ArcGIS Engine Developer Guide

JAVA APPLICATION PROGRAMMING INTERFACE
Instanceof
The instanceof operator in Java allows a developer to determine if its first operand
is an instance of its second.
operand1 instanceof operand2
You can use instanceof in ArcObjects when the logic behind the type is held in
Java. You cannot use instanceof when the type is held in ArcObjects, as the logic
of determining whether an object is an instance of a specified type resides in the
constructors of that object type and not the JVM.
Point point = new Point();
point.putCoords(10, 10);
if(point instanceof IGeometry){
System.out.println(" point is a IGeometry");
geom = point;
}
if(point instanceof IClone){
System.out.println(" point is a IClone");
}
The above code works since the type information is held in Java for Point.Java.
When you construct a Point object, a proxy class for each implemented interface is
also constructed. This allows you to use instanceof on any of these types. Developers
would have access to any methods on Point implementing the IGeometry or
IClone interfaces.
This is backwards compatible as well:
if(geom instanceof Polyline){
System.out.println(" geom is a Polyline");
}
else if(geom instanceof Point){
System.out.println(" geom is a Point");
pnt = (IPoint)geom; // allowable cast as the type is held in JVM
}
Since a direct cast of the geom object into Point was created, the geom object is of
type Point and instanceof can be used to check this information. However, since
the type information was known before it was checked above, it is not extremely
useful. What would be useful is to apply the above logic on methods that return
objects of superinterfaces.
Consider the IWorkspaceFactory.openFromFile method, which returns an IWorkspace.
Since the object returned is a Java object that implements IWorkspace, you cannot
check if the returned object is of any of the known implementing classes that
implement IWorkspace. In this case, to check for type information, you should call
a method on the returned object that is expected. If the method does not throw
an exception, it is of that type. This occurs because the logic on this object is
declared at runtime and is held inside the underlying ArcObjects component.
RasterWorkspaceFactory rasterWkspFactory = new RasterWorkspaceFactory();
IWorkspace wksp = rasterWkspFactory.openFromFile( aPath, 0 );
if(wksp instanceof RasterWorkspace){
/* Code does not execute as logic is in ArcObjects. */
Chapter 4 • Developer environments • 191