org.aspectj/docs/adk15ProgGuideDB/enumeratedtypes.xml

<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>