An object-oriented program consists of objects that communicate by sending messages. Objects represent components like numbers, files, and processes, while messages represent interactions between components. Each object holds attributes to store information and can respond to messages. A class defines a template for objects, ensuring each has the same attributes and understands/responds to the same messages in the same way. The protocol of a class is the set of messages its objects understand.

1. Object Concepts Object-oriented software Objects Messages Attributes and state Sending messages Using messages to make objects do things Grouping objects into classes Grouping messages into a protocol Attributes of a software object Messages that do not alter an objects state Inspecting state using an inspector

2. Classes and protocols Initial state of an object Objects of different classes Classes and writing software Polymorphism Classes and subclasses State-dependent behavior Subclasses Programming and subclasses

3. Message arguments Keyword messages and arguments Message selectors More than one argument in a message Message answers, enquiry messages and collaborating objects Enquiry messages and message answers Collaborating objects

4. New concepts An object is able to find out about its own state. The initial state of an object has to be prescribed. A class groups together objects with the same characteristics (the same attributes and potential behavior). When programming, a class is used to define how objects (‚instances) of the class will be created. One class description serves to describe all the objects of that class Œ the information each can hold and the set of messages to which each can respond.

5. The class description is a template for its members, so that each member has the same attributes to hold information and responds to the same messages with the same resultant behavior. The ‚state of an object is a set of values representing the attributes of objects of a given class.

6. Protocols The protocol described by a class is the set of messages understood by the member objects. The same message sent to objects of different classes may provoke a different behaviour in each class. A class may have a subclass. Every message has a message answer. Message answers may be used in collaborations between objects.

7. Objects Object-oriented software consists of objects that communicate by sending messages to each other. The user can also communicate with an object by sending it a message via a user interface. Objects represent the components of a Smalltalk system such as the numbers, files and processes. Messages represent the interactions between the components of a Smalltalk system for example, arithmetic, file creation and text manipulation.

8. Messages and attributes In an object-oriented application, each action is achieved by sending a message to an object. In order to carry out the action requested by a received message, an object may need to hold information. Different kinds of object hold different kinds of information called attributes

9. A class A class ensures that all instances it creates have the same attributes and respond to the same set of messages in an identical manner. understand the same messages; respond in the same way to each message; have the same attributes.