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POJava: why Array (w index) *order of magnitude* faster than Map (key) access?
 

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  1. POJava: why Array (w index) *order of magnitude* faster than Map (key) access?
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    <p>We ran the attached test. The results consistently show that access via Array by index is 10x faster than access via Map by key. This order of magnitude difference surprised us.</p> <p>Our key for the Map is java.lang.String ... is the cost of computing the Map key's java.lang.String.hashcode() implementation the exclusive reason? in the attached code, I exercised using only one key </p> <pre><code>java.lang.String key = 1; </code></pre> <p>Doesn't the compiler/runtime cache in this case? Or does it recompute on every invoke?</p> <p>Thanks for any insights.</p> <pre><code>public class PerfTest { static java.util.HashMap&lt;String, Double&gt; map; static Double[] array = {1.0,2.0,3.0,4.0,5.0,6.0,7.0,8.0,9.0,10.0}; static long nTimes = 1000000; static{ map = new java.util.HashMap&lt;String, Double&gt;(); map.put("1", new Double(1)); map.put("2", new Double(2)); map.put("3", new Double(3)); map.put("4", new Double(4)); map.put("5", new Double(5)); map.put("6", new Double(6)); map.put("7", new Double(7)); map.put("8", new Double(8)); map.put("9", new Double(9)); map.put("10", new Double(10)); } public static void main(String[] args){ PerfTest tester = new PerfTest(); long timeInMap = tester.testHashMap(); long timeInArray = tester.testArray(); System.out.println("Corrected time elapsed in map(in seconds): " + (timeInMap)/1000000000.0); System.out.println("Corrected time elapsed in array(in seconds): " + (timeInArray)/1000000000.0); } private long testHashMap(){ int sz = map.size(); long startTime = System.nanoTime(); String key = "1"; for (int i=0; i &lt;nTimes; i++){ double sum = 0; for (int j =1; j&lt;=sz; j++){ sum += map.get(key); } } return (System.nanoTime() - startTime); } private long testArray(){ long startTime = System.nanoTime(); for (int i=0; i &lt;nTimes; i++){ double sum = 0; for (int j=0; j&lt; array.length; j++) { sum += array[j]; } } return (System.nanoTime() - startTime); } } </code></pre>
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    1. COWhich VM did you use (vendor, version)?
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    2. COWhy would you possibly be surprised that a direct array lookup is notably slower than a hash table query?
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    3. COwe used Oracle's 1.7 on windows 7 (64 bit)
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