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copied!<p>This is fascinating. Well, first of all, that is not a fair test; you aren't warming up the JVM when you do it that way. Benchmarking is typically very difficult to do. I refactored your code to use <a href="http://code.google.com/p/caliper/" rel="nofollow">Google Caliper</a>, and got similar, yet different results; the immutable class was only 3 times slower. Not sure why yet. Anyway here's the work so far:</p> <h2>TestImmutableSpeed.java</h2> <pre><code>import com.google.caliper.Runner; import com.google.caliper.SimpleBenchmark; public class TestImmutableSpeed { static final class MutableInt { private int value; public int getValue() { return value; } public void setValue(int value) { this.value = value; } public MutableInt() { this(0); } public MutableInt(int value) { this.value = value; } } static final class ImmutableInt { private final int value; public ImmutableInt(int value) { this.value = value; } public int getValue() { return value; } } public static class TestBenchmark extends SimpleBenchmark { public void timeMutable(final int arrLen) { MutableInt[] arrMutable = new MutableInt[arrLen]; for (int i = 0; i < arrMutable.length; ++i) { arrMutable[i] = new MutableInt(i); for (int j = 0; j < arrMutable.length; ++j) { arrMutable[i].setValue(arrMutable[i].getValue() + j); } } long sumMutable = 0; for (MutableInt item : arrMutable) { sumMutable += item.getValue(); } System.out.println(sumMutable); } public void timeImmutable(final int arrLen) { ImmutableInt[] arrImmutable = new ImmutableInt[arrLen]; for (int i = 0; i < arrImmutable.length; ++i) { arrImmutable[i] = new ImmutableInt(i); for (int j = 0; j < arrImmutable.length; ++j) { arrImmutable[i] = new ImmutableInt(arrImmutable[i].getValue() + j); } } long sumImmutable = 0; for (ImmutableInt item : arrImmutable) { sumImmutable += item.getValue(); } System.out.println(sumImmutable); } } public static void main(String[] args) { Runner.main(TestBenchmark.class, new String[0]); } } </code></pre> <h2>Caliper Output</h2> <pre><code> 0% Scenario{vm=java, trial=0, benchmark=Immutable} 78574.05 ns; σ=21336.61 ns @ 10 trials 50% Scenario{vm=java, trial=0, benchmark=Mutable} 24956.94 ns; σ=7267.78 ns @ 10 trials benchmark us linear runtime Immutable 78.6 ============================== Mutable 25.0 ========= vm: java trial: 0 </code></pre> <h2>String update</h2> <p>So I was thinking about this more, and I decided to try changing the wrapped class from an <code>int</code> to an object, in this case a <code>String</code>. Changing the static classes to <code>String</code>s, and loading the strings with <code>Integer.valueOf(i).toString()</code>, and instead of adding, appending them in a <code>StringBuilder</code>, I got these results:</p> <pre><code> 0% Scenario{vm=java, trial=0, benchmark=Immutable} 11034616.91 ns; σ=7006742.43 ns @ 10 trials 50% Scenario{vm=java, trial=0, benchmark=Mutable} 9494963.68 ns; σ=6201410.87 ns @ 10 trials benchmark ms linear runtime Immutable 11.03 ============================== Mutable 9.49 ========================= vm: java trial: 0 </code></pre> <p>However, I think in this case the difference is dominated by all the array copying that would have to happen, and not the fact that it was using <code>String</code>s.</p>

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