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<chapter id="covariance" xreflabel="Covariance">
<title>Covariance</title>
<sect1 id="covariance-inJava5">
<title>Covariance in Java 5</title>
<para>
Java 5 (and hence AspectJ 5) allows you to narrow the return type
in an overriding method. For example:
</para>
<programlisting><![CDATA[
class A {
public A whoAreYou() {...}
}
class B extends A {
// override A.whoAreYou *and* narrow the return type.
public B whoAreYou() {...}
}
]]></programlisting>
</sect1>
<sect1 id="covariance-and-join-point-matching">
<title>Covariant methods and Join Point matching</title>
<para>The join point matching rules for <literal>call</literal>
and <literal>execution</literal> pointcut designators are extended
to match against covariant methods.</para>
<para>
Given the classes <literal>A</literal> and <literal>B</literal>
as defined in the previous section, and the program fragment
</para>
<programlisting><![CDATA[
A a = new A();
B b = new B();
a.whoAreYou();
b.whoAreYou();
]]></programlisting>
<para>The signatures for the call join point <literal>a.whoAreYou()</literal> are
simply:</para>
<programlisting><![CDATA[
A A.whoAreYou()
]]></programlisting>
<para>The signatures for the call join point <literal>b.whoAreYou()</literal> are:
</para>
<programlisting><![CDATA[
A A.whoAreYou()
B B.whoAreYou()
]]></programlisting>
<para>Following the join point matching rules given in <xref linkend="jpsigs"/>,</para>
<variablelist>
<varlistentry>
<term>call(* whoAreYou())</term>
<listitem>
<para>Matches both calls, (since each call join point has at least
one matching signature).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>call(* A.whoAreYou())</term>
<listitem>
<para>Matches both calls, (since each call join point has at least
one matching signature).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>call(A whoAreYou())</term>
<listitem>
<para>Matches both calls, (since each call join point has at least
one matching signature).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>call(A B.whoAreYou())</term>
<listitem>
<para>Does not match anything - neither of the call join points
has a signature matched by this pattern. A lint warning is
given for the call <literal>a.whoAreYou()</literal> ("does not match
because declaring type is A, if match required use target(B)").
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>call(A+ B.whoAreYou())</term>
<listitem>
<para>Matches the call to <literal>b.whoAreYou()</literal> since
the signature pattern matches the signature <literal>B B.whoAreYou()</literal>.
A lint warning is given for the call <literal>a.whoAreYou()</literal> ("does not match
because declaring type is A, if match required use target(B)").
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>call(B A.whoAreYou())</term>
<listitem>
<para>Does not match anything since neither join point has a
signature matched by this pattern.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>call(B whoAreYou())</term>
<listitem>
<para>Matches the call to <literal>b.whoAreYou()</literal> only.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>call(B B.whoAreYou())</term>
<listitem>
<para>Matches the call to <literal>b.whoAreYou()</literal> only.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>The rule for signature matching at call and execution join points
is unchanged from AspectJ 1.2: a call or execution pointcut matches if
the signature pattern matches at least one of the signatures of the
join point, and if the modifiers of the method or constructor are matched
by any modifier pattern or annotation pattern that may be present.</para>
</sect1>
</chapter>
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