Editing Aspect-oriented programming (section)

===Inter-type declarations===
''Inter-type declarations'' provide a way to express cross-cutting concerns affecting the structure of modules. Also known as ''open classes'' and ''[[extension method]]s'', this enables programmers to declare in one place members or parents of another class, typically to combine all the code related to a concern in one aspect. For example, if a programmer implemented the cross-cutting display-update concern using visitors, an inter-type declaration using the [[visitor pattern]] might look like this in AspectJ:

<syntaxhighlight lang="aspectj">
  aspect DisplayUpdate {
    void Point.acceptVisitor(Visitor v) {
      v.visit(this);
    }
    // other crosscutting code...
  }
</syntaxhighlight>

This code snippet adds the <code>acceptVisitor</code> method to the <code>Point</code> class.

Any structural additions are required to be compatible with the original class, so that clients of the existing class continue to operate, unless the AOP implementation can expect to control all clients at all times.