Java performance in creating objects

I'm trying to improve my knowledge in Java performance optimization and I tried multiple approach to create an object.
I came across this behaviour I'm not familiar with regarding usage of final members in a class: the creation of an object is far less expensive (in terms of time) if the members are not final. Is it correct or is there some floaws in my code?

The object with not final members:

public class ComplexNumber {
    private double re, im;

    public ComplexNumber(double _re, double _im) {
        re = _re;
        im = _im;
    }

    public void setRe (double _re) {
        re = _re;
    }

    public void setIm (double _im) {
        im = _im;
    }

    @Override
    public String toString() {
        return re + " + i" + im;
    }

    @Override
    public int hashCode() {
        return 47 + 31*(int)re + 31*(int)im;
    }
}  

The object with final members:

public class FinalComplexNumber {
    private final double re, im;

    public FinalComplexNumber(double _re, double _im) {
        re = _re;
        im = _im;
    }

    @Override
    public String toString() {
        return re + " + i" + im;
    }

    @Override
    public int hashCode() {
        return 47 + 31*(int)re + 31*(int)im;
    }
}  

The main class

public class PerformanceTest {

    private static final long ITERATIONS = 100000000l;

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {


        ComplexNumber nr = new ComplexNumber(0, 0);
        System.out.println(nr);

        long time = System.currentTimeMillis();
        for (int i = 0; i < ITERATIONS; i++) {
            ComplexNumber num = new ComplexNumber(i, i);
        }
        System.out.println(System.currentTimeMillis() - time);

        time = System.currentTimeMillis();
        for (int i = 0; i < ITERATIONS; i++) {
            nr.setIm(i);
            nr.setRe(i);
        }
        System.out.println(System.currentTimeMillis() - time);

        time = System.currentTimeMillis();
        for (int i = 0; i < ITERATIONS; i++) {
            FinalComplexNumber num = new FinalComplexNumber(i, i);
        }
        System.out.println(System.currentTimeMillis() - time);
    }
}  

The results:

run:
0.0 + i0.0
953
219
7875
BUILD SUCCESSFUL (total time: 9 seconds)