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POWhy doesn't String's hashCode() cache 0?
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  1. POWhy doesn't String's hashCode() cache 0?
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    copied!<p>I noticed in the Java 6 source code for String that hashCode only caches values other than 0. The difference in performance is exhibited by the following snippet:</p> <pre><code>public class Main{ static void test(String s) { long start = System.currentTimeMillis(); for (int i = 0; i &lt; 10000000; i++) { s.hashCode(); } System.out.format("Took %d ms.%n", System.currentTimeMillis() - start); } public static void main(String[] args) { String z = "Allocator redistricts; strict allocator redistricts strictly."; test(z); test(z.toUpperCase()); } } </code></pre> <p><a href="http://www.ideone.com/Ho9lfOKy" rel="noreferrer">Running this in ideone.com</a> gives the following output:</p> <pre><code>Took 1470 ms. Took 58 ms. </code></pre> <p>So my questions are:</p> <ul> <li>Why doesn't String's hashCode() cache 0?</li> <li>What is the probability that a Java string hashes to 0?</li> <li>What's the best way to avoid the performance penalty of recomputing the hash value every time for strings that hash to 0?</li> <li>Is this the best-practice way of caching values? (i.e. cache all except one?)</li> </ul> <hr> <p>For your amusement, each line here is a string that hash to 0:</p> <pre><code>pollinating sandboxes amusement &amp; hemophilias schoolworks = perversive electrolysissweeteners.net constitutionalunstableness.net grinnerslaphappier.org BLEACHINGFEMININELY.NET WWW.BUMRACEGOERS.ORG WWW.RACCOONPRUDENTIALS.NET Microcomputers: the unredeemed lollipop... Incentively, my dear, I don't tessellate a derangement. A person who never yodelled an apology, never preened vocalizing transsexuals. </code></pre>
 

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