COMP 401 DYNAMIC DISPATCH AND ABSTRACT METHODS. Instructor: Prasun Dewan

Transcription

1 COMP 401 DYNAMIC DISPATCH AND ABSTRACT METHODS Instructor: Prasun Dewan

2 A POINTHISTORY IMPLEMENTATION public class APointHistory implements PointHistory { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { return contents[index]; protected boolean isfull() {return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = new ACartesianPoint(x, y); contents[size] = p; size++; 2

3 A POLARPOINTHISTORY IMPLEMENTATION public class APolarPointHistory implements PointHistory { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { return contents[index]; Reducing code duplication? protected boolean isfull() {return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = new APolarPoint(x, y); contents[size] = p; size++; 3

4 A POLARPOINTHISTORY IMPLEMENTATION public class APolarPointHistoryWithAddMethod extends APointHistory { public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = new APolarPoint(x, y); contents[size] = p; size++; Only line changed, still code duplication 4

5 A POINTHISTORY IMPLEMENTATION public class ACartesianPointHistoryWithDynamicallyDispatchedMethod implements PointHistory { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { return contents[index]; protected boolean isfull() { return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = createpoint(x, y); contents[size] = p; size++; protected Point createpoint(int x, int y) { return new ACartesianPoint(x, y); 5

6 OVERRIDING METHOD public class APolarPointHistoryWithDynamicallyDispatchedMethod extends ACartesianPointHistoryWithDynamicallyDispatchedMethod { protected Point createpoint(int x, int y) { return new ACartesianPoint(x, y); public static void main (String[] args) { PointHistory pointhistory = new APolarPointHistoryWithDynamicallyDispatchedMethod(); pointhistory.addelement(50, 100); 6

7 A POINTHISTORY IMPLEMENTATION public class ACartesianPointHistoryWithDynamicallyDispatchedMethod implements PointHistory { public public class final APolarPointHistoryWithDynamicallyDispatchedMethod MAX_SIZE = 50; protected extends Point[] ACartesianPointHistoryWithDynamicallyDispatchedMethod contents = new Point[MAX_SIZE]; { protected protected int Point size createpoint(int = 0; x, int y) { public return int new size() ACartesianPoint(x, { y); return size; public Point elementat (int index) { return contents[index]; PointHistory pointhistory = new APolarPointHistoryWithDynamicallyDispatchedMethod(); protected boolean isfull() { pointhistory.addelement(50, 100); return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = createpoint(x, y); contents[size] = p; size++; protected Point createpoint(int x, int y) { return new ACartesianPoint(x, y); Static dispatching: call most specific method known when caller method was compiled Dynamic dispatching: call most specific method for the calling object 7

8 VIRTUAL VS. REGULAR METHOD AND DIFFERENT LANGUAGES Virtual method: Call to it is dynamically dispatched Regular method: Call to it is statically dispatched Java: All instance methods are virtual methods and all class (static)methods are regular methods C++: Instance methods can be declared to be regular or virtual and all class (static)methods are regular methods Smalltalk: All methods are virtual 8

9 INHERITANCE RELATIONSHIPS? APolarPointHistoryWithDynamicallyDispatchedMethod IS-A ACartesianPointHistoryWithDynamicallyDispatchedMethod ACartesianPointHistoryWithDynamicallyDispatchedMethod IS-A APolarPointHistoryWithDynamicallyDispatchedMethod 9

10 CORRECT INHERITANCE AnAbstractPointHistory IS-A ACartesianPointHistory IS-A APolarPointHistory 10

11 CORRECT INHERITANCE AnAbstractPointHistory IS-A ACartesianPointHistory IS-A APolarPointHistory 11

12 A CATERSIAN HISTORY IMPLEMENTATION public class ACartesianPointHistoryWithDynamicallyDispatchedMethod implements PointHistory { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { return contents[index]; Changed how? protected boolean isfull() { return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = createpoint(x, y); contents[size] = p; size++; protected Point createpoint(int x, int y) { return new ACartesianPoint(x, y); 12

13 REMOVING CREATEPOINT public abstract class AnAbstractPointHistory implements PointHistory { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { return contents[index]; protected boolean isfull() { return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = createpoint(x, y); contents[size] = p; size++; 13

14 NULL OVERRIDDEN METHOD public abstract class AnAbstractPointHistory implements PointHistory { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { return contents[index]; protected boolean isfull() { return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = createpoint(x, y); contents[size] = p; size++; protected Point createpoint(int x, int y) { 14

15 OVERRIDING DYNAMICALLY DISPATCHED METHOD public class ACartesianPointHistory extends AnAbstractPointHistory{ protected Point createpoint(int x, int y) { return new ACartesianPoint(x, y); 15

16 ERRONEOUS IMPLEMENTATION WITH NO ERRORS public class ACartesianPointHistory extends AnAbstractPointHistory{ 16

17 ABSTRACT METHOD public abstract class AnAbstractPointHistory implements PointHistory { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { return contents[index]; protected boolean isfull() { return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = createpoint(x, y); contents[size] = p; size++; Only header of method provided as in interface protected abstract Point createpoint(int x, int y); 17

18 ERRONEOUS IMPLEMENTATION public class ACartesianPointHistory extends AnAbstractPointHistory{ Java complains abstract method not implemented in concrete class 18

19 NULL VS. ABSTRACT METHODS Null methods could replace abstract methods. Abstract method force overriding implementations in subclasses. Provide better documentation, indicating to programmer that subclass will implement them. 19

20 ABSTRACT METHOD Declared only in abstract classes Keyword: abstract No body Each (direct or indirect) subclass must implement abstract methods defined by an abstract class. Much like each class must implement the methods defined by its interface(s). 20

21 ABSTRACT CLASS VS. METHODS Abstract method => containing class abstract Cannot have an unimplemented method in an instance Abstract class may not contain abstract method 21

22 A COURSE WITH ABSTRACT METHOD public abstract class ACourse { String title, dept; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; public String gettitle() { return title; public String getdepartment() { return dept; abstract public int getnumber(); public String tostring() { return "Title:" + title + " Dept:" + dept + " Number: " + getnumber(); Abstract Method 22

23 EQUIVALENT WITH INTERFACE public abstract class ACourse implements Course { String title, dept; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; public String gettitle() { return title; public String getdepartment() { return dept; public String tostring() { return "Title:" + title + " Dept:" + dept + " Number: " + getnumber(); An abstract class can implement any interface This means all sub classes implement the interface Interface implementation check is made for concrete classes 23

24 ANOTHER COURSE package courses; public abstract class ACourse implements Course { String title, dept; public ACourse (String thetitle, String thedept) { super(); title = thetitle; dept = thedept; public String gettitle() { return title; public String getdepartment() { return dept; public String tostring() { return "Title:" + title + " Dept:" + dept + " Number: " + getnumber(); This means all sub Interface implementation An abstract class can classes implement the check is made for concrete implement any interface classes interface 24

25 ALTERNATIVE AREGULARCOURSE public class ARegularCourse extends ACourse { int coursenum; public ARegularCourse (String thetitle, String thedept, int thecoursenum) { super (thetitle, thedept); coursenum = thecoursenum; public int getnumber() { return coursenum; Saying ARegularCourse implements Course is redundant 25

26 ALTERNATIVE AFRESHMANSEMINAR public class AFreshmanSeminar extends ACourse { public AFreshmanSeminar (String thetitle, String thedept) { super (thetitle, thedept); public int getnumber() { return SEMINAR_NUMBER; No need for implementation clause 26

27 WHEN ABSTRACT METHOD REQUIRED public abstract class AnAbstractPointHistory implements PointHistory { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { return contents[index]; protected boolean isfull() { return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = createpoint(x, y); contents[size] = p; size++; protected abstract Point createpoint(int x, int y); Not public 27

28 ADDING CREATEPOINT TO INTERFACE public interface PointHistoryWithExtraPublicMethod { public Point createpoint(int x, int y); Factory method public abstract class AnAbstractPointHistoryWithExtraPublicMethod implements PointHistoryWithExtraPublicMethod { public final int MAX_SIZE = 50; protected Point[] contents = new Point[MAX_SIZE]; protected int size = 0; public int size() { return size; public Point elementat (int index) { Least privilege violated return contents[index]; protected boolean isfull() { return size == MAX_SIZE; public void addelement(int x, int y) { if (isfull()) System.out.println("Adding item to a full history"); else { Point p = createpoint(x, y); contents[size] = p; size++; 28

29 FACTORY ABSTRACT METHOD abstract <T> <M>( ) ; extends <T> <M>( ) { return new <C1> ( ) <T> <M> ( ){ return new <C2> ( ) Abstract method that returns an instance of some type T like a factory, it creates and initializes an object. 29

30 ABSTRACT METHODS VS. INTERFACES Unimplemented methods become abstract methods to be implemented by concrete subclasses. Do not need explicit implements clause in subclass if no additional public methods implemented, public class ARegularCourse extends ACourse {. A class with only abstract methods simulates interfaces. No multiple inheritance in Java Separation of abstract and concrete methods with interfaces. A class implements but does not inherit a specification! 30

31 ABSTRACT METHODS VS. INTERFACES Non-public abstract methods relevant even in Java All interface methods must be public. May want non public abstract methods.e.g. abstract boolean isfull() ; 31

32 ABSTRACT METHODS VS. VIRTUAL METHODS Abstract methods must be virtual methods. Virtual methods need not be abstract methods 32

33 FACTORY METHODS VS. ABSTRACT METHODS Some abstract methods may be factory methods Factory methods may not be abstract methods but often this means bad programming practice APolarPointHistoryWithDynamicallyDispatchedMethod IS-A ACartesianPointHistoryWithDynamicallyDispatchedMethod 33

34 OVERLOADING, POLYMORPHISM, AND DYNAMIC DISPATCH Polymorphic method ARegularCourse static void print (Course course) { System.out.println( course.gettitle() + " " + course.getdepartment() + course.getnumber() ); AFreshmanSeminar getnumber() in ARegularCouse static void print (CourseList courselist) { Overloaded method print(course); getnumber() in AFreshmanSeminar Dynamically dispatched method 34

35 OVERLOADING, POLYMORPHISM, AND DYNAMIC DISPATCH Same method name works on different types. Overloading Multiple (static or instance) methods with same name but different parameter types. Resolved at compile time based on parameter types Polymorphism Same method implementation works on multiple object types. Dynamic dispatch (or virtual methods) Multiple instance methods with same name in different classes Resolved at execution time based on type of target object 35

36 POLYMORPHISM VS. OVERLOADING AND DYNAMIC DISPATCH Polymorphism vs. Overloading Polymorphism: single print (Course course) Overloading: print (ARegularCourse course) and print (AFreshmanSeminar course) with same implementation. Polymorphism vs. Dynamic Dispatch Polymorphism: single gettitle() in ACourse Dynamic dispatch: gettitle() in AFreshmanSeminar() and gettitle() in ARegularCourse() with same implementation. Create polymorphic code when you can In overloading and dynamic dispatch, multiple implementations associated with each method name. In polymorphic case, single implementation. Use interfaces rather than classes as types of parameters Use supertypes rather than subtypes as types of parameters 36

37 POLYMORPHISM VS. OVERLOADING AND DYNAMIC DISPATCH Cannot always create polymorphic method. getnumber() for ARegularCourse and AFreshmanSeminar do different things. print(course course) and print (CourseList courselist) do different things. When polymorphism not possible try overloading and dynamic dispatch. 37

38 MANUAL INSTEAD OF DYNAMIC DISPATCH static void print (Course course) { if (course instanceof ARegularCourse) number = ((ARegularCourse)course). getcoursenumberregularcourse (); else if (course instanceof AFreshmanSeminar) number = ((AFreshmanSeminar)course). getcoursenumberfreshmanseminar (); System.out.println( course.gettitle() + " " + course.getdepartment() + number ); More Work Must update code as subtypes added 38

39 CAN WE GET RID OF MANUAL DISPATCH/INSTANCEOF? static void print (Course course) { if (course instanceof RegularCourse) System.out.print( Regular Course: ); else if (course instanceof AFreshmanSeminar) System.out.println( Freshman Seminar ); System.out.println( course.gettitle() + " " + course.getdepartment() + course.getnumber() ); 39

40 CANNOT ALWAYS GET RID OF MANUAL DISPATCH/INSTANCOF static void print (Course course) { printheader (course); System.out.println( course.gettitle() + " " + course.getdepartment() + course.getnumbert() ); static void printheader (ARegularCourse course) { System.out.print( Regular Course: ); static void printheader (AFreshmanSeminar course) { System.out.print( Freshman Seminar: ); At compile time, do not know if regular course or freshman seminar Actual parameter cannot be supertype of formal parameter 40

41 PRINTHEADER() IN AREGULARCOURSE package courses; public class ARegularCourse extends ACourse implements Course { public void printheader () { System.out.print ( Regular Course: ) 41

42 PRINTHEADER() IN AFRESHMANSEMINAR package courses; public class AFreshmanSeminar extends ACourse implements FreshmanSeminar { public void printheader () { System.out.print ( Freshman Seminar: ) 42

43 SHOULD NOT ALWAYS GET RID OF MANUAL DISPATCH/INSTANCOF static void print (Course course) { printheader (course); System.out.println( course.gettitle() + " " + course.getdepartment() + course.getnumbert() ); static void printheader (ARegularCourse course) { System.out.print( Regular Course: ); static void printheader (AFreshmanSeminar course) { Should not System.out.print( Freshman put Hence Seminar: need to use ); instanceof printheader() in ARegularCourse or AFreshmanSeminar as it is UI code. Used in parsers 43

44 EXTRA SLIDES 44

45 A STRINGHISTORY IMPLEMENTATION public class AStringHistory implements StringHistory { public static final int MAX_SIZE = 50; String[] contents = new String[MAX_SIZE]; int size = 0; public int size() { return size; public String elementat (int index) { return contents[index]; boolean isfull() { return size == MAX_SIZE; public void addelement(string element) { if (isfull()) System.out.println("Adding item to a full history"); else { contents[size] = element; size++; 45

46 A BIG STRINGHISTORY IMPLEMENTATION public class ABigStringHistory implements StringHistory { public static final int MAX_SIZE = 500; String[] contents = new String[MAX_SIZE]; int size = 0; public int size() { return size; public String elementat (int index) { return contents[index]; boolean isfull() { return size == MAX_SIZE; public void addelement(string element) { if (isfull()) System.out.println("Adding item to a full history"); else { contents[size] = element; size++; 46

47 NULL OVERRIDDEN METHOD public abstract class AnAbstractStringHistory { boolean isfull(){; void uncheckedaddelement(object element){; public void addelement(object element) { if (!isfull()) uncheckedaddelement(element); else handleerror(); void handleerror () { System.out.println("Adding item to a full history"); ; 47

48 A STRINGHISTORY IMPLEMENTATION public class AStringHistory extends AnAbstractStringHistory implements StringHistory { public final int MAX_SIZE = 50; String[] contents = new String[MAX_SIZE]; int size = 0; public int size() { return size; public String elementat (int index) { return contents[index]; boolean isfull() { return size == MAX_SIZE; void uncheckedaddelement(string element) { contents[size] = element; size++; 48

49 DYNAMIC DISPATCH public abstract class ACollection { boolean isfull(){; void uncheckedaddelement(object element){; public void addelement(object element) { if (!isfull()) uncheckedaddelement(element); else public class AStringHistory handleerror(); extends ACollection implements StringHistory { void boolean handleerror isfull() { () return { size == MAX_SIZE; void uncheckedaddelement(string System.out.println("Adding element) item to { a full history"); ; contents[size] = element; size++; Which isfull() called when addelement is invoked on an instance of AStringHistory()? Regular method: called method statically dispatched at compile time, so overridden isfull() Virtual method: most specific implementation of called method dynamically dispatched at execution time, so overriding isfull() 49

50 REGULAR VS. VIRTUAL METHOD Smalltalk: had only virtual methods With each object a table kept which associates method header with most specific implementation. Table accessed at execution time. C++: Had both virtual and regular methods More efficient with regular methods Java: Has only virtual methods Abstract and interface method by definition are virtual methods 50

51 CORRECT A BIG STRINGHISTORY IMPLEMENTATION public class ABigStringHistory extends AnAbstractStringHistory implements StringHistory { public final int MAX_SIZE = 500; String[] contents = new String[MAX_SIZE]; int size = 0; public int size() { return size; public String elementat (int index) { return contents[index]; boolean isfull() { return size == MAX_SIZE; void uncheckedaddelement(string element) { contents[size] = element; size++; 51

52 AN ERRONEOUS BIG STRINGHISTORY IMPLEMENTATION public class ABigStringHistory extends AnAbstractStringHistory implements StringHistory { public final int MAX_SIZE = 500; String[] contents = new String[MAX_SIZE]; int size = 0; public int size() { return size; public String elementat (int index) { return contents[index]; boolean isfull() { return size == MAX_SIZE; //void uncheckedaddelement(string element) { // contents[size] = element; // size++; // 52

53 NULL OVERRIDDEN METHOD public abstract class AnAbstractStringHistory { boolean isfull(){; void uncheckedaddelement(object element){; public void addelement(object element) { if (!isfull()) uncheckedaddelement(element); else handleerror(); void handleerror () { System.out.println("Adding item to a full history"); ; 53

54 ABSTRACT METHODS public abstract class AnAbstractStringHistory { abstract boolean isfull(); abstract void uncheckedaddelement(object element) public void addelement(object element) { if (!isfull()) uncheckedaddelement(element); else handleerror(); void handleerror () { System.out.println("Adding item to a full history"); ; Only header of method provided as in interface 54

55 AN ERRONEOUS BIG STRINGHISTORY IMPLEMENTATION public class ABigStringHistory implements StringHistory { public final int MAX_SIZE = 500; String[] contents = new String[MAX_SIZE]; int size = 0; public int size() { return size; public String elementat (int index) { return contents[index]; boolean isfull() { return size == MAX_SIZE; //void uncheckedaddelement(string element) { // contents[size] = element; // size++; // Java complains abstract method not implemented in concrete class 55

56 NULL VS. ABSTRACT METHODS public abstract class AnAbstractStringHistory { boolean isfull() { void uncheckedaddelement(object element) { public void addelement(object element) { if (!isfull()) uncheckedaddelement(element); else handleerror(); void handleerror () { System.out.println("Adding item to a full history"); ; Null methods could replace abstract methods in this example. Abstract method force overriding implementations in subclasses. Provide better documentation, indicating to programmer that subclass will implement them. 56

57 ACARTESIANPLANE CONSTRUCTOR public class ACartesianPlane implements CartesianPlane { int originx, originy; int axeslength; Line xaxis; Line yaxis; StringShape xlabel; StringShape ylabel; public ACartesianPlane (int theaxeslength, int theoriginx, int theoriginy ) { axeslength = theaxeslength; originx = theoriginx; originy = theoriginy; xaxis = new ALine(toXAxisX(), toxaxisy(), axeslength, 0); yaxis = new ALine(toYAxisX(), toyaxisy(), 0, axeslength); xlabel = new AStringShape ("X", toxlabelx(), toxlabely()); ylabel = new AStringShape ("Y", toylabelx(), toylabely()); 57

58 NOBSERVABLE CARTESIANPLANE CONSTRUCTOR 58

59 ACARTESIANPLANE 59

60 ANOBSERVABLE CARTESIANPLANE 60