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copied!<p>This question is flawed - an extended <code>Object</code> is always an instance of its super class.</p> <p>In your example, the following is valid...</p> <pre><code>public class PartialFile extends File{ // methods } PartialFile partFile = new PartialFile(); // do operations on partFile File file = partFile; // do operations on file </code></pre> <p>Because <code>PartialFile</code> extends <code>File</code>, you don't really need to 'turn it into its super class' - it already is an instance of the super class.</p> <p>You can think of the <code>PartialFile</code> object as being a <code>PartialFile</code>, a <code>File</code>, an <code>Object</code>, and any other classes that it extends, all at the same time. You don't need to turn it between these different class types - just use it directly. For the above code, if you want to call the <code>File.rename()</code> method, both the following statements will do the same thing...</p> <pre><code>partFile.rename(); file.rename(); </code></pre> <p>You don't need to change the <code>partFile</code> into a <code>file</code> to use the methods of the <code>File</code> object - just use them directly, as the Java VM knows that a <code>PartialFile</code> is also a <code>File</code>.</p> <p>You can't really change a sub-type to be its super type - the Java VM will always know what type it really is - but you can trick it. If you use the following code...</p> <pre><code>PartialFile partFile = new PartialFile(); // do operations on partFile File file = partFile; // do operations on file </code></pre> <p>and then only every use <code>file</code> instead of <code>partFile</code>, you won't be able to use any of the <code>PartialFile</code> methods on it. It kinda disguises that its a <code>PartialFile</code>, but it doesn't actually convert it.</p> <p>If you do something like this...</p> <pre><code>public File createFile(){ PartialFile partFile = new PartialFile(); // operations on partFile return partFile; } File file = createFile(); // operations on file </code></pre> <p>You can create and use a <code>PartialFile</code> in the method, but when you're finished you return it as a simple <code>File</code>. Whenever you refer to this object from now on, the Java VM will pretend that its a <code>File</code>. However, technically it will always be a <code>PartialFile</code>, and it doesn't stop you casting it back into a <code>PartialFile</code> if you want to, like so...</p> <pre><code>PartialFile partFile = (PartialFile)file; // operations on partFile </code></pre> <p>So its just a disguise.</p>

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