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AOS’03 session 1

AOS’03 session 1. Kasper B. Graversen. Today. Who we are and who you are. Introduction Chapter 1 of “Interpretation…” How well do we really know Java? Walkthrough of Simula 67 method binding calling mechanisms co-routines. Who’s who. Kasper G. Context-dependent objects (roles)

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AOS’03 session 1

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  1. AOS’03session 1 Kasper B. Graversen

  2. Today • Who we are and who you are. • Introduction • Chapter 1 of “Interpretation…” • How well do we really know Java? • Walkthrough of Simula 67 • method binding • calling mechanisms • co-routines (C) Kasper B. Graversen

  3. Who’s who • Kasper G. • Context-dependent objects (roles) • Aspect-oriented programming • OO programming languages • Kasper • You (C) Kasper B. Graversen

  4. Who’s who • What’s your opinion on • the curriculum. • the form of evaluation. • any additional bright ideas? (C) Kasper B. Graversen

  5. Chapter 1 of “interpretation…” • The extremes of OO languages • prototypical languages (no classes exist) • class-based languages (as we know from Java) • 4 basic properties of OO languages • Encapsulation (confined data+functionality) • Inheritance (conceptual modeling + code re-use) • Polymorphism (references can refer to many forms) • Dynamic method binding (calls have dynamic ‘destination’)(can be made to break in some languages) • Notice the notion of types is not included! (C) Kasper B. Graversen

  6. Chapter 1 of “interpretation…” • Pure vs. Impure languages • Pure: All constructs relate to object orientation. • Impure: Allows procedural programs • OK characterization, but • words carries too much value • pure lang. may be impractical ie. Java’s maiin method which is difficult to explain and seems not to belong in a class. • Languages are not just “Object Oriented”, but degrees of Object Orientednes. (C) Kasper B. Graversen

  7. How well do we know Java • An understanding of a language can be founded at levels • Syntactic • Valid names • Reserved names • Lexing conventions (chars => tokens) • Semantic • Inheritance, Name spaces, Method calls,Types, casting rules, … (C) Kasper B. Graversen

  8. How well do we know Java • How well do YOU think you know Java? • Which of these are valid identifier names? • Identifier • $my_identifier • $123 • 1booIdentifier • doodi-doo • %popID (C) Kasper B. Graversen

  9. How well do we know Java • Knowing a language one should at least know the meaning of its keywords. • What does these reserved names mean? • finally • strictfp • transient • volatile (C) Kasper B. Graversen

  10. How well do we know Java • How are these lines read by Java?(1) is operator priorities, (2) is lexer rules • 2+3*4 -> 2+(3*4) or (2+3)*4 • i+++j -> i+(++j) or (i++)+j • Why does = always have lower priority than +,-,*, … ? (C) Kasper B. Graversen

  11. How well do we know Java • What is printed on the screen when n=10 if(n > 100) if(n > 200) print(“a”); else print(“b”); (C) Kasper B. Graversen

  12. How well do we know Java • Java has the notion of name spaces. Will this program compile? why/why not. • ns: packages, classes, methods and fields public class String { String String; String(){String = null;} String(String String){this.String = String;} String String(String String) { return new String(String); } } (C) Kasper B. Graversen

  13. How well do we know Java • Method call semantics. Do we know what happens when we do a method call? • During the execution of foo, will “bar” always be printed on the screen? class A { void foo(){ bar() }; } void bar(){print(“bar”); } } (C) Kasper B. Graversen

  14. How well do we know Java class A { void foo(){bar(); } void bar(){print(“bar”); } } class B extends A { void foo(){super(); } void bar(){print(“surprise!”); } } B b = new B(); b.foo(); • No. Here “surprise” is printed Why?? This is revealed later on in - stay tuned :-) (C) Kasper B. Graversen

  15. Programming Language history (C) Kasper B. Graversen

  16. Simula 67 in general • Classes • No destructor method (Java has destroy()) • inspect = instanceof + if-else • qua = cast+error check • in = instanceof • Simula is an impure OO language (C) Kasper B. Graversen

  17. Simula 67 in general • Block structures • Nested classes and procedures • Java only has nested classes and classes in methods • Methods in methods are great • decomposes a method in several parts without having them running around in the class definition • inner methods has access to the outside arguments and variables - no passing of arguments… (C) Kasper B. Graversen

  18. Simula 67 in general • Qualified references • Not just pointing into memory • better readability (a reference is now more than a name) • enables type check (Limits what it can refer to) • improves speed of code • textual prefixing of classes = inheritance • The least specific class code is executed first, inner controlled the order of subclass execution. • No super--either reuse old procedure or completely rewrite it (C) Kasper B. Graversen

  19. Simula 67 in general • Any block can be prefixed, ie. a procedure. • Is used for importing libraries/modules • more detailed and controlled usage • ie. the class Simulation contains functionality for simulation. Any procedure or class which needed the simulation functionality thus were prefixed with ‘simulation’. (C) Kasper B. Graversen

  20. Simula 67 in general • Access modifiers (applies to fields only) • protected (as in Java) • hidden protected (as private in Java) • Appeared Jan. 1973 • An example of encapsulation without hiding (many CS books seems not to be able to make this distinction). (including [Craig02,p3: interpretation of oopl]) • One needs encapsulation before any hiding can take place. (C) Kasper B. Graversen

  21. Simula 67 in general • Imperatives: for, while, if-else • Labels and goto’s • Expressions can contain alternatives • Garbage collector • Appeared in LISP in 1958 (C) Kasper B. Graversen

  22. Simula 67 in general • Method binding • Static/early (non-virtual) • Dynamic/late (virtual, like in Java) • Argument passing in method calls • byvalue • by reference • or by name (C) Kasper B. Graversen

  23. Simula 67 types • Value Types (simple types in Java) • Integer, Short Integer, Long, Real, Boolean, Character • Reference Types • Object Reference, none, text, ”textconstant”, Notext • Notice that text is not an object • Conclusion • VERY similar to Java - “its a matter of a few name changes” (C) Kasper B. Graversen

  24. Simula 67 statements • Assignment: :=(i.e. x := 2) • Reference Assignment: :-(i.e. Queue :- New Head) • Easier to read than = and == some argue • Go to: Go to • Block: Begin … End; • If… Then…; • If… Then… Else…; (C) Kasper B. Graversen

  25. Simula 67 statements • Easy loop specification due to step and until • While: While … do … ; • For: For … do … ; Begin Integer i; For i:= 1 step 10 until 40 do OutInt(i,5); End; (C) Kasper B. Graversen

  26. Simula 67 expressions • IntVal := if A=B then IntVal + 1 else IntVal = 2 • (if A=B then Obj1 else Obj2).Val:= 6 • Similar to Javas ? operator: • IntVal= (a==b ? intVal+1 : 2) • (a==b? obj1 : obj2).val = 6 (C) Kasper B. Graversen

  27. Simula 67 procedures Begin Integer Procedure GCD(M, N); Integer M, N; Begin While M<>N do If M<N then N := N - M else M := M - N; GCD := M End of GCD; Integer A, B; OutText("Enter an integer number: "); OutImage; A := InInt; OutText("Enter an integer number: "); OutImage; B := InInt; OutText("Greatest Common Divisor of your numbers is "); OutInt(GCD(A,B), 4); OutImage; End of Program; (C) Kasper B. Graversen

  28. Simula 67 class • Simula objects consists of • parameters • attributes • procedures • a body (C) Kasper B. Graversen

  29. Simula 67 class ! Class with two parameters; Class Rectangle (Width, Height); Real Width, Height; Begin Real Area, Perimeter; ! Attributes; Procedure Update; ! Void procedure; Begin Area := Width * Height; Perimeter := 2*(Width + Height) End of Update; Boolean Procedure IsSquare; IsSquare := Width=Height; Update; ! Body of rectangle; OutText("Rectangle created: "); OutFix(Width,2,6); OutFix(Height,2,6); OutImage End of Rectangle; (C) Kasper B. Graversen

  30. method bindings & calls

  31. Polymorphism one out of many definitions • A reference can point to objects of different types. The types must be the type of the reference or its subtypes. • ie. Object o = new Car() • but not Car c = new Object() (C) Kasper B. Graversen

  32. Method binding • When a method is called, it must be located. • Static/early binding:Method definitions and calls are explicit and statically defined (at compile-time). • “Works on types” • Dynamic/late binding:Method calls are determined by the currently bound object to the reference at run-time. • Possible due to polymorphism. (C) Kasper B. Graversen

  33. Static/early binding • A continuation of how method calls works in procedural languages (Fortran, Algol and later Pascal and C, …) => may seem more “natural” • C++ considers it an “optimization technique as it yields faster method calls • “most modern languages adopt dynamic binding while older statically typed one generally prefer static binding” [Craig02,p140: interpretation of oopl] • Simula is both old and statically typed! (C) Kasper B. Graversen

  34. Dynamic/late binding • Methods are considered to be virtual. • In many languages the keyword virtual must be denoted in front of the method declaration (ie. Simula, C++, C#) • Ensures it is the most specialized version of the method which is invoked. • two exceptions to this rule • Beta works differently (invokes the LEAST specific first!) • In multiple inheritance it is not clear in case of a conflict which is the most specific (C) Kasper B. Graversen

  35. Dynamic/late binding • so why is it nice? • Instead of a general call, we call on the ‘actual object’. Eg. in a chess game we can Piece[] arr…; for(i=0..n) arr[i].move() • Supports the inheritance concept by enabling an inheritance hierarchy to change implementation in subclasses. • Strengthens the concept of re-use. (C) Kasper B. Graversen

  36. Dynamic/late binding class A { void foo(){bar(); } void bar(){print(“bar”); } } class B extends A { void foo(){super(); } void bar(){print(“surprise!”); } } B b = new B(); b.foo(); • we had a weird example (C) Kasper B. Graversen

  37. Dynamic/late binding class Player{ void play(int no){ … // sound stuff info(); } void info(){ … } } class VideoPlayer extends Player { void play(int no){… super(); } void info(){… // show on screen } } • But it makes more sense when changing it (C) Kasper B. Graversen

  38. Simula method call semantics Class A begin procedure p; begin v() end virtual prodecure v; … end A Class B ! B extends A begin procedure p; begin v; end virtual prodecure v; … end A a :- new B(); a.p(); • Has both kinds of bindings - so what happens here? (C) Kasper B. Graversen

  39. Simula/java method call semantics • What happens here? (from a procedure in B)(this qua A).v() • or in Java A a = new B()((B) a).v() • Or from within a method in Bsuper.v() (C) Kasper B. Graversen

  40. Java method call semantics • All methods are virtual. What is the consequence of that? • Unambiguous method calls / uniformity! • It is always the most specific (most specialized) method which is invoked. • Unambiguous in what fashion? Basically, that every method call is indeterminable -- not just some of them :-) (C) Kasper B. Graversen

  41. Binding of fields • Most languages access methods and fields differently. Are fields virtual in Simula/Java? class A { int x = 0; void foo(){print(x);} } class B extends A { int x = 1; void foo(){ super(); } } B b = new B(); b.foo(); (C) Kasper B. Graversen

  42. Binding of fields • Fields are not virtual in Simula/Java. • B’s x is said to shadowA‘s x, as it is unreachable from outside B (and within unless super is used). • Why is it “natural” to have virtual methods but not virtual fields? … Why is it defined in such a way? • The Java Language Specification (2nd ed) seems not to give any answer. • What do you think? (C) Kasper B. Graversen

  43. Binding of fields • My interpretation is to view methods as “actions” and fields as “data”. • We want the most specific action (unless declared using super) • An action must be able to rely on its data • The object should be ‘encapsulated’ -- data and actions are confined. • Others are be that • there is no need for it • efficiency (C) Kasper B. Graversen

  44. Virtual fields • If we have virtual fields, the only new functionality introduced is the ability to change the type of fields. • However, using accessor methods fields are accessed using methods => field access becomes virtual. • Can increase reusability since we have functionality. (C) Kasper B. Graversen

  45. Virtual fields class Vehicle { int weight, gas; int getWeight(){ return weight; } setWeight(int w){…} drive(int km) {gas-=km*(getWeight()/30)} } class Truck extend Vehicle{ Trailer trailer; int getWeight(){ return super.getWeight()+ trailer.getWeight() } loadTruck() {…} loadTrailer() {…} } • No need to redefine drive() • We can get into trouble, ie. setWeight is that the truck or both truck and trailer? - how do to load the truck only? (C) Kasper B. Graversen

  46. Binding of inner classes • So how about inner classes? • Example: an auto completer in Jedit for different langu-ages (C) Kasper B. Graversen

  47. Virtual inner classes • Each language requires to listening to different things: < for html, \, @ for LaTeX,… class Popup { KeyListener kl; abstract class KeyListener {} Popup(){kl = new KeyListener()} } class LaTeXPopup extends Popup { class Keylistener {…} …} class HTMLPopup extends Popup { class Keylistener {…} …} (C) Kasper B. Graversen

  48. Virtual inner classes • Inner classes are not virtual in Java. • Solution is to use a (factory) method which is virtual, which returns the proper listener. • But does it make sense than fields not being virtual? (C) Kasper B. Graversen

  49. Virtual labels • Labels can be declared virtual in Simula. What do you expect this to mean? • A goto jumps to the most specific label definition (C) Kasper B. Graversen

  50. Virtual labels class SafeMths; begin class SafeDivide(Dividend,Divisor); real Dividend, Divisor; virtual: label ZeroAction,Perform; begin real Result; if Divisor=0 then go to ZeroAction; go to Perform; ZeroAction: Divisor := 0.0001; Inner; Perform: Result := Dividend/Divisor; Complete: end; end; • chapter 18 from “An Introduction to Programming in Simula”, Rob Pooley (C) Kasper B. Graversen

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