotation but does not actually
override an inherited role.
</p>
<p>It is an error to override a role class with an interface or vice versa. A final role cannot be overridden.<br />
Unlike regular inheritance, <strong>constructors</strong> are also inherited
along implicit inheritance, and can be overridden just like normal methods.
</p>
<div class="codecomment">
In <a href="#l1.3.1-1" class="listing">Listing 1.3.1-1</a><code> R1</code> in <code>T</code> implicitly inherits all features of
<code>R1</code> in <code>S</code>. This is, because its enclosing team
<code>T</code> extends the team <code>S</code> (line 10) and the role
definition uses the same name <code>R1</code> (line 11).
Hence the attribute <code><strong>ok</strong></code> is available in the method
<code>m()</code> in <code>T.R1</code> (line 13). <code>T.R1</code> also overrides <code>S.R1</code>
which is marked by the <code>@Override</code> annotation in line 11.
</div>
</div>
<div class="subsect depth4" id="s1.3.1.d">
<h4 class="subsect">(d)&nbsp;<span class="title">Lack of subtyping</span><a class="img" href="s1.3.1.d.html"
title="PermaLink to (d)&nbsp;Lack of subtyping"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>Direct acquisition of roles from a super-team and implicit inheritance
do not establish a <strong>subtype</strong> relation.
A role of a given team is never conform (i.e., substitutable)
to any role of any <em>other</em> team.
<code>S.R</code> and <code>T.R</code> are always incommensurable.<br /><span class="underline">Note,</span> that this rule is a direct consequence of <a href="s1.2.2.e.html" title="&sect;1.2.2.(e)&nbsp;Conformance" class="sect">&sect;1.2.2.(e)</a>.
</p>
</div>
<div class="subsect depth4" id="s1.3.1.e">
<h4 class="subsect">(e)&nbsp;<span class="title">Dynamic binding of types</span><a class="img" href="s1.3.1.e.html"
title="PermaLink to (e)&nbsp;Dynamic binding of types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>Overriding an acquired role by a new role class has the following
implication: If an expression or declaration, which is evaluated on behalf of
an instance of team <code>T</code> or one of its contained roles,
refers to a role <code>R</code>, <code>R</code> will always
resolve to <code>T.R</code> even if <code>R</code> was introduced in
a super-team of <code>T</code> and even if the specific line of code
was inherited from a super-team or one of its roles.
Only the dynamic type of the enclosing team-instance is used to determine
the correct role class (see below for an example).
</p>
<p>A special case of dynamically binding role types relates to so-called class literals
(see <a href="http://java.sun.com/docs/books/jls/second_edition/html/expressions.doc.html#251530"
class="ext">JLS &sect;15.8.2</a>).
Role class literals are covered in <a href="s6.1.c.html" title="&sect;6.1.(c)&nbsp;Class literals for roles"
class="sect">&sect;6.1.(c)</a>.
</p>
<p>The above is strictly needed only for cases involving implicit inheritance.
It may, however, help intuition, to also consider the directly acquired
role <code>T.R</code> in (b) to override the given role <code>S.R</code>.
</p>
<div class="codecomment">
In line 17 of <a href="#l1.3.1-1" class="listing">Listing 1.3.1-1</a> the implicitly inherited method <code>n</code> is called
with the result of an invocation of <code>m</code>. Although
<code>n</code> was defined in <code>S</code> (thus with argument type
<code>S.R2, see line 6</code>) in the context of <code>T</code> it
expects an argument of <code>T.R2</code>. This is correctly provided by
the invocation of <code>m</code> in the context of <code>T</code>.
</div>
</div>
<div class="subsect depth4" id="s1.3.1.f">
<h4 class="subsect">(f)&nbsp;<span class="title">tsuper</span><a class="img" href="s1.3.1.f.html" title="PermaLink to (f)&nbsp;tsuper"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<div class="syntaxlink"><a href="sA.html#sA.5.4" title="&sect;A.5.4&nbsp;TSuperCall" class="syntax">&rarr;&nbsp;Syntax&nbsp;&sect;A.5.4</a></div>
<p>Super calls along implicit inheritance use the new keyword
<strong>tsuper</strong>. While <code>super</code> is still available
along regular inheritance, a call <code>tsuper.m()</code>
selects the version of <code>m</code> of the corresponding role
acquired from the super-team.
</p>
<p>See <a href="s2.4.2.html"
title="&sect;2.4.2&nbsp;Role creation via a regular constructor"
class="sect">&sect;2.4.2</a> for <code>tsuper</code>
in the context of role constructors.
</p>
<p><code>tsuper</code> can only be used to invoke a corresponding
version of the enclosing method or constructor, i.e., an expression
<code>tsuper.m()</code> may only occur within the method <code>m</code>
with both methods having the same signature
(see <a href="s2.3.2.b.html"
title="&sect;2.3.2.(b)&nbsp;Super in the context of declared lifting"
class="sect">&sect;2.3.2.(b)</a> for an exception, where both methods have slightly different signatures).
</p>
<div class="codecomment">
In <a href="#l1.3.1-1" class="listing">Listing 1.3.1-1</a> the role <code>R1</code> in team <code>T</code>
overrides the implicitly inherited method <code>m()</code> from <code>S</code>. <code><strong>tsuper</strong>.m()</code> calls the overridden method <code>m()</code>
from <code>S.R1</code> (line 13).
</div>
</div>
<div class="subsect depth4" id="s1.3.1.g">
<h4 class="subsect">(g)&nbsp;<span class="title">Implicitly inheriting super-types</span><a class="img" href="s1.3.1.g.html"
title="PermaLink to (g)&nbsp;Implicitly inheriting super-types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>If a role class has an explicit super class (using <code>extends</code>)
this relation is inherited along implicit inheritance.
</p>
<div class="codecomment">
In <a href="#l1.3.1-1" class="listing">Listing 1.3.1-1</a> the role <code>R1</code> in <code>T</code> has <code>T.R0</code>
as its implicitly inherited super class, because the corresponding role in the super-team
<code><strong>extends R0</strong></code> (line 3).
</div>
<p>Overriding an implicitly inherited super class is governed by
<a href="s1.3.2.b.html"
title="&sect;1.3.2.(b)&nbsp;Inheriting and overriding the extends clause"
class="sect">&sect;1.3.2.(b)</a>, below.<br />
The list of implemented interfaces is merged along implicit
inheritance.
</p>
</div>
<div class="subsect depth4" id="s1.3.1.h">
<h4 class="subsect">(h)&nbsp;<span class="title">Preserving visibility</span><a class="img" href="s1.3.1.h.html"
title="PermaLink to (h)&nbsp;Preserving visibility"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>A role class must provide at least as much access as the implicit super role,
or a compile-time error occurs (this is in analogy to <a href="http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#227965"
class="ext">JLS &sect;8.4.6.3</a>).
Access rights of methods overridden by implicit inheritance follow
the same rules as for normal overriding.
</p>
</div>
<div class="subsect depth4" id="s1.3.1.i">
<h4 class="subsect">(i)&nbsp;<span class="title">Dynamic binding of constructors</span><a class="img" href="s1.3.1.i.html"
title="PermaLink to (i)&nbsp;Dynamic binding of constructors"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>When creating a role instance using <code>new</code> not only the
type to instantiate is bound dynamically (cf. <a href="#s1.3.1.e" title="&sect;1.3.1.(e)&nbsp;Dynamic binding of types"
class="sect">&sect;1.3.1.(e)</a>), but also the constructor to
invoke is dynamically bound in accordance to the concrete
type.<br />
Within role constructors all <code>this(..)</code> and
<code>super(..)</code> calls are bound statically with respect to explicit inheritance
and dynamically with respect to implicit inheritance. This means the target role name is
determined statically, but using that name the suitable role type is determined
using dynamic binding.
<br />
See also <a href="s2.5.a.html"
title="&sect;2.5.(a)&nbsp;Using abstract classes for creation"
class="sect">&sect;2.5.(a)</a> on using constructors of abstract role classes.
</p>
</div>
<div class="subsect depth4" id="s1.3.1.j">
<h4 class="subsect">(j)&nbsp;<span class="title">Overriding and compatibility</span><a class="img" href="s1.3.1.j.html"
title="PermaLink to (j)&nbsp;Overriding and compatibility"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>The rules of <a href="http://java.sun.com/docs/books/jls/second_edition/html/classes.doc.html#228745"
class="ext">JLS &sect;8.4.6</a>
also apply to methods <em>and constructors</em> inherited via implicit inheritance.
</p>
</div>
<div class="subsect depth4" id="s1.3.1.k">
<h4 class="subsect">(k)&nbsp;<span class="title">Covariant return types</span><a class="img" href="s1.3.1.k.html"
title="PermaLink to (k)&nbsp;Covariant return types"><img style="vertical-align:text-top;margin-left:5px;" src="../images/permalink.png"
alt="" /></a></h4>
<p>Given a team <code>T1</code> with two roles <code>R1</code> and <code>R2</code> where <code>R2</code> explicitly inherits from <code>R1</code>, both roles defining
a method <code>m</code> returning some type <code>A</code>.
Given also a sub-team of <code>T1</code>, <code>T2</code>, where <code>T2.R1</code> overrides <code>m</code> with a covariant return type <code>B</code>
(sub-type of <code>A</code>):
</p>
<div class="listing plain"><pre> <b>public</b> <b>team</b> <b>class</b> T1 {
<b>protected</b> <b>abstract</b> <b>class</b> R1 {
<b>abstract</b> A m();
}
<b>protected</b> <b>class</b> R2 <b>extends</b> R1 {
A m() { <b>return</b> <b>new</b> A(); }
}
}
<b>public</b> <b>team</b> <b>class</b> T2 <b>extends</b> T1 {
<b>protected</b> <b>class</b> R1 {
@Override B m() { <b>return</b> <b>new</b> B(); } <span class="error">// this declaration renders <b>class</b> T2.R2 illegal</span>
}
}</pre></div>
<p>
In this situation role <code>T2.R2</code> will be illegal unless also overriding <code>m</code> with a return type that is at least <code>B</code>.
Note, that the actual error occurs at the implicitly inherited method <code>T2.R2.m</code> which is not visible in the source code,
even <code>T2.R2</code> need not be mentioned explicitly in the source code.
A compiler should flag this as an imcompatibility at the team level, because a team must specialize inherited roles
in a consistent way.
</p>
</div>
<h5 class="listing">Example code (Teams and Roles):</h5>
<div class="listing example frame" id="l1.3.1-3">
<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>protected</b> <b>class</b> MyRole {</pre></td>
</tr>
<tr class="line odd">
<td class="ln">3</td>
<td><pre> String name;</pre></td>
</tr>
<tr class="line even">
<td class="ln">4</td>
<td><pre> <b>public</b> MyRole (String n) { name = n; }</pre></td>
</tr>
<tr class="line odd">
<td class="ln">5</td>
<td><pre> <b>public</b> <b>void</b> print() { System.out.println("id="+name); }</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>protected</b> MyRole getRole() { <b>return</b> <b>new</b> MyRole("Joe"); }</pre></td>
</tr>
<tr class="line even">
<td class="ln">8</td>
<td><pre>}</pre></td>
</tr>
</table>
</div>
<div class="listing example frame" id="l1.3.1-4">
<table class="listing">
<tr class="line odd">
<td class="ln">10</td>
<td><pre><b>public</b> <b>team</b> <b>class</b> MySubTeam <b>extends</b> MyTeamA {</pre></td>
</tr>
<tr class="line even">
<td class="ln">11</td>
<td><pre> <b>protected</b> <b>class</b> MyRole {</pre></td>
</tr>
<tr class="line odd">
<td class="ln">12</td>
<td><pre> <b>int</b> age;</pre></td>
</tr>
<tr class="line even">
<td class="ln">13</td>
<td><pre> <b>public</b> <b>void</b> setAge(<b>int</b> a) { age = a; }</pre></td>
</tr>
<tr class="line odd">
<td class="ln">14</td>
<td><pre> <b>public</b> <b>void</b> print() {</pre></td>
</tr>
<tr class="line even">
<td class="ln">15</td>
<td><pre> tsuper.print();</pre></td>
</tr>
<tr class="line odd">
<td class="ln">16</td>
<td><pre> System.out.println("age="+age);</pre></td>
</tr>
<tr class="line even">
<td class="ln">17</td>
<td><pre> }</pre></td>
</tr>
<tr class="line odd">
<td class="ln">18</td>
<td><pre> }</pre></td>
</tr>
<tr class="line even">
<td class="ln">19</td>
<td><pre> <b>public</b> <b>void</b> doit() {</pre></td>
</tr>
<tr class="line odd">
<td class="ln">20</td>
<td><pre> MyRole r = getRole();</pre></td>
</tr>
<tr class="line even">
<td class="ln">21</td>
<td><pre> r.setAge(27);</pre></td>
</tr>
<tr class="line odd">
<td class="ln">22</td>
<td><pre> r.print();</pre></td>
</tr>
<tr class="line even">
<td class="ln">23</td>
<td><pre> }</pre></td>
</tr>
<tr class="line odd">
<td class="ln">24</td>
<td><pre>}</pre></td>
</tr>
<tr class="line even">
<td class="ln">25</td>
<td><pre>...</pre></td>
</tr>
<tr class="line odd">
<td class="ln">26</td>
<td><pre>MySubTeam myTeam = <b>new</b> MySubTeam();</pre></td>
</tr>
<tr class="line even">
<td class="ln">27</td>
<td><pre>myTeam.doit();</pre></td>
</tr>
</table>
</div>
<h5 class="listing">Program output</h5>
<div class="listing example frame"><pre>id=Joe
age=27</pre></div>
<div class="codecomment">
<h5>Effects:</h5>
<ul>
<li>According to <a href="s1.3.html"
title="&sect;1.3&nbsp;Acquisition and implicit inheritance of role classes"
class="sect">&sect;1.3</a>, <code>MyTeamA</code> implements
<code>ITeam</code> (line 1).
</li>
<li>An implicit role inheritance is created for
<code>MySubTeam.MyRole</code> (<a href="#s1.3.1.c"
title="&sect;1.3.1.(c)&nbsp;Overriding and implicit inheritance"
class="sect">&sect;1.3.1.(c)</a>; line&nbsp;11).<br />
If we visualize this special inheritance using a fictitious keyword
<code>overrides</code> the compiler would see a declaration:
<div class="listing plain"><pre><b>protected</b> <b>class</b> MyRole <em>overrides MyTeamA.MyRole</em> { ... }</pre></div>
</li>
<li>Invoking <code>getRole()</code> on <code>myTeam</code> (line&nbsp;27, 20)
creates an instance of <code>MySubTeam.MyRole</code> because the
acquired role <code>MyTeamA.MyRole</code> is overridden by
<code>MySubTeam.MyRole</code>
following the rules of implicit inheritance (cf. <a href="#s1.3.1.e" title="&sect;1.3.1.(e)&nbsp;Dynamic binding of types"
class="sect">&sect;1.3.1.(e)</a>).
</li>
<li>Overriding of role methods and access to inherited features works as usual.
</li>
<li>As an example for <a href="#s1.3.1.f" title="&sect;1.3.1.(f)&nbsp;tsuper" class="sect">&sect;1.3.1.(f)</a> see the call <code>tsuper.print()</code>
(line&nbsp;15), which selects the implementation of <code>MyTeamA.MyRole.print</code>.
</li>
</ul>
</div>
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