Factory.java

enum CoordinateSystem
{
  CARTESIAN,
  POLAR
}

class Point
{
  private double x, y;

  protected Point(double x, double y)
  {
    this.x = x;
    this.y = y;
  }

  public Point(double a,
               double b, // names do not communicate intent
               CoordinateSystem cs)
  {
    switch (cs)
    {
      case CARTESIAN:
        this.x = a;
        this.y = b;
        break;
      case POLAR:
        this.x = a * Math.cos(b);
        this.y = a * Math.sin(b);
        break;
    }
  }

  // steps to add a new system
  // 1. augment CoordinateSystem
  // 2. change ctor

  // singleton field
  public static final Point ORIGIN = new Point(0,0);

  // factory method
  public static Point newCartesianPoint(double x, double y)
  {
    return new Point(x,y);
  }

  public static Point newPolarPoint(double rho, double theta)
  {
    return new Point(rho*Math.cos(theta), rho*Math.sin(theta));
  }

  public static class Factory
  {
    public static Point newCartesianPoint(double x, double y)
    {
      return new Point(x,y);
    }
  }
}

class PointFactory
{
  public static Point newCartesianPoint(double x, double y)
  {
    return new Point(x,y);
  }
}

class FactoryDemo
{
  public static void main(String[] args)
  {
    Point point = new Point(2, 3, CoordinateSystem.CARTESIAN);
    Point origin = Point.ORIGIN;

    Point point1 = Point.Factory.newCartesianPoint(1, 2);
  }
}