Program	This class is the root of the program hierachy and defines a program in which input and output are performed using the system console, which is redirected to the standard Java I/O streams System.in and System.out.
ConsoleProgram	This class is similar to Program but fills the program frame with an interactive console (as defined in the IOConsole class in the acm.io package).
DialogProgram	This class extends the basic functionality of the Program class so that input operations are performed using the IODialog class in the acm.io package.
CommandLineProgram	This class is similar to ConsoleProgram but uses System.in and System.out instead of an IOConsole. This class is typically used only for Java programs that are invoked from a command line and make no use of Java’s graphical capabilities.

The principal advantages of using the Program class are:

• The conventional pattern of use associated with the acm.program package moves students away from the imperative style of public static void main into a more pedagogically defensible framework in which students are always working in the context of an object.
• The Program class allows Java applications to double as applets, thereby making it possible for intructors to use either paradigm in a consistent way. Even for those instructors that choose to focus on the application paradigm, using the Program class makes it much easier for students to make their code available on the web. Moreover, because the Program class is defined to be a subclass of Applet, applications that run in that domain can take advantage of such applet-based features as loading audio clips and images from the code base.
• The Program class includes several features that make instruction easier, such as the definition of standard menu bars that support operations like printing and running programs with a test script.
• The classes in the acm.program package offer a compelling example of an inheritance hierarchy that introductory students can understand and appreciate right from the beginning of their first course.

In most environments, the only thing students need to do to create a new program is to define a Program subclass that implements a run method. For example, the following class definition creates a traditional, stream-oriented program that reads in two integers and computes their sum:

     public class Add2 extends ConsoleProgram {
        public void run() {
           println("This program adds two numbers.");
           int n1 = readInt("Enter n1: ");
           int n2 = readInt("Enter n2: ");
           int total = n1 + n2;
           println("The total is " + total + ".");
        }
     }