JTS "points must form a closed linestring"

Question

I'm trying to create a polygon and calculate it's area using

    final GeometryFactory gf = new GeometryFactory();

    final ArrayList<Coordinate> points = new ArrayList<Coordinate>();
    points.add(new Coordinate(0,0));
    points.add(new Coordinate(0,1));
    points.add(new Coordinate(1,0));
    points.add(new Coordinate(1,1));
    points.add(new Coordinate(0,0));


    final Polygon polygon = gf.createPolygon(new LinearRing(new CoordinateArraySequence(points
        .toArray(new Coordinate[points.size()])), gf), null);

    System.out.println(polygon.getArea());

And I get the error:

 points must form a closed linestring

When I tried to use the points :

points.add(new Coordinate(0,0));
points.add(new Coordinate(0,1));
points.add(new Coordinate(1,0));
points.add(new Coordinate(1,1));
points.add(new Coordinate(0,0));

I've received area = 0 .

I'd like to know what I've got to do in order to calculate the area correctly without getting any errors.

Answer 1

In the conversion world, what you've built is known as a "bowtie".

If you really want that shape, you need to conform to topology rules by making a multipart polygon with "left hand rule" part vertices {0,0},{0.5,0.5},{0,1},{0,0} and {0.5,0.5},{1,0},{1,1},{0.5,0.5} (reverse the order [or just swap vertices 2 & 3 in each part] to generate "right hand rule" polygons).

If you wanted a rectangle, then you need to order the vertices so that no line segment crosses another (in the direction appropriate to your software):

//Left hand rule 
points.add(new Coordinate(0,0));
points.add(new Coordinate(1,0));
points.add(new Coordinate(1,1));
points.add(new Coordinate(0,1));
points.add(new Coordinate(0,0));

//Right Hand Rule
points.add(new Coordinate(0,0));
points.add(new Coordinate(0,1));
points.add(new Coordinate(1,1));
points.add(new Coordinate(1,0));
points.add(new Coordinate(0,0));