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

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. We will see how a simple, uniform object model enables live, dynamic, interactive software development.

© 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?

© 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?

2. Smalltalk Basics

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.

© 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?

© 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?

3. Standard Classes

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!

© 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?

© 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?

4. Smalltalk Coding Idioms

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.

© 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?

© 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?

5. Seaside

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.

6. Debugging

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.

© Oscar Nierstrasz ST — xxx X.21 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?

© Oscar Nierstrasz ST — xxx X.22 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?

7. Best Practice Patterns

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.

© Oscar Nierstrasz ST — xxx X.25 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?

© Oscar Nierstrasz ST — xxx X.26 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?

8. Refactoring and Design Patterns

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

© Oscar Nierstrasz ST — xxx X.29 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?

© Oscar Nierstrasz ST — xxx X.30 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?

9. Understanding Classes and Metaclasses

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

© Oscar Nierstrasz ST — xxx X.33 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?

© Oscar Nierstrasz ST — xxx X.34 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?

10. Reflection

© Oscar Nierstrasz ST — xxx X.36 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?

© Oscar Nierstrasz ST — xxx X.37 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?

11. Working with Bytecode

© Oscar Nierstrasz ST — xxx X.39 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?

© Oscar Nierstrasz ST — xxx X.40 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?

12. Virtual Machines

13. Traits and Classboxes

© Oscar Nierstrasz ST — xxx X.43 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?

© Oscar Nierstrasz ST — xxx X.44 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?

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