I created two files:
PreConditionExample.java
public class PreConditionExample {
    public double sqrt(double p) {
        return Math.sqrt(p);
    }

    public static void main(String[] args) {
        PreConditionExample t = new PreConditionExample();
        System.out.println("sqrt -4 : "+t.sqrt(-4));
        System.out.println("sqrt 9 : "+t.sqrt(9));
    }
}

PreconditionAspect.aj
public aspect PreconditionAspect
{
    pointcut sqrtPc(double param) : call ( * Math.sqrt(double)) &&
args(param) ;

    before(double param ) : sqrtPc(param)
    {

        if (param < 0)
        {
            System.out.println("illegal parameter");
            //throw new RuntimeException();
        }
    }

}

but it doesn't work in this way. "illegal parameter " was not printed.
but I put them in the same file, it works.
Why?
help/