ArcGIS Engine Developer Guide

JAVA APPLICATION PROGRAMMING INTERFACE
The return value is an Object, specified by the javadoc as “variant”. Therefore, the
value can be downcasted to String or Short, depending on their type in the
geodatabase being queried.
Casting
ArcObjects follows an interface-based programming style. Many methods use
interface types as parameters and have interfaces as return values. When the
return value of a method is an interface type, the method returns an object
implementing that interface. When a method takes an interface type as parameter,
it can take in any object implementing that interface. This style of programming
has the advantage that the same method can work with many different object
types, provided they all implement the same interface.
For example, IFeature.getShape() method returns an object implementing IGeometry.
The object returned could potentially be any one of the following classes that
implement IGeometry: BezierCurve, CircularArc, EllipticArc, Envelope,
GeometryBag, Line, MultiPatch, Multipoint, Path, Point, Polygon, Polyline, Ray,
Ring, Sphere, TriangleFan, Triangles, or TriangleStrip.
Casting is used to convert between types. There are three types of potential casts
you, as a developer, may be tempted to use with the Java API:
1. Interface to concrete class casting
2. Interface cross-casting
3. Interface downcasting
It is important to understand that objects returned from methods within
ArcObjects can behave differently than objects implicitly defined in your code
because the object reference is not held in the JVM.
If you have a method, doSomeProcessingOnPolygon(Polygon p), that operates only on
Polygon objects, and you want to pass the object obtained as a result of
IFeature.getShape, you need a way to convert the “type” of the object from
IGeometry to Polygon. In Java, this is done using a class cast operation:
/* incorrect usage: will give ClassCastException */
Polygon poly = (Polygon)geom;
However, if you use the same code with the ArcGIS API for Java, you will get a
ClassCastException. The reason for the exception is that the “geom” object reference
is actually a reference to the native ArcObjects component. As a consequence
of the interoperability between Java and the native ArcObjects components,
the logic of casting this object reference to the Polygon object resides in the
constructor of the Polygon object and not in the JVM.
Every class in the ArcGIS API for Java has a constructor that takes in a single
object as a parameter. This constructor can create the corresponding object using
the reference to the ArcObjects component. Therefore, to achieve the equivalent
of a class casting when using the ArcGIS API for Java, use the “object constructor”
of the class being casted to.
Polygon poly = new Polygon(geom);
The following code illustrates the object constructor being used to cast the geom
object to a Polygon:
Chapter 4 • Developer environments • 189