CSCI 383

1. CSCI 383 Object-Oriented Programming & Design Lecture 3 Martin van Bommel

2. Imperative Programming • So, what are the paradigms of programming? • Imperative programming is the “traditional” model of computation • State • Variables • Assignment • Loops • A processing unit is separate from memory, and “acts” upon memory CSCI 383 Lecture 3 M. van Bommel

3. Visualization of IP CSCI 383 Lecture 3 M. van Bommel

4. Recursive Design • Why Not Build Program out of Computers? • Alan Kay wondered why conventional computers were constructed out of pieces that were useless by themselves • Why not build a whole out of pieces that were similar at all levels of detail? (Think of fractals) • Idea: A program can be build out of little computing agents CSCI 383 Lecture 3 M. van Bommel

5. Recursive Design (cont’d) • The structure of the part mirrors the structure of the larger unit CSCI 383 Lecture 3 M. van Bommel

6. Kay's Description of OOP • Object-oriented programming is based on the principle of recursive design • Everything is an object • Objects perform computation by making requests of each other through the passing of messages • Every object has its own memory, which consists of other objects • Every object is an instance of a class. • A class groups similar objects • The class is the repository for behavior associated with an object • Classes are organized into singly-rooted tree structure, called an inheritance hierarchy • We can illustrate these principles by considering how to go about solving a problem in real life CSCI 383 Lecture 3 M. van Bommel

7. Illustration of OOP Concepts • To illustrate the concepts of OOP in an easily understood framework, consider Chris sending flowers to Robin. • Chris can't deliver them directly. So Chris uses the services of the local Florist • Chris tells the Florist (named Fred) the address for Robin, how much to spend, and the type of flowers to send • Fred contacts a florist in Robin’s city, who arranges the flowers, then contacts a driver, who delivers the flowers • If we start to think about it, there may even be other people involved in this transaction. There is the flower grower, perhaps somebody in charge of arrangements, and so on CSCI 383 Lecture 3 M. van Bommel

8. Agents and Communities • Our first observation is that results are achieved through the interaction of agents, which we will call objects • Furthermore, any nontrivial activity requires the interaction of an entire community of objects working together • Each object has a part to play, a service they provide to the other members of the community CSCI 383 Lecture 3 M. van Bommel

9. Elements of OOP - Objects • So we have Kay's first principle • Everything is an object • Actions in OOP are performed by agents, called instances or objects • There are many agents working together in the scenario • Chris, Robin, the florist, the florist in Robin’s city, the driver, the flower arranger, and the grower. • Each agent has a part to play, and the result is produced when all work together in the solution of a problem CSCI 383 Lecture 3 M. van Bommel

10. Elements of OOP - Messages • And principle number 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 send the flowers, Chris gives a message to Fred. Fred in turn gives a message to the florist in Robin’s city, who gives another message to the driver, and so on CSCI 383 Lecture 3 M. van Bommel

11. Information Hiding • Notice that the user of a service being provided by an object need only know the name of the messages that the object will accept • I need not have any idea how the actions performed in response to my request will be carried out • Having accepted a message, an object is responsible for carrying it out CSCI 383 Lecture 3 M. van Bommel

12. Elements of OOP - Receivers • Messages differ from traditional function calls in two very important respects: • In a message, there is a designated receiver that accepts the message • The interpretation of the message may be different, depending upon the receiver CSCI 383 Lecture 3 M. van Bommel

13. OOP: The General Principle • An object oriented program is structured as a community of interacting agents called objects. • Action is initiated by the transmission of a message to an agent (an object). • The message encodes the request for action and is accompanied by additional information (arguments) needed to carry out the request. • The receiver is the agent to which the message is sent. • If the receiver accepts the message it accepts the responsibility to carry out the indicated action. • In response to a message the receiver will perform some method to satisfy the request. CSCI 383 Lecture 3 M. van Bommel

14. OOP: A Way to View the World • Chris wants to send some flowers to his friend Robin who lives in a distant city. • Because of the distance, Chris cannot pick up the flowers and bring them to Robin in person. • Chris goes down to Fred, a local florist, and tells him the number and type of flowers he wants together with the address they need to be delivered to. • Chris can rest assured that the flowers will be delivered. CSCI 383 Lecture 3 M. van Bommel

15. Agents, Messages, Methods • Chris finds an appropriate agent (Fred) • And passes to him a message containing a request • It is the responsibility of Fred to satisfy the request • There is some method (a set of operations) used by Fred to do this • Chris does not need to know the particular method he will use, this information is hidden from inspection CSCI 383 Lecture 3 M. van Bommel

16. Behavior and Interpretation • Although different objects may accept the same message, the actions (behavior) the object will perform will likely be different • If Fred asked his wife Beth to send some flowers to Robin for her birthday, she might use a different agent than the florist Fred • determination of what behavior to perform may be made at run-time (late binding) • The fact that the same name can mean two entirely different operations is one form of polymorphism, a topic we will discuss later CSCI 383 Lecture 3 M. van Bommel

17. OOP Element - Recursive Design • Every object has its own memory, which consists of other objects • Each object is like a miniature computer itself - a specialized processor performing a specific task CSCI 383 Lecture 3 M. van Bommel

18. Non-interference • It is important that objects be allowed to perform their task however they see fit, without unnecessary interactions or interference with other objects • “Instead of a bit-grinding processor ... plundering data structures, we have a universe of well-behaved objects that courteously ask each other to carry out their various desires” -- Dan Ingalls • “Ask not what you can do to your data structures, but ask what your data structures can do for you” CSCI 383 Lecture 3 M. van Bommel

19. Elements of OOP - Classes • Every object is an instance of a class. A class groups similar objects • The class is the repository for behavior associated with an object • The behavior I expect from Fred is determined from a general idea I have of Florists • We say Fred is an instance of the class Florist • Behavior is associated with classes, not with individual instances. • All objects that are instances of a class use the same method in response to similar messages CSCI 383 Lecture 3 M. van Bommel

20. Hierarchies of Categories • But there is more that I know about Fred than just that he is a Florist. • I know he is a ShopKeeper, and a Human, and a Mammal, and a Material Objects, and so on • At each level of abstraction I have certain information recorded. • That information is applicable to all lower (more specialized) levels CSCI 383 Lecture 3 M. van Bommel

21. Elements of OOP - Inheritance • The principle that knowledge of a more general category is also applicable to a more specific category is called inheritance • Classes can be organised into a hierarchical inheritance structure. • A child class (subclass) will inherit attributes from a parent class (super class) higher up in the hierarchy. • An abstract parent class is a class for which there are no direct instances, it is only introduced to group subclasses CSCI 383 Lecture 3 M. van Bommel

22. Hierarchies of Categories CSCI 383 Lecture 3 M. van Bommel

23. Class Hierarchies CSCI 383 Lecture 3 M. van Bommel

24. Elements of OOP - Overriding • Subclasses can alter or override information inherited from parent classes: • All mammals give birth to live young • A platypus is an egg-laying mammal • The search for a method to use in response to a message starts with the receiver’s class and continues up the parent chain. When methods with the same name are available higher in the class hierarchy the method that executes is said to override the inherited behavior • Inheritance combined with overriding are where most of the power of OO originates CSCI 383 Lecture 3 M. van Bommel

25. Problem Solving • Because the OOP view is similar to the way in which people go about solving problems in real life (finding another agent to do the real work!), intuition, ideas, and understanding from everyday experience can be brought to bear on computing • However, infinite recursion does not solve the problem • Someone has to do the work! • On the other hand, common sense was seldom useful when computers were viewed in the process-state model, since few people solve their everyday problems using pigeon-holes CSCI 383 Lecture 3 M. van Bommel

26. From Newsweek • “Unlike the usual programming method – writing software one line at a time – NeXT's ‘object-oriented’ system offers larger building blocks that developers can quickly assemble the way a kid builds faces on Mr. Potato Head.” CSCI 383 Lecture 3 M. van Bommel