ArcGIS Engine Developer Guide

JAVA APPLICATION PROGRAMMING INTERFACE
with an interface. Objects use type inheritance; the code associated with an
interface comes from the class implementation.
This model shares some features of the Java interface model. An interface in the
Java language is a specification of methods that an object declares it implements.
A Java interface does not include instance variables or implementation code.
The ArcGIS API for Java has two objects for every ArcObjects interface: a
corresponding interface and an interface proxy class. The interface is named in the
ArcObjects style, prefixed with an I. The interface proxy class appends the term
proxy to the name. An example of this mapping is provided below:
ArcObjects Interface
interface IArea : IUnknown
Java Representation
public interface IArea{}
public class IAreaProxy implements IArea{}
The proxy classes are used internally by the ArcGIS API for Java to provide
implementation to respective interfaces. An application developer should never
use the default constructor of these classes as it holds no implementation.
ArcObjects requires developers to go through an interface to access objects. The
Java language does not use this model; subsequently, the ArcGIS API for Java has
two ways of accessing objects—by interface or by class.
/* use the class implementing com.esri.arcgis.geometry.Point();
IPoint iPoint = new Point();
/* access object through class */
Point cPoint = new Point();
You cannot access objects through the default interface proxy class:
IPointProxy proxyPoint = new IPointProxy(); // incorrect usage
This will be discussed in more depth in subsequent sections.
ArcObjects interfaces are immutable and subsequently never versioned. An
interface is never changed once it is defined and published. When an interface
requires additional methods, the API defines a new interface by the same name
with a version number appended to it as described in the following table.
ArcObjects Interface
interface IGeometry : IUnknown
interface IGeometry2 : IGeometry
interface IGeometry3 : IGeometry2
interface IGeometry4 : IGeometry3
Java Representation
public interface IGeometry{}
public interface IGeometry2 extends IGeometry{}
public interface IGeometry3 extends IGeometry2{}
public interface IGeometry4 extends IGeometry3{}
Classes
In the ArcObjects model, classes provide the implementation of the defined
interfaces. ArcObjects provides three types of classes: abstract classes, classes, and
coclasses. These class types can be distinguished through the object model diagrams
provided in ArcGIS Developer Help. It is important to be familiar with them
before you begin to use the three class types.
In ArcObjects, an abstract class cannot be used to create new objects and are
absent in the ArcGIS API for Java. These classes are specifications in ArcObjects
for instances of subclasses through type inheritance. An abstract class enumerates
what interfaces are to be implemented by the implementing subclass but does not
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