13.4.2.1 几何类层次结构
The geometry classes define a hierarchy as follows:
-
Geometry
(noninstantiable)-
Point
(instantiable) -
Curve
(noninstantiable)-
LineString
(instantiable)-
Line
-
LinearRing
-
-
-
Surface
(noninstantiable)-
Polygon
(instantiable)
-
-
GeometryCollection
(instantiable)-
MultiPoint
(instantiable) -
MultiCurve
(noninstantiable)-
MultiLineString
(instantiable)
-
-
MultiSurface
(noninstantiable)-
MultiPolygon
(instantiable)
-
-
-
It is not possible to create objects in noninstantiable classes. It is possible to create objects in instantiable classes. All classes have properties, and instantiable classes may also have assertions (rules that define valid class instances).
Geometry
is the base class. It is an abstract class. The instantiable subclasses of Geometry
are restricted to zero-, one-, and two-dimensional geometric objects that exist in two-dimensional coordinate space. All instantiable geometry classes are defined so that valid instances of a geometry class are topologically closed (that is, all defined geometries include their boundary).
The base Geometry
class has subclasses for Point
, Curve
, Surface
, and GeometryCollection
:
-
Point
represents zero-dimensional objects. -
Curve
represents one-dimensional objects, and has subclassLineString
, with sub-subclassesLine
andLinearRing
. -
Surface
is designed for two-dimensional objects and has subclassPolygon
. -
GeometryCollection
has specialized zero-, one-, and two-dimensional collection classes namedMultiPoint
,MultiLineString
, andMultiPolygon
for modeling geometries corresponding to collections ofPoints
,LineStrings
, andPolygons
, respectively.MultiCurve
andMultiSurface
are introduced as abstract superclasses that generalize the collection interfaces to handleCurves
andSurfaces
.
Geometry
, Curve
, Surface
, MultiCurve
, and MultiSurface
are defined as noninstantiable classes. They define a common set of methods for their subclasses and are included for extensibility.
Point
, LineString
, Polygon
, GeometryCollection
, MultiPoint
, MultiLineString
, and MultiPolygon
are instantiable classes.