Re: Hash table performance

I profiled his example in net beans.

That's my JVM
C:\Users\Rze=BCnik\Documents\java>java -version
java version "1.6.0_17"
Java(TM) SE Runtime Environment (build 1.6.0_17-b04)
Java HotSpot(TM) Client VM (build 14.3-b01, mixed mode, sharing)

Here is the code I used:

package hashmapexample;

import java.util.HashMap;

/**
 *
 * @author Rze=BCnik
 */
public class Main {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        HashMap<Double, Double> hashtable = new HashMap<Double,=

();
        for (int i = 1; i <= 1000000; ++i) { /* changed upper=

1m - sorry no, patience */
            double x = i;
            hashtable.put(x, 1.0 / x);
        }

        System.out.println("hashtable(100.0) = " + hashtable.ge=

(100.0));
    }

}

I used -Xms512m -Xmx512m to eliminate extensive collections.

The results of profiling are as follows:
54.2% of time spent in java.util.HashMap.put(Object, Object) (1m
invocations)
of which:
* * 19.5% in java.util.HashMap.addEntry(int, Object, Object, int)
* * * * 11.1% in java.util.HashMap.resize(int) (17 invocations)
<--- !!!
* * * * 3.3% self-time
* * * * 1.4% in java.util.HashMap$Entry.<init>(int, Object, Object,
java.util.HashMap.Entry) <-- so the cost of allocating entries is
negligible
* * 8.1% in java.lang.Double.hashCode() <--- that's too much (?)
... rest of put omitted, circa 1%

Now, the interesting part is
30.3% of time spent in java.lang.Double.valueOf(double) <--- that's
boxing
Furthermore, there were 2m + 1 calls to new Double meaning that no
caching occurred.