The GeoAPI Feature class is used to represents "something that can be drawn on a map".

We are not doing anything creative here in terms of information modelling. GeoAPI provides a normal Data Model, Query Model, Metadata separation.

References:

• GeoAPI javadocs
• ISO19107
• OGC General Feature Model
• Geographic Markup Language

Feature Model

Data Model

We store information in a data structure composed of Feature, Attributes, and Associations.

To create a feature we make use of a SimpleFeatureFactory

Query Model

Use Expression to access information stored in the data model. Use Filter to select features that are of interest.

• Filter Interfaces

final FilterFactory ff = CommonFactoryFinder.getFilterFactory( GeoTools.getDefaultHints );
Filter filter = ff.propertyLessThan( ff.property( "AGE"), ff.literal( 12 ) );

FeatureCollection<SimpleFeature> features = featureSource.getFeatures( filter );
features.accepts( new FeatureVisitor<SimpleFeature>() ){
    public void visit( SimpleFeature feature ){
          Expression query = ff.property( "NAME" );
          String name = query.evaulate( feature, String.class );
          System.out.println( feature.getID() + " is named: "+ name );
    }
}, null );

Metadata Model

Describe the represented information. You can review this description in order to figure out the attribute names when making a request. If you are creating new content you should check the feature type to ensure your content is valid.

Descriptors

A SimpleFeatureType is defined in terms of a List of AttributeDescriptors. Each AttributeDescriptor has a Name and a AttributeType; you can use the same AttributeType for several AttributeDescriptors.

Simple Feature

We have created an extension of Feature and FeatureType to provide a few helper methods when the following conditions are met.

• a feature's properties are limited to mandatory GeometryAttribute and Attribute (no complex attributes or multiplicity allowed).
• attributes values may be null; but each attribute must be represented

These restrictions match the abilities of a simple shapefile or database table.

Discussion

Comparison to Java

It may help you to compare the ideas presented here to something you are already familiar with ...

• The data structure Feature is used to hold information. Each feature "belongs to" a FeatureType which is used to describe valid contents. This is a dynamic type system because FeatureType is a data structure we can define at runtime.
• The data structure Object is used to hold information. Each Object "belongs to" a Class which is used to describe valid contents. This is a static type system because Class is a data structure we need to compile before the application is started.

(This table is a quick comparison please forgive and correct any mistakes)

class Flag {
  Point location;
  String name;
  int classification;
  double height;
}

SimpleFeatureTypeBuilder b = new SimpleFeatureTypeBuilder();
b.setName( "Flag" );
b.setCRS( DefaultGeographicCRS.WSG84 );
b.add( "location", Point.class );
b.add( "name", String.class );
b.add( "classification", Integer.class );
b.add( "height", Double.class );

Comparison to Java Beans