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POIs string concatenaion really that slow?
 

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  1. POIs string concatenaion really that slow?
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    <p>I'm currently looking into String concat options and the penalty they have on the overall performance. And my test-case creates results that blow my mind, I'm not sure if I'm overlooking something.</p> <p>Here is the deal: Doing <code>"something"+"somethingElse"</code> in java will (at compile-time) create a new <code>StringBuilder</code> every time this is done.</p> <p>For my test-case, I'm loading a file from my HDD that has <strong>1661 lines of example data</strong> (classic "Lorem Ipsum"). This question is <strong>not about the I/O performance</strong>, but about the performance of the different string concat methods.</p> <pre><code>public class InefficientStringConcat { public static void main(String[] agrs) throws Exception{ // Get a file with example data: System.out.println("Starting benchmark"); // Read an measure: for (int i = 0; i &lt; 10; i++){ BufferedReader in = new BufferedReader( new InputStreamReader(new FileInputStream(new File("data.txt"))) ); long start = System.currentTimeMillis(); // Un-comment method to test: //inefficientRead(in); //betterRead(in); long end = System.currentTimeMillis(); System.out.println("Took "+(end-start)+"ms"); in.close(); } } public static String betterRead(BufferedReader in) throws IOException{ StringBuilder b = new StringBuilder(); String line; while ((line = in.readLine()) != null){ b.append(line); } return b.toString(); } public static String inefficientRead(BufferedReader in) throws IOException { String everything = "", line; while ((line = in.readLine()) != null){ everything += line; } return everything; } } </code></pre> <p>As you can see, the setup is the same for both tests. Here are the results:</p> <p>Using <strong><code>inefficientRead()</code>-method</strong>:</p> <pre><code>Starting benchmark #1 Took 658ms #2 Took 590ms #3 Took 569ms #4 Took 567ms #5 Took 562ms #6 Took 570ms #7 Took 563ms #8 Took 568ms #9 Took 560ms #10 Took 568ms </code></pre> <p>Using <strong><code>betterRead()</code>-method</strong></p> <pre><code>Starting benchmark #1 Took 42ms #2 Took 10ms #3 Took 5ms #4 Took 7ms #5 Took 16ms #6 Took 3ms #7 Took 4ms #8 Took 5ms #9 Took 5ms #10 Took 13ms </code></pre> <p>I'm running the tests with <strong>no extra parameters</strong> to the <code>java</code>-command. I'm running a <a href="http://en.wikipedia.org/wiki/Mac_Mini#Specifications_2">MacMini3,1 from early 2009</a> and Sun JDK 7:</p> <pre><code>[luke@BlackBox ~]$ java -version java version "1.7.0_09" Java(TM) SE Runtime Environment (build 1.7.0_09-b05) Java HotSpot(TM) Client VM (build 23.5-b02, mixed mode) </code></pre> <p>This strikes me as a very heavy difference. Am I doing something wrong in measuring this, or is this supposed to happen?</p>
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    1. USLukas Knuth
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    1. COWell, 10 iterations is not nearly enough for a benchmark, but apart from that it *seems* fine. Try increasing the number of iterations to, say, 100,000 and add a "warm-up" phase where you call the method 10,000 times (without timing it) to let the JIT do some inlining.
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      1. POIs string concatenaion really that slow?
      1. USCameron Skinner
    2. CO@CameronSkinner Apparently you don't take into account that each high-level iteration already consists of 1661 method invocations.
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      1. POIs string concatenaion really that slow?
      1. USMarko Topolnik
    3. CO@MarkoTopolnik: The benchmark is being measured across the `betterRead` or `inefficientRead` methods, so those are the methods that you should call multiple times. So you are correct: You **don't** take into account the implementation details of the methods under test.
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