Smalltalk80LanguageImplementation:Part 02
- PART TWO
Part One provided an overview of the Smalltalk-80 language both from the semantic view of objects and message sending and from the syntactic view of the form that expressions take. The Smalltalk-80 programmer must first understand the semantics of the language: that all information is represented in the form of objects and that all processing is done by sending messages to objects. Every object is described by a class; every class, with the exception of class Object, is a subclass of another class. Programming in the Smalltalk-80 system involves the description of new classes Of objects, the creation of instances of classes, and the sequencing of messages to the instances. The Smalltalk,80 syntax defines three forms that messages can take: unary, binary, and keyword messages. Successful use of the language requires that the programmer have a general knowledge of each of the basic kinds of objects in the system and of the messages that can be sent to them.
The semantics and syntax of the language are relatively simple. Yet the system is large and powerful due to the numbers of and kinds of available objects. There are eight significant categories of classes in the Smalltalk-80 system: kernel and kernel support, linear measures, numbers, collections, streams, classes, independent processes, and graphics. The protocol of these kinds of objects is reviewed in 12 chapters of Part Two. In each of these chapters, the diagram of the class hierarchy given in Chapter 1 is re-presented in order to highlight the portion of the hierarchy discussed in that chapter. Three additional chapters in Part Two provide examples of Smalltalk-80 expressions and class descriptions.
The classes in the Smalltalk-80 system are defined in a linear hierarchy. The chapters in Part Two take an encyclopedic approach to reviewing class protocol: categories of messages are defined, each message is annotated, and examples are given. In presenting the protocol of a class, however, only those messages added by the class are described. The complete message protocol is determined by examining the protocol specified in the class and in each of its superclasses. Thus it is useful to present the classes starting with a description of class Object and to proceed in a mostly depth-first manner so that inherited protocol can be understood in conjunction with the new protocol.