build Interface to define two final classes for typing a field
<p>I have two <code>final</code> classes : <code>Scanner</code> and <code>CsvReader</code>.</p>

<p>I want a field : <code>private MyInterface scan;</code> able to receive a <em>Scanner instance</em> or a <em>CsvReader instance</em>, and only thoses types.</p>

<p>How build <em>MyInterface</em> ?</p>
