An object-oriented program is build from several classes.  Each class defines a type of object and the program works through the interactions of these objects.  Objects interact by invoking methods.  A method defines an action that an object can carry out.  It may be as simple as reporting a data value or it may be a complex computation that changes the state of that object.  Objects know about other objects defined by different classes and can invoke their methods.  In this way complex interactions that comprise the execution of the program take place.

Where do all these classes come from?  How are the objects created and stated?  Often we use classes that are part of the standard Java library or are part of another special purpose library.  We also define our own classes.  We define objects using these classes.  We will use a library called objectdraw , developed at Williams College.  The objectdraw library provides graphical objects and objects that can "sense" mouse actions from the system.

A program always has one class that acts as a driver or initiator of actions.  This special class is responsible for creating enough objects defined by other classes to get the program started.  Once the first few objects are created, their interactions drive the program.  The driver class may or may not have a significant role, once the program is under way; this depends on the design.

There are two types of driver in Java, an application and an applet. In addition, there are two common types of program. A stand-alone program carries out its function independently, once the application or applet has started. An interactive or event-driven program sets up the initial configuration, then reacts to events from outside the program, such as mouse clicks or keystrokes by the user, or events from sensors.

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