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![20
Which one does Java use?
public class Foo {
public String toString() {
return "Foo's toString()";
}
public static void main (String args[]) {
Object bar = new Foo();
System.out.println (bar);
}
}
• Java uses dynamic binding](https://image.slidesharecdn.com/15-oop-231219175457-c9444dfb/85/10-Encapsulation-object-oriented-programming-java-ppt-20-320.jpg)
![20
Which one does Java use?
public class Foo {
public String toString() {
return "Foo's toString()";
}
public static void main (String args[]) {
Object bar = new Foo();
System.out.println (bar);
}
}
• Java uses dynamic binding](https://image.slidesharecdn.com/15-oop-231219175457-c9444dfb/75/10-Encapsulation-object-oriented-programming-java-ppt-20-2048.jpg)
10 - Encapsulation(object oriented programming)- java . ppt
- 1. 1 Chapter 10: DataAbstraction and Object Orientation Aaron Bloomfield CS 415 Fall 2005
- 2. 2 Fundamental OO Concepts •Encapsulation • Inheritance • Dynamic Method Binding
- 3. 3 Encapsulation • Encapsulation – Encapsulationallows the programmer to group data and the subroutines that operate on them together in one 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. 4 Modules • If amodule 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 type. var Database : module exports (tuple with (:=, name)) … type tuple = record var name : packed array 1..80 of char … end tuple … • What can the code outside the Database module do?
- 5. 5 Module Changing • Bodyis Changed • Private Part of Header is Changed • Public Part of Header is Changed
- 6. 6 Classes can limitvisibility • Private • Protected • Public • Package (in some languages, e.g. Java)
- 7. 7 Derived class canrestrict visibility • Private – Protected and public members of base class are private in derived class. • Protected – Protected and public members of base class are protected in derived class. • Public – Protected and public members of base class are protected and public in derived class. • Private members of base class aren’t visible in derived class.
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- 9. 9 Four Important Issues •Choosing a Constructor • References and Values • Execution Order • Garbage Collection – We’ve seen that already
- 10. 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. 11 Constructors • Eiffel code: •class COMPLEX creation new_cartesian, new_polar … new_cartesian(x_val, y_va; : REAL) is … new_polar(rho, theta : REAL) is … • class mydata { public: mydata(string data); mydata(int data); mydata();
- 12. 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 make 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. 13 Execution Order • Ifclass B is derived from class A, A constructor is called before B constructor – To get arguments to the A constructor, you must use an intializer list class foo : bar { ... } foo::foo (foo_params) : bar(bar_params) { … – The part after the colon is a call to bar’s constructor
- 14. 14 Destructors and GarbageCollection • When an object is destroyed, the destructor is called for the derived class first, 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. 15 Java’s finalize() method •In Java, you can override the finalize() method • This allows code to be executed when the object is about to be deleted – But you shouldn’t extend the object’s lifetime by doing this – As the finalize() method is only called once per object
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- 17. 17 Polymorphism • A derivedclass (D) has all the members of its base class (C) – 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. 18 Polymorphism Example class person{ … class student : public person { … class professor : public person { … student s; professor p; … person *x = &s; person *y = &p;
- 19. 19 Dynamic vs. Staticbinding • 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_label();
- 20. 20 Which one doesJava use? public class Foo { public String toString() { return "Foo's toString()"; } public static void main (String args[]) { Object bar = new Foo(); System.out.println (bar); } } • Java uses dynamic binding
- 21. 21 Dynamic method binding •Dynamic method binding: calls to virtual methods are dispatched 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. 22 Dynamic method binding –C++: keyword “virtual” prefixes function declaration class 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. 23 Abstract Methods • Bodylessvirtual methods In C++: called pure virtual method, created by following a procedure declaration with an assignment to zero. class person { … public: virtual void print_mailing_label() = 0;
- 24. 24 Abstract Classes • Classthat 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 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. 25 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 object’s class • Our objects are being more complicated for the compiler to manage – Virtual method tables – Reference counts – Etc…
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- 29. 29 Multiple Inheritance • Derivedclass with two or more base classes • E.g. - Student class • C++: class student : public person, public gp_list_node { … }
- 30. 30 Multiple Inheritance • Supportedin C++, Eiffel, CLOS • Single Inheritance only in Simula, Smalltalk, Modula- 3, Ada 95 & Oberon • Java provides limited support – more on this later
- 31. 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. 32 Multiple inheritance types •Normal (non-repeated) • Repeated • Shared • Mix-in
- 33. 33 Normal (non-repeated) MI •Recall “views” of objects – data members – vtables • Compile-time constant offset d
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- 35. 35 Efficiency (or lackthereof) • May have to determine view dynamically • Results in less time-efficient code • An example implementation may have: – 3 extra cycles, 1 extra memory access over single inheritance – 5 extra cycles, 3 extra memory accesses over static methods
- 36. 36 Semantic Ambiguities • Whatif two base classes have implementations of a shared method? – Won’t work in Eiffel or C++ – In other languages, you must call methods explicitly, i.e. class::method()
- 37. 37 Semantic Ambiguities • Whatif 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. 38 Replicated Multiple Inheritance •Default 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
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- 40. 40 Shared Multiple Inheritance •Default in Eiffel • Ex. Person • Still have problem when inheriting overridden methods
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- 42. 42 Mix-in Inheritance • Onlyone 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. 43 Java Interfaces • publicclass String extends Object implements Serializable, CharSequence, Comparable; • Java interfaces can contain definition prototypes and static variables