comp dynamic dispatch and abstract methodsdewan/comp114/f12/lectures/... · comp 401 dynamic...
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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++;
}
}
}
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];
}
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++;
}
}
}
Reducing code duplication?
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
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);
}
}
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);
}
7
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);
}
}
public class APolarPointHistoryWithDynamicallyDispatchedMethod
extends ACartesianPointHistoryWithDynamicallyDispatchedMethod {
protected Point createPoint(int x, int y) {
return new ACartesianPoint(x, y);
}
}
PointHistory pointHistory =
new APolarPointHistoryWithDynamicallyDispatchedMethod();
pointHistory.addElement(50, 100);
Static dispatching: call most specific method
known when caller method was compiled
Dynamic dispatching: call most specific
method for the calling object
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
9
INHERITANCE RELATIONSHIPS?
APolarPointHistoryWithDynamicallyDispatchedMethod
ACartesianPointHistoryWithDynamicallyDispatchedMethod
IS-A
APolarPointHistoryWithDynamicallyDispatchedMethod
ACartesianPointHistoryWithDynamicallyDispatchedMethod
IS-A
10
CORRECT INHERITANCE
AnAbstractPointHistory
ACartesianPointHistory
IS-A
APolarPointHistory
IS-A
11
CORRECT INHERITANCE
AnAbstractPointHistory
ACartesianPointHistory
IS-A
APolarPointHistory
IS-A
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];
}
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);
}
}
Changed how?
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++;
}
}
}
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) {}
}
15
OVERRIDING DYNAMICALLY DISPATCHED METHOD
public class ACartesianPointHistory
extends AnAbstractPointHistory{
protected Point createPoint(int x, int y) {
return new ACartesianPoint(x, y);
}
}
16
ERRONEOUS IMPLEMENTATION WITH NO ERRORS
public class ACartesianPointHistory extends AnAbstractPointHistory{
}
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++;
}
}
protected abstract Point createPoint(int x, int y);
}
Only header of method provided as in interface
18
ERRONEOUS IMPLEMENTATION
Java complains abstract method not implemented in concrete class
public class ACartesianPointHistory extends AnAbstractPointHistory{
}
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.
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).
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
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
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
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();
}
}
An abstract class can
implement any
interface
This means all sub
classes implement the
interface
Interface implementation
check is made for concrete
classes
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
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
27
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);
}
WHEN ABSTRACT METHOD REQUIRED
Not public
28
ADDING CREATEPOINT TO INTERFACE public interface PointHistoryWithExtraPublicMethod {
public Point createPoint(int x, int y);
}
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) {
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++;
}
}
}
Least privilege violated
Factory method
29
FACTORY ABSTRACT METHOD
…
abstract <T> <M>(…) ;
…
…
<T> <M>(…) {
return new <C1> (…)
}
…
extends
…
<T> <M> (…){
return new <C2> (…)
}
…
Abstract method that returns an instance of
some type T – like a factory, it creates and
initializes an object.
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!
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() ;
32
ABSTRACT METHODS VS. VIRTUAL METHODS
Abstract methods must be virtual methods.
Virtual methods need not be abstract methods
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
ACartesianPointHistoryWithDynamicallyDispatchedMethod
IS-A
34
OVERLOADING, POLYMORPHISM, AND
DYNAMIC DISPATCH
static void print (Course course) {
System.out.println(
course.getTitle() + " " +
course.getDepartment() +
course.getNumber()
);
}
static void print (CourseList courseList) {
…
print(course);
…
} Overloaded method
Dynamically dispatched
method
getNumber() in
ARegularCouse
getNumber() in
AFreshmanSeminar
ARegularCourse
AFreshmanSeminar
Polymorphic method
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
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
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.
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
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() );
}
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
41
PRINTHEADER() IN AREGULARCOURSE
package courses;
public class ARegularCourse extends ACourse implements Course {
…
public void printHeader () {
System.out.print (“Regular Course: ”)
}
}
42
PRINTHEADER() IN
AFRESHMANSEMINAR
package courses;
public class AFreshmanSeminar extends ACourse implements
FreshmanSeminar {
…
public void printHeader () {
System.out.print (“Freshman Seminar: ”)
}
}
43
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: ”);
}
SHOULD NOT ALWAYS GET RID OF MANUAL
DISPATCH/INSTANCOF
Should not put printHeader() in ARegularCourse or AFreshmanSeminar as it is UI code.
Hence need to use instanceof
Used in parsers
44
EXTRA SLIDES
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++;
}
}
}
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++;
}
}
}
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");
};
}
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++;
}
}
49
DYNAMIC DISPATCH public abstract class ACollection {
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");
};
}
public class AStringHistory extends ACollection implements StringHistory {
…
boolean isFull() { return size == MAX_SIZE; }
void uncheckedAddElement(String element) {
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()
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
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++;
}
}
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++;
//}
}
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");
};
}
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
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
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.
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()); }
58
ANOBSERVABLE CARTESIANPLANE CONSTRUCTOR
59
ACARTESIANPLANE
60
ANOBSERVABLE CARTESIANPLANE