My Quaternion class

import org.bukkit.util.Vector;

public class Quaternion {

    private final double w, x, y, z;

    public Quaternion(double w, double x, double y, double z) {
        this.w = w;
        this.x = x;
        this.y = y;
        this.z = z;
    }

    // Rotate a vector using a rotation quaternion.
    public static Vector rotate(Vector vector, Quaternion r) {
        return r.mul(Quaternion.from(vector)).mul(r.inverse()).toVector();
    }

    // Rotate a vector theta degrees around an axis.
    public static Vector rotate(Vector vector, Vector axis, double deg) {
        return Quaternion.rotate(vector, Quaternion.rotation(deg, axis));
    }

    // Create quaternion from a vector.
    public static Quaternion from(Vector vector) {
        return new Quaternion(0, vector.getX(), vector.getY(), vector.getZ());
    }

    // Create a rotation quaternion from theta and an axis.
    public static Quaternion rotation(double deg, Vector axis) {
        axis = axis.normalize();
        deg = Math.toRadians(deg / 2);
        double sd = Math.sin(deg);
        return new Quaternion(Math.cos(deg), axis.getX() * sd, axis.getY() * sd, axis.getZ() * sd);
    }

    public Vector toVector() {
        return new Vector(x, y, z);
    }

    public Quaternion inverse() {
        double l = w*w + x*x + y*y + z*z;
        return new Quaternion(w/l, -x/l, -y/l, -z/l);
    }

    public Quaternion conjugate() {
        return new Quaternion(w, -x, -y, -z);
    }

    // Multiply this quaternion and another.
    // Returns the Hamilton product of this quaternion and r.
    public Quaternion mul(Quaternion r) {
        double n0 = r.w*w - r.x*x - r.y*y - r.z*z;
        double n1 = r.w*x + r.x*w + r.y*z - r.z*y;
        double n2 = r.w*y - r.x*z + r.y*w + r.z*x;
        double n3 = r.w*z + r.x*y - r.y*x + r.z*w;
        return new Quaternion(n0, n1, n2, n3);
    }

    public double getW() {
        return w;
    }

    public double getX() {
        return x;
    }

    public double getY() {
        return y;
    }

    public double getZ() {
        return z;
    }
}