用蒙特卡洛方法算浦丰投针问题，为什么算出来的结果pi是在分子上的？

Question

public class randomPoints {
double x;
double y;
static int intersect = 0;
static int total = 0;

public double getx() {
    return this.x;
}

public double gety() {
    return this.y;
}

randomPoints() {
    x = (Math.random() );
    y = (Math.random() );
    randomPoints points = sndP(new randomPoints(x, y));
    double a = points.getx();
    double b = points.gety();
    if (b>1&&y>0) {
        intersect++;
    }
    total++;
}

randomPoints(double xy, double yx) {
    x = xy;
    y = yx;
}

public static randomPoints sndP(randomPoints p) {
    double a = Math.random() + p.x - 0.5;
    boolean c = Math.random() - 0.5 > 0;
    double b;
    if (c) {
        b = p.y + (double) Math.sqrt(0.25 - (a - p.x) * (a - p.x));
    } else {
        b = p.y - (double) Math.sqrt(0.25 - (a - p.x) * (a - p.x));
    }
    return new randomPoints(a, b);
}

public static void main(String[] args) {
    for (int i = 0; i <= 999999999; i++) {
        randomPoints points = new randomPoints();

// System.out.print(points.x+" "+points.y+"\n" );
}
float c = (float)intersect / (float)total;
System.out.print(4*c);
}
}