Smalltalk-80: The Interactive Programming Environment (Addison-Wesley series in computer science)

The Smalltalk-80 system consists of an object-oriented programming language and an integrated collection of tools for interacting with components of that language. In the Smalltalk-80 language, the fundamental way to indicate that something should happen is by sending a message to an object. An object is a representation of information consisting of private memory, and a set of operations to manipulate information stored in the private memory or to carry out some actions relative to that information. Sending a message is the Smalltalk-80 way of asking the object to carry out one of its operations.
All information in the Smalltalk-80 system is represented as an object. Each object knows the messages it can understand. Associated with each such message is a method that describes how the object should respond to the message. The user interface to the Smalltalk-80 system can be viewed as a graphical way in which to identify objects and to choose messages to send to objects. When an object is sent a message, the appropriate method is invoked and some action is taken. The purpose of this book is to describe the varied ways in which objects and messages can be identified and methods invoked; its purpose is also to describe some useful ways in which information about objects can be retrieved.