Writing a New Reader

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In this section contains an example of an IlvMapFeatureIterator that you can use to read polylines that were saved in an ASCII file.

• It has a header specifying its format.
• There is one pair of coordinates (latitude and longitude) per line. These coordinates are expressed in degrees.
• Lines can contain comments. These comments, when they exist, are merged to form an attribute.
• Polylines are separated by a blank line.
• The file has the .pol extension.

ascii polylines
-1.0 40.0   A 1x1 degree rectangle centered on the 
 1.0 40.0    (0,39) point
 1.0 38.0
-1.0 38.0
-1.0 40.0

0.0  90.0   A meridian extending from the North pole to the South pole
0.0 -90.0

<installdir>/jviews-maps81/codefragments/newreader/srchtml/java/SimplePolylineReader.java

As shown below, the SimplePolylineReader implements the IlvMapFeatureIterator interface:

class SimplePolygonReader implements IlvMapFeatureIterator
{
 ... member variables ...
 /**
 * Constructor
 */
 public SimplePolygonReader (String fileName) throws FileNotFoundException
 {
 Initializing member variables
 }
 ... methods ...
}

Since latitude and longitude in the polyline file are expressed in degrees, the coordinate system is geographic. This is why the isGeoreferenced method returns true and the getCoordinateSystem method returns IlvGeographicCoordinateSystem.WGS84. The getCoordinateSystem method would return null if the projection of the file to be read was unknown. See the description of the isGeoreferenced method in The IlvMapFeatureIterator Interface.

public boolean isGeoreferenced()
  {
    return true;
  }
 public IlvCoordinateSystem getCoordinateSystem()
  {
    return IlvGeographicCoordinateSystem.WGS84;
  }

Because of the data format, the bounding box of the polyline cannot be retrieved until the data has been read. Here, the methods getUpperLeftCorner and getLowerRightCorner return null to indicate that these points are not known. Another option is to read the data, place it in an array, and then compute the bounding box.

public IlvCoordinate getLowerRightCorner()
  {
    return null;
  }

The getDefaultFeatureRenderer method must return a renderer able to transform into graphic objects all the map features read by this feature iterator. The IlvDefaultCurveRenderer can process map features whose geometry is of type IlvMapLineString.

public IlvFeatureRenderer getDefaultFeatureRenderer()
  {
    return new IlvDefaultCurveRenderer();
  }

If the geometries of the returned map features are not predefined but instead are instances of a derived class, or if the map feature attributes store drawing parameters to be used in rendering operations such as color or line width, it is necessary to provide renderers that can process these attributes or derived geometries. See the section Creating a Colored Line Renderer.

The getNextFeature method reads the geometry of a map feature and creates an IlvMapFeature object that will hold all the information required to process the geometry. The geometry read in the code example that follows is an IlvMapLineString, which is the class to define polyline geometries.

public IlvMapFeature getNextFeature()
  throws IOException
{
  return readPolyline();
}

0. A geometry of the type IlvMapLineString is created, which will be associated with the map feature.

   private IlvMapFeature readPolyline() throws IOException { // Concatenates all the comment lines. StringBuffer buffer = new StringBuffer(); // Reads the current map feature. IlvMapFeature feature = new IlvMapFeature(); // Reads the current line string geometry. IlvMapLineString geometry = new IlvMapLineString();

1. The points making up this line string are read and the related comment is stored as an attribute.

   // Stores the line of text that is read. String line; // Reads a line. while ((line = file.readLine()) != null) { [...]

2. The longitude coordinate values are read. Note that an exception of type IlvMapFormatException is thrown if a format error is detected while reading.

   // Process longitude. IlvCoordinate c = new IlvCoordinate(); if (tokenizer.hasMoreElements() == false) throw new IlvMapFormatException("Longitude coordinate expected"); try { currentToken = (String)tokenizer.nextElement(); c.x = decimalParser.parse(currentToken).doubleValue(); } catch (ParseException e) { throw new IlvMapFormatException("Error while parsing longitude"); }

These comments also apply to latitude coordinates.
3. Each point read from the file is added to the line string geometry.

   // Add this point to geometry. geometry.addPoint(c); [...]

4. The following if statement tests whether the end of the file has been reached. In this case, the getNextFeature method should return a null pointer.

   // End of file. if ((line == null) && (geometry.getPointCount() == 0)) return null;

5. The geometry is associated with the map feature.

   // Initialize the map feature. feature.setGeometry(geometry);

6. The comments are extracted to form attributes, which are associated with the map feature. The attributeInfo object, which is shared by the attributes of the map features, was initialized in the reader's constructor.

   // Set attribute. IlvStringAttribute[] attribute = new IlvStringAttribute[1]; if (buffer.length() > 0) { attribute[0] = new IlvStringAttribute(); attribute[0].setString(buffer.toString()); } else { attribute[0] = null; } feature.setAttributeInfo(attributeInfo); feature.setAttributes(new IlvFeatureAttributeProperty(attributeInfo, attribute));

7. The read map feature is returned.

   // Returns the read map feature. return feature; }