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copied!<p>You can revert your thinking - try to fail as quickly as possible when the String <em>definitely</em> is no date:</p> <ul> <li>it's <code>null</code></li> <li>its <code>length</code> is not 8 (based on your example date format!)</li> <li>it <a href="http://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/StringUtils.html#isNumeric%28java.lang.CharSequence%29" rel="nofollow noreferrer">contains anything other that a number</a> (if your date format is only for numerical dates)</li> </ul> <p>If none of those apply, then try to parse it - preferably with a pre-made static <code>Format</code> object, don't create one on every method run.</p> <hr> <p><em>EDIT after comments</em></p> <p>Based on <a href="https://stackoverflow.com/questions/10013998/fastest-way-to-parse-a-yyyymmdd-date-in-java/10014172#10014172">this neat trick</a>, I wrote a fast validation method. It looks ugly, but is significantly faster than the usual library methods (which should be used in any standard situation!), because <em>it relies on your specific date format</em> and does not create a <code>Date</code> object. It handles the date as an <code>int</code> and goes on from that.</p> <p>I tested the <code>daysInMonth()</code> method just a little bit (<a href="https://stackoverflow.com/a/1021332/1273080">the leap year condition taken from Peter Lawrey</a>), so I hope there's no apparent bug.</p> <p>A quick (estimated!) microbenchmark indicated a speedup by a factor of 30.</p> <pre><code>public static boolean isValidDate(String dateString) { if (dateString == null || dateString.length() != "yyyyMMdd".length()) { return false; } int date; try { date = Integer.parseInt(dateString); } catch (NumberFormatException e) { return false; } int year = date / 10000; int month = (date % 10000) / 100; int day = date % 100; // leap years calculation not valid before 1581 boolean yearOk = (year >= 1581) && (year <= 2500); boolean monthOk = (month >= 1) && (month <= 12); boolean dayOk = (day >= 1) && (day <= daysInMonth(year, month)); return (yearOk && monthOk && dayOk); } private static int daysInMonth(int year, int month) { int daysInMonth; switch (month) { case 1: // fall through case 3: // fall through case 5: // fall through case 7: // fall through case 8: // fall through case 10: // fall through case 12: daysInMonth = 31; break; case 2: if (((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0)) { daysInMonth = 29; } else { daysInMonth = 28; } break; default: // returns 30 even for nonexistant months daysInMonth = 30; } return daysInMonth; } </code></pre> <p>P.S. Your example method above will return <code>true</code> for "99999999". Mine will only return true for existent dates :).</p>

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