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POFastest way to tell if a string is a valid date
 

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  1. POFastest way to tell if a string is a valid date
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    <p>I am supporting a common library at work that performs many checks of a given string to see if it is a valid date. The Java API, commons-lang library, and JodaTime all have methods which can parse a string and turn it in to a date to let you know if it is actually a valid date or not, but I was hoping that there would be a way of doing the validation without actually creating a date object (or DateTime as is the case with the JodaTime library). For example here is a simple piece of example code:</p> <pre><code>public boolean isValidDate(String dateString) { SimpleDateFormat df = new SimpleDateFormat("yyyyMMdd"); try { df.parse(dateString); return true; } catch (ParseException e) { return false; } } </code></pre> <p>This just seems wasteful to me, we are throwing away the resulting object. From my benchmarks about 5% of our time in this common library is spent validating dates. I'm hoping I'm just missing an obvious API. Any suggestions would be great!</p> <p><strong>UPDATE</strong></p> <p>Assume that we can always use the same date format at all times (likely yyyyMMdd). I did think about using a regex as well, but then it would need to be aware of the number of days in each month, leap years, etc...</p> <hr> <p><strong>Results</strong></p> <p>Parsed a date 10 million times</p> <pre><code>Using Java's SimpleDateFormat: ~32 seconds Using commons-lang DateUtils.parseDate: ~32 seconds Using JodaTime's DateTimeFormatter: ~3.5 seconds Using the pure code/math solution by Slanec: ~0.8 seconds Using precomputed results by Slanec and dfb (minus filling cache): ~0.2 seconds </code></pre> <p>There were some very creative answers, I appreciate it! I guess now I just need to decide how much flexibility I need what I want the code to look like. I'm going to say that dfb's answer is correct because it was purely the fastest which was my original questions. Thanks!</p>
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    1. COThrowing Exceptions tends to be "heavy". Have you considered using regular expressions for validation ?
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    2. COWell, one thing you could do, if you end up having many of the same strings to validate, is use some kind of memoization technique.
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    3. COEuropean date format (DD/MM/YYYY) or US date format (MM/DD/YYYY)? Good luck.
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