This document discusses object-oriented programming principles and concepts. It outlines six key principles of OOP: everything is an object, objects communicate by passing messages, objects have their own memory made up of other objects, objects are instances of classes, classes define common behavior, and classes are organized into inheritance hierarchies. It provides an example of sending flowers to illustrate how objects representing different roles interact to complete a task. The document also explains concepts like classes, messages, inheritance, and polymorphism.

1. Lecture 2: Thinking Object-Oriented (2) CSCI-383 Object-Oriented Programming & Design Ji Ruan 2009/09/16

6. A Way of Viewing the World!

7. Recursive Design The structure of the part mirrors the structure of the larger unit.

11. Agents and Communities Definition: An object-oriented program is structured as a community of interacting agents called objects . Each object has a role to play. Each object provides a service or performs an action that is used by other members of the community.

12. Elements of OOP - Objects So we have Kay's principle No. 1: Everything is an object. Actions in OOP are performed by agents, called instances or objects. There are many agents working together in our scenario. Chris, Robin, the florist in Robins city, the delivery person, the flower arranger, and the grower etc. Each agent has a part to play, and the result is produced when all work together in the solution of a problem.

13. Elements of OOP - Messages Principle No. 2: Objects perform computation by making requests of each other through the passing of messages. Actions in OOP are produced in response to requests for actions, called messages . An instance may accept a message, and in return will perform an action and return a value. To begin the process of sending the flowers, Chris gives a message to Fred. Fred in turn gives a message to the florist in Robins city, who gives another message to the driver and so on.

16. Different Receivers, Same Message, Different Actions Var Fred : Florist; Elizabeth : Friend; Ken : Dentist; begin Fred.sendFlowersTo(myFriend); { will work } Elizabeth.sendFlowersTo(myFriend); { also work } Ken.sendFlowersTo(myFriend); { probably not work } end; The same message will result in different actions, depending upon who it is given to.

18. Elements of OOP - Recursive Design Principle No. 3: Every object has it's own memory, which consists of other objects. Each object is like a miniature computer itself - a specialized processor performing a specific task.

22. Class Hierarchies

23. Class Hierarchies link to larger size. Charles Darwin (1809-1882) Tree of Life

24. Elements of OOP - Inheritance Principle No. 6: Classes are organized into a singly-rooted tree structure, called an inheritance hierarchy Information (data and/or behavior) we associate with one level of abstraction in a class hierarchy is automatically applicable to lower levels of the hierarchy.