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copied!<p>Maybe not the answer you wanted, but as you gave us very little information ...</p> <p>In unixoid systems (Linux, Mac, *BSD) you have the <a href="http://linux.die.net/man/1/file" rel="nofollow">file</a> command, that</p> <blockquote> <p>tests each argument in an attempt to classify it. There are three sets of tests, performed in this order: filesystem tests, magic tests, and language tests. The first test that succeeds causes the file type to be printed.</p> </blockquote> <p>E.g.</p> <pre><code>$ file linux-image-3.1.0-030100rc10-generic_3.1.0-030100rc10.201110200610_amd64.deb linux-image-3.1.0-030100rc10-generic_3.1.0-030100rc10.201110200610_amd64.deb: Debian binary package (format 2.0) </code></pre> <p>Using <a href="http://download.oracle.com/javase/6/docs/api/java/lang/Runtime.html#exec%28java.lang.String%5b%5d,%20java.lang.String%5b%5d,%20java.io.File%29" rel="nofollow">Runtime.exec(...)</a> you could invoke that program and parse its output.</p> <h3>Edit 1:</h3> <p>To determine if a given file is a <a href="http://en.wikipedia.org/wiki/Portable_Network_Graphics" rel="nofollow">PNG</a>:</p> <pre><code>import java.io.*; public class IsPng { public static void main(String ...filenames) throws Exception { if(filenames.length == 0) { System.err.println("Please supply filenames."); return; } for(String filename : filenames) { if(isPng(new File(filename))) { System.out.println(filename + " is a png."); } else { System.out.println(filename + " is _not_ a png."); } } } private static final int MAGIC[] = new int[] { 0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a }; private static boolean isPng(File filename) throws Exception { FileInputStream ins = new FileInputStream(filename); try { for(int i = 0; i < MAGIC.length; ++i) { if(ins.read() != MAGIC[i]) { return false; } } return true; } finally { ins.close(); } } } </code></pre> <h3>Edit 2:</h3> <p>Sometimes <a href="http://download.oracle.com/javase/6/docs/api/java/net/URLConnection.html#getContentType%28%29" rel="nofollow">URLConnection.getContentType()</a> works, too, for local files:</p> <pre><code>new File(name).toURI().toURL().openConnection().getContentType() </code></pre> <p>But your comments sound like you <em>have to</em> implement the method by yourself, not using external programs (?).</p>

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