How can i draw a line surrounding all the polygons in a single layer in GeoTools?

Question

I have written some code in Java using the GeoTools (v.22) library that opens a shapefile, takes all the polygons in it and divides them in different FeatureCollections based on a value associated to each; it then creates a layer for every FeatureCollection and adds it to a map, resulting in all the polygons each divided from the others by a black stroke.

how can i draw another colored line surrounding all the polygons in the same layer?

Answer 1

You are looking for the Concave Hull of your features. So first you need to get a GeometryCollection of your features. You can then call Eric Grosso's Concave Hull implementation.

So something like:

    File f = new File("/home/ian/Data/states/states.shp");
    FileDataStore ds = FileDataStoreFinder.getDataStore(f);

    Filter filter = ECQL.toFilter("strEndsWith(\"STATE_NAME\",'a')=true");
    SimpleFeatureCollection collection = ds.getFeatureSource().getFeatures(filter);
    ArrayList<Geometry> geoms = new ArrayList<>();
    try (SimpleFeatureIterator it = collection.features()) {
      while (it.hasNext()) {
        SimpleFeature feature = it.next();
        Geometry geom = (Geometry) feature.getDefaultGeometry();

        geoms.add(geom);
      }
    }
    GeometryFactory gf = new GeometryFactory();
    GeometryCollection gc = gf.createGeometryCollection(geoms.toArray(new Geometry[] {}));
    gc = (GeometryCollection) Densifier.densify(gc, 5);
    double threshold = 10;
    ConcaveHull ch = new ConcaveHull(gc, threshold);
    Geometry concaveHull = ch.getConcaveHull();
    System.out.println(gc);
    System.out.println(concaveHull);

Which in this case generates the following map:

while a threshold of 1 gives: