A class can inherit from (extend) only one other class. However, a class can be extended by any number of other classes (see ). A class hierarchy represents all the classes that a class inherits either directly or indirectly through multiple levels of inheritance. The root of a class hierarchy is a class that does not extend any other class. The entire class hierarchy is treated as a single object type. You can logically think of a class as the merging of its own members with all of the non-private members of all the classes above it in the hierarchy, including the members of the built-in root class, Progress.Lang.Object.

To illustrate this, consider the hierarchy for the sample user-defined classes (see Sample classes), where acme.myObjs.Common.CommonObj is the top user-defined super class (base class), acme.myObjs.CustObj inherits from acme.myObjs.Common.CommonObj, and acme.myObjs.NECustObj inherits from acme.myObjs.CustObj, represented as follows:

Progress.Lang.Object             <--- Top of hierarchy
  acme.myObjs.Common.CommonObj
    acme.myObjs.CustObj
      acme.myObjs.NECustObj      <--- Bottom of hierarchy

A class definition can override a non-private method in its class hierarchy by providing a method definition with the same name, return type, number of parameters, and corresponding parameter types as the method in its class hierarchy. For example the sample acme.myObjs.CustObj class has a method GetCustomerName( ) that returns the name of a customer, and the acme.myObjs.NECustObj class overrides the GetCustomerName( ) method to provide both the name and E-mail address of the customer. In addition, any non-private data is available to all subclasses of the super class that defines it.

Also, the acme.myObjs.Common.CommonObj class is abstract and all of its abstract members (which can include properties, methods, or class events) must be overridden and implemented by the first derived non-abstract class, in this case acme.myObjs.CustObj. For example, acme.myObjs.CustObj overrides and implements three abstract members, including the OutputGenerated event and the two methods, PublishOutputGenerated( ) and MessageHandler( ).

Finally, the data type of an object can be referenced as any class or interface type that is part of its class hierarchy. For example, an object instantiated as the acme.myObjs.NECustObj class can be referenced as the acme.myObjs.Common.CommonObj class. However, if it is so referenced, only the accessible class members defined within CommonObj or its inherited class hierarchy can be referenced using the object. Note that if CommonObj is an abstract class (as in the sample classes), you can also reference any of its accessible abstract members, because they are guaranteed to be implemented in some derived class.