Sine in Java between three points

Sinus.java

import java.awt.geom.Point2D;

public class Sinus{
  /**
	 * Computes the sine of the angle between three points.
	 * Use the properties of the cross product between two vectors in a 2D euclidean space.
	 * The positive angles are counter-clockwise.
	 * @param pt1 Point defined by a location in a coordinate space
	 * @param pt2 Point defined by a location in a coordinate space
	 * @param pt3 Point defined by a location in a coordinate space
	 * @return the value of sin between the vector p2p3 and p2p1
	 */
	public static double sine(Point2D pt1, Point2D pt2, Point2D pt3){
		// Arrays to store the coordinates of the vectors
		double a[] = new double[2];
	        double b[] = new double[2];

		// Computing p3p2 and p1p2 vectors
		a[0] = pt3.getX() - pt2.getX();
		a[1] = pt3.getY() - pt2.getY();
		b[0] = pt1.getX() - pt2.getX();
		b[1] = pt1.getY() - pt2.getY();
   
		return (a[0]*b[1] - a[1]*b[0])/(pt3.distance(pt2) * pt1.distance(pt2));
	}

	public static void main(String[] args) {
		Point2D pt1 = new Point2D.Double(0, 10);
		Point2D pt2 = new Point2D.Double(0, 0);
		Point2D pt3 = new Point2D.Double(10, 0);

		System.out.println(sine(pt1, pt2, pt3));

	}
}