1. Dynamic Object-Oriented Programming with
Smalltalk
1. Introduction
Prof. O. Nierstrasz

2. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.2
Smalltalk is still today one of the
few fully reflective, fully dynamic,
object-oriented development
environments.
Smalltalk is still today one of the
few fully reflective, fully dynamic,
object-oriented development
environments.
We will see how a simple, uniform
object model enables live, dynamic,
interactive software development.
We will see how a simple, uniform
object model enables live, dynamic,
interactive software development.

3. © Oscar Nierstrasz
ST — xxx
X.3
What you should know!
How does Smalltalk differ from Java or C++?
Where are Smalltalk programs stored?
Where are objects stored?
What was the Dynabook?
Is a class an object?
What is dynamic binding?
What is the difference between a message and a
method?

4. © Oscar Nierstrasz
ST — xxx
X.4
Can you answer these questions?
What ideas did Smalltalk take from Simula? From Lisp?
Is there anything in Smalltalk which is not an object?
What exactly is stored in the changes file?
If objects have private state, then how can an Inspector
get at that state?
How do you create a new class?
What is the root of the class hierarchy?
If a class is an object, then what is its class? The class
of its class? …
If you don’t know, how would you find out?

5. 2. Smalltalk Basics

6. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.6
Less is More — simple syntax and semantics
uniformly applied can lead to an expressive and
flexible system, not an impoverished one.
Less is More — simple syntax and semantics
uniformly applied can lead to an expressive and
flexible system, not an impoverished one.

7. © Oscar Nierstrasz
ST — xxx
X.7
What you should know!
How can you indicate that a method is “private”?
What is the difference between a comment and a
string?
Why does 1+2*3 = 9?
What is a cascade?
How is a block like a lambda expression?
How do you create a new class?
How do you inspect an object?

8. © Oscar Nierstrasz
ST — xxx
X.8
Can you answer these questions?
Why does Smalltalk support single (and not multiple)
inheritance?
Is the cascade strictly necessary?
Why do you need to declare local variables if there are
no static types?
How can you discover the class of GUI object?
How does SUnit differ from JUnit?

9. 3. Standard Classes

10. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.10
Reify everything — by reifying its entire
implementation model, Smalltalk succeeds in being open,
and extensible.
New features can be added without changing the syntax!
Reify everything — by reifying its entire
implementation model, Smalltalk succeeds in being open,
and extensible.
New features can be added without changing the syntax!

11. © Oscar Nierstrasz
ST — xxx
X.11
What you should know!
How are abstract classes defined in Smalltalk?
What’s the difference between a String and a
Symbol?
Where are class names stored?
What is the difference between self and super?
Why do we need Blocks?
How is a Block like a lambda?
How would you implement Boolean>>and:?
What does inject:into: do?

12. © Oscar Nierstrasz
ST — xxx
X.12
Can you answer these questions?
How are Numbers represented internally?
Is it an error to instantiate an abstract class in Smalltalk?
Why isn’t the assignment operator considered to be a
message?
What happens if you send the message #new to
Boolean? To True or False?
Is nil an object? If so, what is its class?
Why does ArrayedCollection>>add: send itself the
message shouldNotImplement?

13. 4. Smalltalk Coding Idioms

14. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.14
Distribute responsibility — in a well-designed object-
oriented system you will typically find many, small,
carefully named methods.
This promotes fluent interfaces, reuse, and maintainability.
Distribute responsibility — in a well-designed object-
oriented system you will typically find many, small,
carefully named methods.
This promotes fluent interfaces, reuse, and maintainability.

15. © Oscar Nierstrasz
ST — xxx
X.15
What you should know!
What does yourself return? Why is it needed?
How is a new instance of a class initialized?
When should you implement invariants and
preconditions?
What happens when we evaluate an expression with
“print it”?
Why should a method never send super a different
message?
How is super static and self dynamic?
How do you make your code self-documenting?

16. © Oscar Nierstrasz
ST — xxx
X.16
Can you answer these questions?
When should you override new?
If instance variables are really private, why can we see
them with an inspector?
When does self = super?
When does super = self?
Which classes implement assert: ?
What does self refer to in the method
SnakesAndLadders class>>example?

17. 5. Seaside

18. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.18
Model your domain with objects — model
domain components as objects. Compose objects, not
text. Strive for fluent interfaces. Build applications by
scripting components.
Model your domain with objects — model
domain components as objects. Compose objects, not
text. Strive for fluent interfaces. Build applications by
scripting components.

19. 6. Debugging

20. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.20
It can be easier to talk to objects than to read
classes — The system is alive. Talk to it.
The debugger can be your best friend. Don’t be afraid of it.
It can be easier to talk to objects than to read
classes — The system is alive. Talk to it.
The debugger can be your best friend. Don’t be afraid of it.

21. © Oscar Nierstrasz
ST — xxx
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What you should know!
When should you explicitly return self?
Why shouldn’t you redefine methods named basic*?
Why are blocks not full closures?
How do you provide access to instance variables that
are collections, without breaking encapsulation?
What is one of the most important uses of super?
How does programming with Smalltalk differ from
programming in a conventional static language?

22. © Oscar Nierstrasz
ST — xxx
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Can you answer these questions?
What will happen if you redefine the method class?
When should you define accessors for instance
variables?
How can explicit references to class names make your
application fragile?
Where is the method halt defined?

23. 7. Best Practice Patterns

24. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.24
Let your code talk — Names matter. Let the
code say what it means.
Introduce a method for everything that needs to be
done. Don’t be afraid to delegate, even to yourself.
Let your code talk — Names matter. Let the
code say what it means.
Introduce a method for everything that needs to be
done. Don’t be afraid to delegate, even to yourself.

25. © Oscar Nierstrasz
ST — xxx
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What you should know!
How should you name instance variables?
Why should you be suspicious of comments?
How does Simple Delegation differ from Self
Delegation?
When would you use Double Dispatch?
Why should you avoid introducing a Converter Method
for an object supporting a different protocol?
How do you sort a Collection?
When should you use Lazy Initialization?

26. © Oscar Nierstrasz
ST — xxx
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Can you answer these questions?
Which patterns would you use to implement a
transactional interface?
How can Method Object help you to decompose long
methods?
Why is it a bad idea to query an object for its class?
Why are you less likely to see Double Dispatch in a
statically-typed language?
How can you avoid Modifying Super?
How can you avoid writing case statements?
What pattern does Object>>-> illustrate?

27. 8. Refactoring and Design Patterns

28. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.28
Beware of misplaced
responsibilities — cluttered code
impacts extensibility.
Beware of misplaced
responsibilities — cluttered code
impacts extensibility.

29. © Oscar Nierstrasz
ST — xxx
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What you should know!
How does the Open-Closed Principle apply to OOP?
What are signs that an object has clearly-defined
responsibilities?
How can you recognize misplaced methods?
How should you refactor long methods?
How can you eliminate duplicated code between
unrelated classes?
Why are type tests a code smell?
When do design patterns themselves turn into code
smells?
Why is it a bad idea to use global variables to store
Singleton instances?

30. © Oscar Nierstrasz
ST — xxx
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Can you answer these questions?
How do the Common Closure and Common Reuse
Principles alter the usual notion of cohesion?
How does refactoring differ from reengineering?
Can refactoring be fully automated?
In what situations does the Law of Demeter not apply?
How do design patterns make use of delegation?
Why are Long Parameter Lists a code smell?
Are isNil tests a code smell? What design pattern
could help you eliminate them?
Is the Smalltalk SystemDictionary a good example of a
Singleton?

31. 9. Understanding Classes and Metaclasses

32. Birds-eye view
© Oscar Nierstrasz
ST — Introduction
1.32
Reify your metamodel — A fully reflective
system models its own metamodel.
Reify your metamodel — A fully reflective
system models its own metamodel.

33. © Oscar Nierstrasz
ST — xxx
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What you should know!
What does is-a mean?
What is the difference between sending a message to
an object and to its class?
What are the responsibilities of a metaclass?
What is the superclass of Object class?
Where is new defined?
What is the difference between class variables and
class instance variables?

34. © Oscar Nierstrasz
ST — xxx
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Can you answer these questions?
Why are there no explicit metaclasses?
When should you override new?
Why don’t metaclasses inherit from Class?
Are there any classes that don’t inherit from Object?
Is Metaclass a Class? Why or why not?
Where are the methods class and superclass defined?
When should you define an indexed class?
Are Java static variables just like class variables or class instance
variables?
Where is the SystemDictionary Smalltalk defined?

35. 10. Reflection

36. © Oscar Nierstrasz
ST — xxx
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What you should know!
What is the difference between introspection and
intercession?
What is the difference between structural and
behavioural reflection?
What is an object? What is a class?
What is the difference between performing a message
send and simply evaluating a method looked up in a
MethodDictionary?
In what way does thisContext represent the run-time
stack?
What different techniques can you use to intercept and
control message sends?

37. © Oscar Nierstrasz
ST — xxx
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Can you answer these questions?
What form of “reflection” is supported by Java?
What can you do with a metacircular architecture?
Why are Behaviour and Class different classes?
What is the class ProtoObject good for?
Why is it not possible to become: a SmallInteger?
What happens to the stack returned by thisContext if you
proceed from the self halt?
What is the metaclass of an anonymous class?

38. 11. Working with Bytecode

39. © Oscar Nierstrasz
ST — xxx
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What you should know!
What are the problems of the old compiler?
How is the new Squeak compiler organized?
What does the Squeak semantic analyzer add to the
parser-generated AST?
What is the format of the intermediate representation?
What kind of virtual machine does the Squeak bytecode
address?
How can you inspect the bytecode of a particular
method?

40. © Oscar Nierstrasz
ST — xxx
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Can you answer these questions?
What different groups of bytecode are supported?
Why is the SmaCC grammar only BNF-“like”?
How can you find out what all the bytecodes are?
What is the purpose of IRBuilder?
Why do we not generate bytecode directly?
What is the responsibility of class InstructionStream?
How would you implement a statement coverage
analyzer?

41. 12. Virtual Machines

42. 13. Traits and Classboxes

43. © Oscar Nierstrasz
ST — xxx
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What you should know!
Why does single inheritance lead to duplicated code?
How does the composing class retain control of trait
composition?
What do “glue” methods do for traits?
What is the “flattening property” and why is it important
for traits?
Why is there “inappropriate inheritance” in the Smalltalk
Collections hierarchy?
What is a “class extension”?
In what way to classboxes ensure locality of changes?
What problems are solved by combined traits and
classboxes?

44. © Oscar Nierstrasz
ST — xxx
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Can you answer these questions?
Why do multiple inheritance and mixins leads to “fragile
class hierarchies”?
C++, Eiffel and Python all offer multiple inheritance – are
they broken?
Why don’t traits specify any state?
How much code is duplicated in the standard Java
libraries?
What problems occur in Java due to the lack of class
extensions?
Can classboxes be “flattened” in the same way that traits
can? Why or why not?

45. © Oscar Nierstrasz
ST — Introduction
1.45
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