1. David Evans http://www.cs.virginia.edu/~evans
Lecture 8: SmallTalking about Objects
Herbert Klaeren: Would you agree that the object-oriented concept has superseded the abstract data type idea. And if you were to redo the CLU development right now, would it become and object-oriented language?
Barbara Liskov: Well, I guess I have a lot of answers to that question. One is that I believe the most important thing about object-oriented programming is data abstraction. It happens to be expressed in a slightly different form in object-oriented languages, with the notion that the operations belong to the objects rather than to the type. But, I don't believe that's a very important difference. I believe the important idea, grouping object and operations together, is supported by both approaches. On the other hand, I am now designing an object-oriented language and it does have an inheritance mechanism and it does have a type hierarchy mechanism. And I have to say that even today I am not a hundred percent convinced about the utility of these mechanisms. But I'm thinking about it.
Question and answer from History of Programming Languages II, 1993.
Background
just got here last week
finished degree at MIT week before
Philosophy of advising students
don’t come to grad school to implement someone else’s idea
can get paid more to do that in industry
learn to be a researcher
important part of that is deciding what problems and ideas are worth spending time on
grad students should have their own project
looking for students who can come up with their own ideas for research
will take good students interested in things I’m interested in – systems, programming languages & compilers, security
rest of talk – give you a flavor of the kinds of things I am interested in
meant to give you ideas (hopefully even inspiration!) but not meant to suggest what you should work on
CS655: Programming Languages
University of Virginia
Computer Science
David Evans

2. University of Virginia CS 655
Menu
What is Object-Oriented Programming?
Surveys: 78 years of C++ experience, 26 years of Java experience
Elevator Speeches (scattered)
Language Features that support OOP
13 November 2018
University of Virginia CS 655

3. University of Virginia CS 655
Stroustrup’s Answer:
The problem is that there is no distinction between the general properties of any shape and the properties of a specific shape. Expressing this distinction and taking advantage of it defines object-oriented programming.
subtype polymorphism?
type hierarchy
O-O P is expressing and taking advantage of the distinction between the general properties of any shape and the properties of a specific shape.
13 November 2018
University of Virginia CS 655

4. University of Virginia CS 655
Can we do this in CLU?
Shape = cluster [spfc_shape: type] is create, draw
where spfc_shape has
draw = proctype (s: spfc_shape, x: int, y: int)
rep = record [shape: spfc_shape, locx: int, locy: int]
create = proc (s: spfc_shape, x: int, y: int) ... end create
draw = proc (s: cvt)
spfc_shape$draw (s.shape, s.x, s.y)
end draw
end Shape
13 November 2018
University of Virginia CS 655

5. University of Virginia CS 655
Using Shape
st: Shape[Triange] = Shape[Triangle]$create (Triangle$create (3,4,5), 10, 20)
sq : Shape[Quadrangle] = Shape[Quadrangle]$create (Quadrangle$create (2,4, 6, 8), 20, 50)
st.draw () % syntactic sugar assumed
sq.draw ()
s := st
s := sq
st and sq are different types – can’t assign them to the same variable
What’s missing?
13 November 2018
University of Virginia CS 655

6. University of Virginia CS 655
Subtype Polymorphism
Shape Triangle
supertype subtype
C++: base class derived class
Eiffel: parent descendant
Java: Triangle extends Shape.
shape
quadrangle
triangle
Triangle is-a Shape.
Triangle is a subtype of Shape.
Triangle  Shape
Subtype can be used where supertype is expected.
Program should still behave as expected if subtype is implemented correctly. ([Liskov & Wing] paper formalizes what this means.)
13 November 2018
University of Virginia CS 655

7. University of Virginia CS 655
Subsumption
Shape s; …
s := new Triangle (...);
...
s := new Quadrangle (...);
B subtype of type A, you can assign B to a reference of type A.
13 November 2018
University of Virginia CS 655

8. University of Virginia CS 655
Subsumption A
, S  T
E : S
[subsumption] A
E : T A
, Triangle  Shape
E : Triangle A
E : Square
13 November 2018
University of Virginia CS 655

9. University of Virginia CS 655
Subtyping Rules
T  T [reflexive- ]
T1  T2; T2  T3 [transitive- ]
T1  T3
T1  T2 [struct- ]
struct [ ..., f: T1, ...]  struct [ ..., f: T2, ...]
(...’s must match)
Structs are monotonic. Struct is a subtype if components are subtypes. (Often called covariant.)
13 November 2018
University of Virginia CS 655

10. University of Virginia CS 655
Arrays
T1  T2 [unsound-array- ]
array[T1]  array [T2]
as : array[Shape]; at : array[Triangle];
...
as := at;
as[2] := new Quadrangle (2, 3, 2, 6);
Triangle t := at[2];
Are we sure about the struct rule’s soundness?
13 November 2018
University of Virginia CS 655

11. University of Virginia CS 655
Procedures
Triangle  Shape
fss = proc (Shape) returns (Shape)
fts = proc (Triangle) returns (Shape)
fst = proc (Shape) returns (Triangle)
ftt = proc (Triangle) returns (Triangle)
fts  fss ? fst  fss ?
ftt  fss ? fss  fts ?
13 November 2018
University of Virginia CS 655

12. University of Virginia CS 655
How to decide?
s: Shape; t: Triangle
s := fss (s); t := fst (s); t := fts (t);
s := fts (s); t := fss (s); t := fss (s);
s := fst (s); t := ftt (s);
s := ftt (s);
13 November 2018
University of Virginia CS 655

13. University of Virginia CS 655
Procedures
fss = proc (Shape) returns (Shape)
fts = proc (Triangle) returns (Shape)
fst = proc (Shape) returns (Triangle)
ftt = proc (Triangle) returns (Triangle)
fts  fss fst  fss
ftt  fss fss  fts
Result can be
more specific
(monotonic = covariant)
Parameters can be
less specific
(anti-monotonic = contravariant)
13 November 2018
University of Virginia CS 655

14. University of Virginia CS 655
Procedure Calls
S  T
 S1 T1 S
Sr  Tr
 Sn Tn
anti-monotonic on parameters
monotonic on results
13 November 2018
University of Virginia CS 655

15. What is Object-Oriented Programming?
Programmer can define subtype relationships
Typing rules that allow subtype to be used in place of supertype (subtype polymorphism)
Type-directed method dispatch
Implementation sharing (inheritance)
Speeches!
13 November 2018
University of Virginia CS 655

16. Type-Directed Method Dispatch
s: Shape := new Triangle (3, 4, 5);
s.draw ();
Static Dispatch:
Calls shape draw method
Dynamic Dispatch:
Calls triangle draw method
13 November 2018
University of Virginia CS 655

17. Dispatching Solutions
Supertype declares methods virtual to allow overriding
Java
Everything is overridable, unless supertype declared it final
Eiffel
Subtype uses explicit redefine clause
Which supports reuse best?
Which is safest?
13 November 2018
University of Virginia CS 655

18. University of Virginia CS 655
Method Binding
s : Shape := new Triangle (3, 4, 5);
h : int := s.getHypotenuse ();
s := new Quadrangle (2, 4, 3, 8);
h : int := s.getHypotenuse (); // no method
Dynamic Checking:
First is okay, second is run-time error.
Static Checking:
Both are compile-time errors.
Smalltalk, Dylan
C++, Java, Eiffel
13 November 2018
University of Virginia CS 655

19. Implementation Reuse: Subclassing, Inheritance
Use implementation of one type to implement another type
Often use implementation of supertype to implement subtype
Commonly used OO languages confuse issue by combining subtyping and inheritance:
Eiffel – cannot separate
Java – cannot separate, can use interfaces for subtyping only
C++ - can use implementation inheritance without subtyping (private, protected inheritance)
13 November 2018
University of Virginia CS 655

20. Language Principle: Getting Defaults Right Matters
Shouldn’t require extra work to hide things, should require extra work to expose them (forgetting something should be safer)
Possible Examples:
Algol60: call-by-value requires extra work (should have been call-by-name)
Java: preventing overriding requires extra work (final) / opposite of C++
C++: preventing subtyping requires extra work (public inheritance is default, need private to reuse implementation without subtyping)
Java access: default is package protected, need private to hide variables and methods
13 November 2018
University of Virginia CS 655

21. A Type and Class Hierarchy
Shape
Quadrangle
Equilateral
Triangle
Parallelogram
EquilateralTriangle
Rhombus
Rectangle
Square
13 November 2018
University of Virginia CS 655

22. University of Virginia CS 655
Add an attribute
Shapes should have a color and set_color method
Change Shape, Quadrangle, Parallelogram, Triangle, Equilateral, EquilateralTriangle, Rhombus, Rectangle, Square, etc.
Change Shape, others inherit new attribute and method automatically
13 November 2018
University of Virginia CS 655

23. University of Virginia CS 655
Add is_equilateral
bool Shape::is_equilateral () {
return false; }
bool Equilateral::is_equilateral () {
return true;
13 November 2018
University of Virginia CS 655

24. Is a Rhombus equilateral?
Shape
is_equilateral () { return false; }
is_equilateral () {
return true; }
Quadrangle
Equilateral
Parallelogram
Multiple inheritance can be tricky!
Rhombus
is_equilateral?
13 November 2018
University of Virginia CS 655

25. University of Virginia CS 655
Solutions
Java, Ada95, BETA
Don’t allow it (Java: interfaces for multiple supertypes, not implementation sharing)
Pro: Safe and Simple, Con: Limits Reuse
Eiffel
Explicit renaming or hiding (error if not done)
Automated Delegation (John Viega, 29 Feb)
13 November 2018
University of Virginia CS 655

26. Smalltalk Design Principles 1
Personal Mastery: If a system is to serve the creative spirit, it must be entirely comprehensible to a single individual.
Good Design: A system should be built with a minimum set of unchangeable parts; those parts should be as general as possible; and all parts of the system should be held in a uniform framework.
Purpose of Language: To provide a framework for communication.
13 November 2018
University of Virginia CS 655

27. Smalltalk Design Principles 2
Objects: A computer language should support the concept of "object" and provide a uniform means for referring to the objects in its universe.
Storage Management: To be truly "object-oriented", a computer system must provide automatic storage management.
Uniform Metaphor: A language should be designed around a powerful metaphor that can be uniformly applied in all areas.
13 November 2018
University of Virginia CS 655

28. Smalltalk Design Principles 3
Modularity: No component in a complex system should depend on the internal details of any other component.
Polymorphism: A program should specify only the behavior of objects, not their representation.
Factoring: Each independent component in a system would appear in only one place.
Operating System: An operating system is a collection of things that don't fit into a language. There shouldn't be one.
Natural Selection: Languages and systems that are of sound design will persist, to be supplanted only by better ones.
13 November 2018
University of Virginia CS 655

29. Stroustrup’s Conclusions
“Object-oriented programming is programming with inheritance. Data abstraction is programming using user-defined types. With few exceptions, object-oriented programming can and ought to be a superset of data abstraction. These techniques need proper support to be effective. Data abstraction primarily needs support in the form of language features and object-oriented programming needs further support from a programming environment. To be general purpose, a language supporting data abstraction or object-oriented programming must enable effective use of traditional hardware.”
13 November 2018
University of Virginia CS 655

30. University of Virginia CS 655
My Conclusions
Object-Oriented Programming is a state of mind.
It is difficult to reach that state of mind if your language doesn’t have a way to declare S  T and the type judgment:
Other language features can help, but we aren’t yet sure what the right ones are: dynamic dispatch, implementation inheritance, mixins, automated delegation, etc. A
, S  T
E : S
[subsumption] A
E : T
13 November 2018
University of Virginia CS 655

31. University of Virginia CS 655
Analogies
Structured Programming is a state of mind.
It is difficult to reach that state of mind if your language doesn’t have structured control statements (e.g., while, for, if, blocks, procedures)
Data Abstraction is a state of mind.
It is difficult to reach that state of mind if your language doesn’t have type checking and mechanisms for restricting access
13 November 2018
University of Virginia CS 655

32. University of Virginia CS 655
Charge
Read Wing & Liskov paper: what must be true about S and T to make S  T is safe?
Continue working on projects – make sure you understand all comments
Position Paper 2 due Monday
Talk: today at 3:30, in the Rotunda:
Evelyn Fox Keller, “Linking Organisms and Computers: Theory and Practice in Contemporary Molecular Biology"
13 November 2018
University of Virginia CS 655