1. Principles of Object-oriented Programming Programming Language Paradigms August 26, 2002

2. Pre-history 40s/50s – von Neumann architecture, stored program concept –Allows the development of general-purpose computing machines –Languages tend to be closely tied to the underlying architecture –Structured programming emerges Verb and task oriented Difficult to scale

3. History Simula-67 – objects, early GUI elements Xerox PARC (70s) – Smalltalk, bitmap displays, multiple windows, mouse, menus, icons, etc. Smalltalk-80 – modern version –released outside PARC. –Used a virtual machine and a bytecode compiler. –Spinoff companies create commercial versions.

4. Squeak History 1995: (Apple) – Alan Kay et al. create an open-source version of Smalltalk. –Development of a platform that can be used by people of all ages and skill levels. Squeak compiler and VM written in Squeak. Support for sound, 2-D and 3-D graphics, networking/web, Flash, etc.

5. Other OO history C++ developed at AT&T labs (Stroustrup:79) –Interested in an efficient language that extended C –Strong typing Objective C –Used in NeXT and OS X – combination of C and Smalltalk. Java developed by Sun (1991-1995) –Initially developed as a language for communicating appliances/set-tops –Modified for use with WWW. –Smalltalk features in a typed environment

6. About Objects An ‘object’ is a way to capture both the features of a data structure and its behavior. Noun-oriented: encourages program design through thinking about pieces Construction of reusable elements

7. Classes and objects A class is a characterization of a type of object. –The Muppet class refers to the set of all muppets. –An object kermit is an instantiation of the Muppet class. Classes have: –Instance variables –Class variables –methods

8. An example Object subclass #Muppet instanceVariableNames: ‘name’ classVariableNames: ‘numberOfSpecies’ category: ‘muppets’ To create a muppet: Kermit <- [Muppet new].

9. Messages In Smalltalk, methods are invoked by sending the object a message. noun verb: arg1 mod: arg2 …. Modeled on speech acts.

10. Inheritance One of the most popular OO features is inheritance. –Can define new classes in terms of existing classes. –This creates a class hierarchy in which subclasses take on the behavior of their parent classes. A subclass can generate new behaviors by –Defining new methods –Overriding a parent method.

11. Inheritance example

12. Polymorphism Polymorphism is the idea that an object or method can perform the same operation on different data types. –Addition is polymorphic in most languages. Allows a programmer to write code without knowing the types of the data involved. [listOfStuff sort] <- don’t need to know what this is a list of.

13. Polymorphism example

14. Encapsulation Encapsulation provides a way of packing all the structure and behavior of an object into a single module. –Interface: the public face of an object –Implementation: How methods are carried out. Prevents users from misusing internal structure. Implementation can be changed without affecting external programs.

15. Encapsulation example