1 OO in Programming and Design

This course is concerned with object-oriented programming (OOP), design (OOD), and analysis (OOA). Clearly, with so much of this OO stuff flying around, it would be nice if we had an idea of what an object is, and just what is meant by “object-oriented”.

1.1 Observations on Pre-OO Programming Languages

The Roots of OOP

• OO programming and design have their roots in simulation.

• Simula (1967) programming language introduced objects & classes to describe the physical world being simulated.

OO programming and design have their roots in simulation. In the late 1960s, the population of programmers was dominated by people who had switched fields, typically from the mathematics or physical sciences, rather than people who had originally trained in computer science. Indeed, academic programs in computer science were far less common than they are today, and tended to treat CS as more of a tool to be applied to application areas rather than as a field of study in its own right. Commonly, a CS degree program would offer two tracks - business programming and scientific programming.

One of the more active and exciting application areas at the time was simulation. Programmers in this area began to recognize common patterns arising over and over in their code. A number of code libraries were designed and distributed that attempted to capture and simplify these common simulation tasks. Eventually, special-purpose simulation programming languages were developed.

There was ample precedence for this kind of specialization. The dominant languages of the time were generally perceived as tailored to specific applications: FORTRAN and APL for science applications, COBOL for business, LISP for A.I., SNOBOL for text/string processing, etc. Any of these languages could have been used for other kinds of applications, but it was certainly easier to work with a language that matched well with the application area.

1.2 Tension in Pre-OO Programming Languages

The history of programming languages and of design can be viewed as a continual contest against increasing complexity of software systems:

1.3 Pre-OO Design Techniques

Observations

• If you Google for ADTs (Abstract Data Types)…
  • Most of the examples you will get are general-purpose data structures (stacks, vectors, lists, etc)
  • It’s easy to get the impression that an ADT is something that someone else provides for you as a convenient little package of code.
  • Misses the big point
• ADTs are an organizing principle for your own programs.
• But how do you design a program in a way that winds up with good ADTs?

OK, let’s agree that a good ADT is a wonderful thing. Just where do good ADTs come from? Or, to put it another way, when you are designing a program, how do you figure out which ADTs you should use?

If you search the web for the term “abstract data type”, you will find lots of examples of things you might pull out of a library of data structures: stacks, queues, lists, etc. It’s easy to get the impression that an ADT is something that someone else provides for you as a convenient little package of code.

But that misses the point that “abstract data type” is an organizing principle for your own programs. ADTs for general purpose data structures are all very nice, but how do you come up with the special-purpose ADTs that organize your program in a way that is specific to the problem it solves?

Pre-OO design techniques were oriented towards functions performed by the program. For example, if you were working on automating the management of book handling in a typical library, you might start by dividing the system into major subsystems such as RecordStorage, CheckInOut, AccountMgmt, and InventoryMgmt. Under each subsystem, you would group functions to be performed, such as CheckOutBook, CheckInBook, and RenewBook under the CheckInOut subsystem, or addBook, removeBook, and transferToBranch under InventoryMgmt.

1.4 Stepwise Refinement & Top-Down Design

• The most fundamental design technique

  • Uses pseudocode

• Start with a single statement describing the desired behavior

• Repeatedly
  • Select a pseudocode statement that is too vague to be coded
  • Replace that single statement with two or more, slightly more detailed, statements that accomplish the same thing.

Stepwise Refinement Example

Automating book handling in a library:

• Might start by dividing the system into major subsystems such as RecordStorage, CheckInOut, AccountMgmt, and InventoryMgmt.

• Under each subsystem, you would group functions to be performed, such as

  • CheckOutBook, CheckInBook, and RenewBook under the CheckInOut subsystem, or
  • addBook, removeBook, and transferToBranch under InventoryMgmt.

If pre-OO designers were asked to design one of these functions, say, addBook, they would probably do so by “stepwise refinement”, a.k.a. “top-down design”.

Stepwise Refinement Example II

Suppose we wanted to design the function to add a new book to the library’s collection.

• Designers would reason that this function must both update the electronic card catalog so that the book can be found, and must also record that the book is present in the inventory of a particular branch:

function addBook (book, branchName)
{
  bookInfo = getBookInfo(book);
  record branchName as location in bookInfo;
  addToCardCatalog (bookInfo);
  addToInventory (bookInfo, branchName);
}

Stepwise Refinement Example III

Next the designers would pick one of those rather vaguely understood lines of code in that function body and expand it, either in place or as a separate function. For example:

function addBook (book, branchName)
{
  bookInfo = getBookInfo(book);
  record branchName as location in bookInfo;
  // addToCardCatalog (bookInfo)
  add to AuthorIndex (bookInfo);
  add to SubjectIndex (bookInfo);
  add to TitleIndex (bookInfo);
  cardCatalog.addToAuthorIndex
  addToInventory (bookInfo, branchName);
}

Then, again, we would pick a vaguely understood line and expand it.

Stepwise Refinement Example IV

function addBook (book, branchName)
{
  bookInfo = getBookInfo(book);
  record branchName as location in bookInfo;
  // addToCardCatalog (bookInfo)
  //   add to AuthorIndex (bookInfo);
  authorList = bookInfo.getAuthors();
  for each author au in authorList {
    catalog.authorIndex.addByAuthor (au, bookInfo);
  }
  add to SubjectIndex (bookInfo);
  add to TitleIndex (bookInfo);
  cardCatalog.addToAuthorIndex
  addToInventory (bookInfo, branchName);
}

• The process of “pick something vague and expand it” continues until the desired level of detail has been reached.

1.5 Top-Down Example: Observations

• In this example, we have mentioned nearly a dozen functions. None of them have been associated with an ADT.

• An experienced designer might spot some ADT candidates: Book, BranchLibrary, CardCatalog, etc.

• But there’s nothing in the process that encourages or supports this.

Now, stepwise refinement is a valuable technique. It’s still probably the technique of choice for low-level design (the design of an algorithm for a single function). But it’s not nearly so useful as a technique for high-level design (the separation fo a large problem into separate modules). Consider that, at this point in our example, we have mentioned about a dozen different function names. None of them have been associated with an ADT. What are the odds that any of them would wind up inside an ADT? Pretty slim, usually.

An experienced designer might take note that the above process gives some hints as to some potential ADTs (Book, branchLibrary, CardCatalog, etc.). But this insight is pretty much separate from the design process itself. A designer has to be somewhat schizoid, actively participating in the design while simultaneously standing aside as a detached observer, watching for hints of possible ADTs.

2 The Object-Oriented Philosophy

2.1 Program = Simulation

The Object-Oriented Philosophy

• Every program is really a simulation.

• The quality of a program’s design is proportional to the faithfulness with which the structures and interactions in the program mirror those in the real world.

Design from the World

• Walk into a library and what do you see?

  • Books, Librarians, Patrons (customers), Shelves, …

  • In an older library you might actually see a card catalog!

  • Of course, on the way in the door you saw the building/Branch Library.

• These things form a set of candidate ADTs.

  • Next we would explore the various properties and relationships that these things exhibit in real life:

Real-World Relationships

• Books have titles, authors, ISBN, …

• Librarians and Patrons have names. Patrons have library cards with unique identifiers. Most librarians are also patrons.

• Patrons return books. Librarians check in the returned books.

• Librarians check books out for patrons. (At some libraries, patrons can also do their own check-out.) Librarians handle the processing of newly acquired books.

• Although books can be returned to any branch, each book belongs to a specific branch and will, if necessary, be delivered there by library staff.

Organizing the World

OO designers would start by organizing these things into ADTs, e.g.:

Simulate the World’s Actions

Only then would OO designers consider the specific steps required to add a book to inventory:

“update the electronic card catalog so that the book can be found, and record that the book is present in the inventory of a particular branch”

Simulate the World’s Actions

function Librarian::acquisition (Book book)
{
  BranchLibrary branch = this.assignment;
  Catalog catalog = mainLibrary.catalog;
  catalog.add (book, branch);
  branch.addToInventiory (book);
}

function Catalog::add (Book book)
{
  authorIndex.add (book);
  titleIndex.add (book);
  subjectIndex.add (book);
}

OO Approach - Observations

Technically, does the same thing as the earlier design,but

• Division into ADTs is a natural consequence of the “observational” approach

• More concern with terminology (“acquisition” rather than “addBook”, “patron” rather than “customer”, “stacks” rather “shelves”

• Avoids programmer-isms such as BookInfo

• Separation of concerns is better (e.g., catalog functions are not built into the librarian code)

2.2 Simulation == Modeling

OO Analysis & Design

• Analysis is the process of determining what a system should do

• Design is the process of figuring out how to do that

Models

We can rephrase that as

• Analysis is the construction of a model of the world in which the program will run.

  • including how the program will interact with that world

• Design is the construction of a model of a program to work within that world.

Steps in Modeling

• Classification: discovering appropriate classes of objects

• Refined by documenting

• interactions between objects

• relations between classes

• Both steps often driven by scenarios

3 Putting the “Programming” in OOP

OOP in Programming Language History

3.1 What is an Object?

Maybe you thought I was never going to get around to this?

Objects

An object is characterized by

• identity

• state

• behavior

Identity

Identity is the property of an object that distinguishes it from all others. We commonly denote identity by

• names, such as Steve, George,

  • in programming, x, y

• references that distinguish without names: this, that,

  • in programming, *p

State

The state of an object consists of the properties of the object, and the current values of those properties.

For example, given this list of properties:

 struct Book {
    string title;
    Author author;
    ISBN isbn;
    int edition;
    int year;
    Publisher publisher;
 };

we might describe the state of a particular book as

 Book cs330Text = {
    "Object Oriented Design & Patterns", horstmann,
    "0-471-74487-5", 2, 2006,
    wileyBooks};

Behavior

Behavior is how an object acts and reacts to operations on it, in terms of

• visible state changes and

• operations on other visible objects.

3.2 Messages & Methods

In OOP, behavior is often described in terms of messages and methods.

A message to an object is a request for service.

A method is the internal means by which the object responds to that request.

Differing Behavior

Different objects may respond to the same message via different methods.

Example: suppose I send a message “send flowers to my grandmother” to …

• a florist

• my sister

• a Department secretary
• a tree

3.3 What is a Class?

A class is a named collection of potential objects.

In the languages we will study,

• all objects are instances of a class, and

• the method employed by an object in response to a message is determined by its class.

• All objects of a given class use the same method in response to similar messages.

Are Objects and Classes New

Classes versus Types

Although “class” and “type” mean nearly the same thing, they do carry a slightly different idiomatic meaning.

• In conventional PLs, each value (object) is an instance of exactly one type.

• In OOPLs, each object (value) may be an instance of several related classes.

Instances of Multiple Types

• Classes can be related via “inheritance”.

  • Inheritance captures an “is-a” or “is a specialized form of” relationship.

Inheritance Example

What makes a PL an OOPL

• It must provide support for variant behavior

  • Usually accomplished via dynamic binding

• It must provide a way to associate common messages with different classes.
  • Usually accomplished via
    • inheritance, and
    • subtyping

.