A Little Smalltalk

Superclasses We did not discuss the classes Collection, KeyedCollection, SequenceableCollection, or ArrayedCollection in the last section, even though they were listed as forms of "collection" in the beginning of this chapter. These classes, along with such classes as Boolean, Magnitude, or Number, are what are known as abstract superclasses. Instances of abstract superclasses are seldom useful by themselves, but the classes are important in providing methods that can be inherited by subclasses. For example, while it is legal in Smalltalk to say: x ~ Collection new and the resulting variable x will indeed be an instance of class Collection, the object is not particularly useful. It has no insertion or deletion protocol, for example. An instance of class Set, however, is very useful, and the messages defined in class Collection are important in providing functionality to objects of this class and to other subclasses of class Collection. The selection and design of abstract superclasses is one of the more important arts in Smalltalk programming. For example, if one were designing a system to manipulate banking accounts, a class Account might be a useful abstract superclass for classes CheckingAccount and SavingsAccount. The actions specific to the individual types of accounts would be in the subclasses, whereas any common behavior, such as the actions necessary for opening and closing an account or querying the balance, might be implemented in the superclass. EXERCISES Basic Classes 33 1. Suppose you created a new instance of the class UndefinedObject, as follows: I~UndefinedObject new How does i respond to print? To isNil or notNil? To the object comparison message = = with nil ? Is i nil? List facts to support your answer. 2. Note that the messages arcSin and arcCos produce an object of type Radian and not of type Float. Furthermore, only objects of type Radian respond to sin and cos. An alternative would have been to eliminate the class Radian and to permit all objects of class Float to respond to the messages sin and cos. Discuss the advantages and disadvantages of these two different arrangements. 3. Suppose you have a Bag containing numbers. How would you go about producing an instance of the class List containing the numbers listed in sorted order? 4. What is the class of Class? what is the superclasss of Class? 5. Many times, two or more different sequences of messages to collections will have the same effect. In each of the following, describe a sequence of messages that will have the same effect ,\S the indicated message. a) implement reject: in terms of select: b) implement size in terms of inject:into: c) implement includes: in terms of inject:into:.