1. Messages, Instances, and Initialization (Methods) CMPS 2143

2. Terms Creation – allocation of memory space for a new object and the binding of that space to a name Initialization – setting initial data values for the object and establishing initial conditions for manipulating it Message passing (method lookup, method call) – dynamic process of asking an object to perform a specific action. 2

3. Messages A message is always given to some object, called the receiver Different objects may accept the same message and yet perform different actions ▫ eg. circle.getArea() and square.getArea() 3 parts to a message-passing expression ▫ aGame.displayCard (aCard, 42, 27) receiver message selector arguments 3

4. Variations in Messages Syntax Most common syntax uses period to separate receiver from selector / some use a space Requiring ( ) when they are empty (known as a unary message) Messages may require keyword notations ▫ aGame displayCard: aCard atLocation: 45 and: 56. Might use brackets ▫ int cardRank = [aCard getRank]; 4

5. Examples: C++, C#, Java, Python, Ruby aCard.flip (); aCard.setFaceUp (true); Pascal, Delphi, Eiffel, Oberon aCard.flip; aCard.setFaceUp (true); Smalltalk aCard flip. aCard setFaceUp: true. Objective-C [aCard flip.] [aCard setFaceUp: true]. CLOS (flip aCard) (setFaceUp aCard true) 5

6. Statically vs Dynamically Typed Languages IMPORTANT in regards to message passing Java, C++, C#, and Pascal are statically typed ▫ Use the type of the receiver to check at compile time that a receiver will understand the message selector Smalltalk, CLOS and Python are dynamically typed ▫ No way to do this at compile time, so may generate a run time error if receiver does not understand the message selector Objective-C – you have a choice on a variable by variable basis 6

7. Accessing Receiver within Method Message is ALWAYS passed to a receiver in OOP Receiver (in most languages) does NOT appear in the arguments - it is implicit within the method When necessary, can be explicit and use a pseudo- variable ▫ Java, C++, C# use this ▫ Eiffel uses Current ▫ Smalltalk, Obj-C, Object Pascal – use self ▫ Python, CLOS, Oberon require explicit 7

8. C++ Example of this PlayingCard::PlayingCard (Suits suitValue, int rankValue) { this.suitValue = suitValue; this.rankValue = rankValue; } Rarely needed, unless you need to pass this to another method. Some Java guidelines suggest this style. 8

9. Java Example class QuitButton extends Button implements ActionListener { public QuitButton ( ) { : //install ourself as a listener for button //events addActionListener (this); } : } 9

10. Creating Primitive objects Most languages, variables created in a declaration statement – some can combine initialization Pascal var sum : integer; begin sum := 0; Java, C++, C# int sum = 0; Primitive variables exist within block they are declared 10

11. Creating Objects Usually process of naming and creating Objects separate Java PlayingCard aCard; aCard = new PlayingCard (Heart, 3); or PlayingCard aCard = new PlayingCard (Heart, 3); 11

12. Creating Objects C++ - you have a choice PlayingCard aCard(Heart, 3); or PlayingCard * aCard; aCard = new PlayingCard (Heart, 3); 12

13. Creation of Array of Objects Create/allocate array Create/allocate array elements C++ - can be combined PlayingCard cardArray [52]; //all will have default values Heart, 0 Java PlayingCard cardArray[ ] = new PlayingCard [52]; for (int i = 0; i < 13; i++) cardArray[i] = new PlayingCard (Spade, i+1); Draw Picture 13

14. Memory Allocation All OO languages use pointers in underlying representation – not all expose this rep to the programmer Java “has no pointers” in contrast to C++ (that is they can’t be seen by the programmer) C++ PlayingCard aCard; //automatic variable PlayingCard * aCard = new PlayingCard; //dynamically allocated 14

15. Memory Recovery Primitive variables are automatically recovered on procedure/method exit Automatic variables are automatically recovered on procedure/method exit Dynamically allocated variables are recovered ▫ Using delete or free  Fast, but programmer could forget and create memory leaks ▫ Automatic garbage collection  Slow, but no memory leaks and not possible to accidently try to use memory after freed or try to free memory already freed 15

16. Examples Ojective-C [acard free]; C++ delete aCard Arrays ▫ C++ delete [ ] cardArray; Java, C#, Smalltalk, CLOS – auto garbage collection 16

17. Constructors Constructor is a special method used to initialize a newly created object Want to guarantee that an object can never be used before it has been initialized Do NOT construct twice!!! ▫ You’ll get a redefinition error or memory leak Constructors have same name as class Never have a return type 17

18. Example Java declaration/definition class PlayingCard { public PlayingCard (int s, int r) { suit = s; rank = r; faceup = true; } : } Use aCard = new PlayingCard (PlayingCard.Diamong, 3); 18

19. Overloaded Constructors C++, C#, Java allow several methods with same name, as long as signature is different class PlayingCard { public: PlayingCard () { suit = Diamond; rank = 1; faceUp = true;} PlayingCard (Suit s, int r) { suit = s; rank = r; faceUp = true;} PlayingCard ( PlayingCard & other) { suit = other.suit; rank = other.rank; } }; 19

20. Calling Constructors in C++ PlayingCard cardOne; //invokes default PlayingCard * cardTwo = new PlayingCard; //default PlayingCard cardThree (PlayingCard.Heart, 3); //param. PlayingCard cardFour (cardThree); //copy CAREFUL!!! //creates a new card PlayingCard cardFive; //forward definition for function called cardSix that //returns a PlayingCard PlayingCard cardSix (); // 20

21. Calling Constructors in Java PlayingCard cardOne = new PlayingCard(); //default PlayingCard cardTwo = new PlayingCard(); //default PlayingCard cardThree = new PlayingCard(PlayingCard.Heart, 3); //param. PlayingCard cardFour = cardThree.clone(); //copy CAREFUL!!! //only creates reference PlayingCard cardFive; //syntax error?? PlayingCard cardSix (); 21

22. Other languages Objective-C constructors do not have to have same name, use + sign Apple Object Pascal has none (create using new) Delphi Pascal closer to C++ Python uses __init__ 22

23. Constant values Data fields that can be assigned once and thereafter are not allowed to change Constructors give us the ability to assign the values Java – it is a final variable class ShippingCo implements IShippingCo { //max packages public final int MAX = 100; : } 23

24. Java Final value can be assigned in the constructor (s) class Game implements IGame { public Game (int maxPlayers ) { MAXPLAYERS = maxPlayers; : } : public final int MAXPLAYERS; } 24

25. C++ Just uses const instead One difference between C++ and Java ▫ C++ are truly constant ▫ Java finals asserts that the associated variable cannot be assigned a new value – nothing prevents it from changing its own internal state final aBox = new Box(); //aBox can be assigned only //once aBox.setHeight (10); //can change internal state 25

26. Orthodox canonical class form C++ Guidelines on Style say you should define 4 important methods 1.A default constructor 2.A copy constructor 3.An overloaded assignment operator 4.A destructor (even if it is an empty method) We have already seen the first two 26

27. Destructors and Finalizers Actions to perform at the end of a variable’s lifetime Performed in C++ with a destructor easily ▫ Invoked automatically  On block exit for automatic variables  On delete for dynamic variables ▫ Destructor is the name of the class preceded by a tilde (~)  Has no arguments Should always provide one, even if empty ▫ There is a system default one 27

28. C++ class ShippingCo { public: : ~ShippingCo ( ) { delete [] items;} private: Package [ ] packages; int numPackages; int coID; } 28

29. Other languages Delphi Pascal uses keyword destructor, called when memory is freed Java and Eiffel have similar facilities, although both languages use garbage collection. 29

30. Java finalizer Class FinalizeExample { : public void finalize () { System.out.printline (“finally doing finalization”); System.exit (0); } Object x = new FinalizeExample (); //Superclass Object x = new Integer (3); //redefining x releases memory. : //at any point the gc will get it 30

31. Metaclasses Hidden classes Ha! Classes are objects ▫ Contain static member data and static methods ▫ To access these members ClassName.methodName (args); double y = Math.sqrt (4.4); 31