curious bob - Page 2 - Developer IT

Java curious Loop Performance

- by user1680583

I have a big problem while evaluate my java code. To simplify the problem I wrote the following code which produce the same curious behavior. Important is the method run() and given double value rate. For my runtime test (in the main method) I set the rate to 0.5 one times and 1.0 the other time. With the value 1.0 the if-statement will be executed in each loop iteration and with the value 0.5 the if-statement will be executed half as much. For this reason I expected longer runtime by the first case but opposite is true. Can anybody explain me this phenomenon?? The result of main: Test mit rate = 0.5 Length: 50000000, IF executions: 25000856 Execution time was 4329 ms. Length: 50000000, IF executions: 24999141 Execution time was 4307 ms. Length: 50000000, IF executions: 25001582 Execution time was 4223 ms. Length: 50000000, IF executions: 25000694 Execution time was 4328 ms. Length: 50000000, IF executions: 25004766 Execution time was 4346 ms. ================================= Test mit rate = 1.0 Length: 50000000, IF executions: 50000000 Execution time was 3482 ms. Length: 50000000, IF executions: 50000000 Execution time was 3572 ms. Length: 50000000, IF executions: 50000000 Execution time was 3529 ms. Length: 50000000, IF executions: 50000000 Execution time was 3479 ms. Length: 50000000, IF executions: 50000000 Execution time was 3473 ms. The Code public ArrayList<Byte> list = new ArrayList<Byte>(); public final int LENGTH = 50000000; public PerformanceTest(){ byte[]arr = new byte[LENGTH]; Random random = new Random(); random.nextBytes(arr); for(byte b : arr) list.add(b); } public void run(double rate){ byte b = 0; int count = 0; for (int i = 0; i < LENGTH; i++) { if(getRate(rate)){ list.set(i, b); count++; } } System.out.println("Length: " + LENGTH + ", IF executions: " + count); } public boolean getRate(double rate){ return Math.random() < rate; } public static void main(String[] args) throws InterruptedException { PerformanceTest test = new PerformanceTest(); long start, end; System.out.println("Test mit rate = 0.5"); for (int i = 0; i < 5; i++) { start=System.currentTimeMillis(); test.run(0.5); end = System.currentTimeMillis(); System.out.println("Execution time was "+(end-start)+" ms."); Thread.sleep(500); } System.out.println("================================="); System.out.println("Test mit rate = 1.0"); for (int i = 0; i < 5; i++) { start=System.currentTimeMillis(); test.run(1.0); end = System.currentTimeMillis(); System.out.println("Execution time was "+(end-start)+" ms."); Thread.sleep(500); } }

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