Intro to CS: Object-Oriented Programming
Dr. Scott Smith

Me

My Courses

600.321/421 (fall)

Smalltalk, object-oriented programming, object-oriented design, team programming

600.426 (odd years spring)

Programming language design & implementation

600.428 (even years spring)

Compiler Writing

My Research

• Better types for object-oriented languages
• Better (faster) compilers for object-oriented languages by using type information

Objects and Programming

Some governing principles behind good programming language design

• PRECISE: language must give unambiguous instructions to computer
• COMPLETE: nothing the computer is capable of doing that can't be easily expressed in the language
• FAST: duuuuh.
• HUMAN-FRIENDLY: its a language amenable to human, not machine instruction
  1. Highly structured: programs divided into many pieces that may be treated independently and fit together precisely. Good for team programming.
  2. Highly intuitive: Programs written in a natural way of thinking

Most advanced languages satisfy 1. of human-friendly by dividing programs into procedures (functions) and modules (groups of functions and other definitions). C & Pascal aren't so good at the module part.

Object-Oriented Programming Languages (OOP): a school of language design.

Aimed at improved intuitions for programs beyond C/Pascal.

OO programming language examples

• C++
• Java
• Smalltalk
• Objective C
• CLOS
• Modula-3
• Self

C++

• most widely used
• extends C so scales up nicely
• has a type system
• relatively fast execution

• easy to not be object-oriented: can ignore all O-O features
• easy to write bad hackish code: too low level (pointers, malloc/free)
• syntax is overly complex

Smalltalk

• very high-level, don't worry about managing memory or pointers
• very flexible, can even change code while program running
• everything is an object, so it forces object view
• language comes with a large, standardized library

• slow
• slow
• slow
• no types

Java

• Slower than C++, faster than Smalltalk
• Does not extend C, like Smalltalk
• Typed
• More high-level than C++ (no pointers) but less high-level than Smalltalk
• Cleaner syntax than C++
• Pretty decent standardized library but not like Smalltalk
• Has built-in security features to allow downloading of code: more appropriate for internet programming.

Why Objects in Programming?

Real-World object properties:

Active, autonomous behavior

not always directly controlled by us on the outside (smoke detector beeps upon smoke)

Communicative

can "send messages" object to object (talk/command/push button)

Collaborative

long-term collaborative relationships between objects will arise (disk drive/CPU/memory/.. of a fixed computer; ant colony)

Encapsulated

some of the properties/behaviors are not externally visible (though some internals uncovered by sending messages: "are you hungry?")

Nested

complex objects have other objects as components (which in turn may have object components etc) (computer example)

Uniquely named

(though not always -- tension between wanting short name vs unambiguous name)

Self-aware

(possibly; note this does not require self-consciousness, dogs are aware of their own itches on their bodies)

Creation/Destruction

May be created or destroyed

Virtual objects are much more precise in their name, borders, interactions, etc.

Examples of Virtual Objects

Bank Account

Keeps balance; responds to messages to deposit, withdraw, and get balance

Set

Elements can be added, deleted, queried for presence.

GUI Window

Ultimately responsible for keeping contents of window. Many operations possible but include resize, move, etc.

Conclusion: Object-oriented programming (OOP) is good because

1. It jibes well with our intuitions: objects inside the computer correspond to real-world objects
2. All programming problems begin in the real world (e.g., for Amazon.com there are invoices, orders, inventory, etc which all exist as objects in the real world) and so as a start on programming, can directly map real-world objects to virtual objects.

Simple Virtual Object Example: a Set

The external view (signature): aSet understands the messages

1. aSet add: anElement
2. aSet remove: anElement
3. aSet contains: anElement
4. aSet size
5. aSet do: [ someCode] -runs someCode on each set member

Messages have both an effect (internal changes or induced messages) and a return value.
Users only need external view of aSet to use it.
Encapsulation protects internals from view.

Internal view:

1. instance variables for aSet: hashArray, valueArray
   instance variables are
   1. private (generally)
   2. for just that object and
   3. have indefinate extent
2. for each message, a method to perform the action

Question:What properties of real-world objects does this set have?
Definition: an object-based programming language is one which easily supports virtual objects (they can be hacked up in any language using the above formula).

Objects and Large Software Systems

Object view in software design is Object-oriented Design

1. Makes code more intuitively understandable (thesis)
2. Unifies design and programming method: initial program thoughts start from the real-world objects involved in the programming problem (invoices for account, computer gates for digital logic simulation) descriptions, even when using a non-OO language.
3. Divides code into logical chunks individuals of a team can be experts in, and outsiders can understand via interfaces
4. Allows "off the shelf" code libraries to be reused
5. Supports code evolution: internals can always be re-written as long as interface stays same

The Brave Future

Purely virtual objects. Applications don't start from real-world objects but objects that are already virtual.
Example: Electronic invoice payment app starts with electronic invoice.

Classes

Classes serve two distinct purposes

• Factories that create new objects (code plus specification)
• Classifications of objects (specification only: e.g. Sizable class is any object with a size method)

Classes as Factories

Recall real-world object properties: a means for creating objects.

Making objects from class templates

aSet := Set new "Set is the class; this makes an object"

Classes and Classification

Other main concept behind classes.

Classification: an object can be "of" a certain class not its creator. In this view, classes are like sets.

• John is a man.
• Mary is a woman.
• Bob is a dog.
  
• All men are people.
• All women are people.
  
• All people are mammals.
• All dogs are mammals.

Mammal is an abstract class: only for classification (and inheritance).