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<chapter id="enumeratedtypes" xreflabel="Enumerated Types">
<title>Enumerated Types</title>
<sect1 id="enums-in-java5">
<title>Enumerated Types in Java 5</title>
<para>Java 5 (and hence AspectJ 5) provides explicit support for
enumerated types. In the simplest case, you can declare an enumerated
type as follows:</para>
<programlisting><![CDATA[
public enum ProgrammingLanguages {
COBOL, C, JAVA, ASPECTJ
}
]]></programlisting>
<para>Enumerated types are just classes, and they can contain method
and field declarations, and may implement interfaces. Enums may only
have private constructors, and may not be extended.</para>
<para>Enumerated types in Java 5 all implicitly extend the type
<literal>java.lang.Enum</literal>. It is illegal to explicitly
declare a subtype of this class.</para>
</sect1>
<sect1 id="enums-in-aspectj5">
<title>Enumerated Types in AspectJ 5</title>
<para>
AspectJ 5 supports the declaration of enumerated types just as Java 5
does. Because of the special restrictions Java 5 places around enumerated
types, AspectJ makes the following additional restrictions:
</para>
<itemizedlist>
<listitem>You cannot use declare parents to change the super type of
an enum.</listitem>
<listitem>You cannot use declare parents to declare java.lang.Enum as
the parent of any type.</listitem>
<listitem>You cannot make inter-type constructor declarations on an
enum.</listitem>
<listitem>You cannot extend the set of values in an enum via any
ITD-like construct.</listitem>
<listitem>You cannot make inter-type method or field declarations on
an enum.</listitem>
<listitem>You cannot use declare parents to make an enum type implement
an interface.</listitem>
</itemizedlist>
<para>In theory, the last of these two items <emphasis>could</emphasis>
be supported. However, AspectJ 5 follows the simple rule that <emphasis>
an enum type cannot be the target of an inter-type declaration or declare
parents statement</emphasis>. This position may be relaxed in a future
version of AspectJ.</para>
<para>If an enum is named explicitly as the target of a
declare parents statement, a compilation error will result. If an enumerated
type is matched by a non-explicit type pattern used in a declare parents
statement it will be ignored (and an XLint warning issued).</para>
</sect1>
</chapter>
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