Chapter 10- Data Abstraction and Object Orientation.ppt

1、1,Chapter 10: Data Abstraction and Object Orientation,Aaron Bloomfield CS 415 Fall 2005,2,Fundamental OO Concepts,Encapsulation Inheritance Dynamic Method Binding,3,Encapsulation,Encapsulation Encapsulation allows the programmer to group data and the subroutines that operate on them together in one

2、place, and to hide irrelevant details from the user.Information Hiding Making objects and algorithms invisible to portions of the system that do not need them.,4,Modules,If a module M exports a type T, the rest of the program can only pass T to subroutines exported from M. T is said to be an opaque

3、type.var Database : moduleexports (tuple with (:=, name)type tuple = recordvar name : packed array 180 of charend tuple What can the code outside the Database module do?,5,Module Changing,Body is ChangedPrivate Part of Header is ChangedPublic Part of Header is Changed,It is a change to the interface

4、 which will often require a user code change,May require user recompile, but never requires a change in user code.,Will never require a change or recompile of the user code,6,Classes can limit visibility,PrivateProtectedPublicPackage (in some languages, e.g. Java),Visible inside the classs methods,V

5、isible inside the methods of the class and its descendants.,Visible anywhere the class declaration is in scope,7,Derived class can restrict visibility,Private Protected and public members of base class are private in derived class. Protected Protected and public members of base class are protected i

6、n derived class. Public Protected and public members of base class are protected and public in derived class. Private members of base class arent visible in derived class.,8,Initialization and Finalization,9,Four Important Issues,Choosing a Constructor References and Values Execution Order Garbage C

7、ollection Weve seen that already,10,Choosing a Constructor,Object-Oriented Languages allow classes to have zero, one or more different constructors. Two ways to distinguish between constructors Different Names Different Number and Types of Arguements,11,Constructors,Eiffel code:class COMPLEXcreation

8、new_cartesian, new_polarnew_cartesian(x_val, y_va; : REAL) isnew_polar(rho, theta : REAL) is class mydata public:mydata(string data);mydata(int data);mydata();,12,References and Values,C+ vs. Java Java uses reference, C+ you can specify Reference Every object is created explicitly so it is easy to m

9、ake sure the correct constructor is called. More elegant, but requires allocation from heap and extra indirections on every access of the object. Value More efficient but harder to control initialization,13,Execution Order,If class B is derived from class A, A constructor is called before B construc

10、tor To get arguments to the A constructor, you must use an intializer listclass foo : bar .foo:foo (foo_params) : bar(bar_params) The part after the colon is a call to bars constructor,14,Destructors and Garbage Collection,When an object is destroyed, the destructor is called for the derived class f

11、irst, then the destructors of the base classes are called. Reverse order of derivation Destructors purpose is to return allocated space back to the heap Many languages provide automatic garbage collection Java, Smalltalk, Eiffel, etc.,15,Javas finalize() method,In Java, you can override the finalize

12、() method This allows code to be executed when the object is about to be deleted But you shouldnt extend the objects lifetime by doing this As the finalize() method is only called once per object,16,Dynamic Method Binding,17,Polymorphism,A derived class (D) has all the members of its base class (C)

13、Class D can be used anytime class C is expected. If class D does not hide any publicly visible members of C then D is a subtype of C. If class D is used in place of class C, this is a form of polymorphism.,18,Polymorphism Example,class person class student : public person class professor : public pe

14、rson student s; professor p; person *x = ,19,Dynamic vs. Static binding,Static method binding uses the type of the reference: s.print_mailing_label(); p.print_mailing_label(); Dynamic method binding uses the class of the object that is referred/pointed to: x-print_mailing_label(); y-print_mailing_la

15、bel();,20,Which one does Java use?,public class Foo public String toString() return “Foos toString()“;public static void main (String args) Object bar = new Foo();System.out.println (bar); Java uses dynamic binding,21,Dynamic method binding,Dynamic method binding: calls to virtual methods are dispat

16、ched to the appropriate implementation at run time based on the class of the object Simula: virtual methods listed at beginning of class declaration CLASS Person; VIRTUAL: PROCEDURE PrintMailingLabel; BEGIN END Person;,22,Dynamic method binding,C+: keyword “virtual” prefixes function declaration cla

17、ss person public: virtual void print_mailing_label (); This requires keeping a virtual method table along with each object More on this in a bit,23,Abstract Methods,Bodyless virtual methods In C+: called pure virtual method, created by following a procedure declaration with an assignment to zero.cla

18、ss person public: virtual void print_mailing_label() = 0;,24,Abstract Classes,Class that contains one or more abstract methods Java: called an interface (which has only abstract methods) Generally not possible to declare object of an abstract class b/c it would be missing at least one member But you

19、 can do so in C+ Serves as a base for concrete classes. Concrete class must provide a definition for every abstract method it inherits Application to dynamic method binding: allows code that calls methods of objects of a base class, assuming that the concrete methods will be invoked at run time.,25,

20、Member Lookup: vtable,In dynamic binding each object is represented with a record whose first field contains the address of a virtual method table (vtable) for that objects classOur objects are being more complicated for the compiler to manage Virtual method tables Reference counts Etc,26,Member Loo

21、kup- vtable,27,Single Inheritance,28,Multiple Inheritance,29,Multiple Inheritance,Derived class with two or more base classes E.g. - Student class C+: class student : public person, public gp_list_node ,30,Multiple Inheritance,Supported in C+, Eiffel, CLOS Single Inheritance only in Simula, Smalltal

22、k, Modula-3, Ada 95 & Oberon Java provides limited support more on this later,31,Why use MI?,Involves a number of tradeoffs Complexity vs. Simplicity Efficiency vs. Scalability How do you decide? Does it satisfy the “is a” relationship? Is object creation speed a constraint?,32,Multiple inheritance

23、types,Normal (non-repeated) Repeated Shared Mix-in,33,Normal (non-repeated) MI,Recall “views” of objects data members vtables Compile-time constant offset d,34,35,Efficiency (or lack thereof),May have to determine view dynamically Results in less time-efficient code An example implementation may hav

24、e: 3 extra cycles, 1 extra memory access over single inheritance 5 extra cycles, 3 extra memory accesses over static methods,36,Semantic Ambiguities,What if two base classes have implementations of a shared method? Wont work in Eiffel or C+ In other languages, you must call methods explicitly, i.e.

25、class:method(),37,Semantic Ambiguities,What if the relationship below occurs?This is repeated multiple inheritance As one of the ancestors is repeated in the parent class of one of the descendents,gp_list_node,person,student,gp_list_node,student_prof,professor,38,Replicated Multiple Inheritance,Defa

26、ult in C+ Ex. gp_list_node Can only directly access one level deep To access a student view of gp_list_node, you must first assign a student_prof pointer into a student or professor pointer,39,40,Shared Multiple Inheritance,Default in Eiffel Ex. Person Still have problem when inheriting overridden m

27、ethods,41,42,Mix-in Inheritance,Only one base class can contain method definitions The other base class(es) contain only abstract methods Only type of MI supported in Java, but not necessarily MI Traditional Java inheritance uses keyword extends Mix-in (interface) inheritance in Java uses keyword implements Done via interfaces,43,Java Interfaces,public class String extends Object implements Serializable, CharSequence, Comparable;Java interfaces can contain definition prototypes and static variables,