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copied!<p>I'm looking through the Java source code, and have found something interesting which is indicative of the <code>IOException</code> reason. <code>InputStream</code> <strong>is an abstract class</strong>. We therefore can't predict the kind of input which will be closed, so it's good to keep information flowing. </p> <p>Whatever code uses that input stream needs to be able to throw an <code>IOException</code>, because there is a chance that closing the input stream may fail. If it fails, whatever's using the implementation needs to know about it, because there's a good chance it needs to be handled. </p> <p>It's important to understand the layout of the Exception structure in Java. Every exception, of course, extends <code>Exception</code>. However, there are also broader categories of exceptions: <code>java.lang.IOException</code> is one of these, and covers all possible input/output exceptions. When we say there has been an I/O error, we're referencing anything which falls under <code>IOException</code>. As a result, many exceptions extends this one, e.g. <code>FileNotFoundException</code>, <code>EOFException</code>, etc. as it's important to have a broad, overarching exception to manage these.</p> <p>As a result, any IO class will need to be able to throw <em>any</em> of the various IOExceptions on close. <code>close()</code> therefore must throw <code>IOException</code> - this gives its implementation the ability to throw any of the extensions of <code>IOException</code> as well. This is why <code>close()</code> throws <code>IOException</code> - exceptions are inherited, and you need to be able to any of the IOExceptions when closing a stream.</p> <hr> <p>Here are a couple scenarios worth noting:</p> <ul> <li>You can't close an IOStream twice, though this generally doesn't throw an exception if you do</li> <li>The content becomes inaccessible (e.g. a disk was unmounted) (The <code>close()</code> is actually critical for the operating system, as it needs to have an indicator of when the file is no longer busy)</li> <li>The generic source has closed</li> <li>Any generic failure not covered by all other subclasses of <code>IOException</code> (e.g. <code>FileNotFoundException</code>)</li> </ul> <p>You can check what caused an <code>IOException</code> by running <code>Exception.getMessage()</code>. </p>

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