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C for Java Programmers

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A pure virtual method must be overridden in subclasses. ... it is the root of its own hierarchy and provides only the behavior defined ... – PowerPoint PPT presentation

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Title: C for Java Programmers

1
C for Java Programmers

Chapter 6
Polymorphism

2
Introduction

A static type is associated with a declaration.
A dynamic type is associated with a value.

3
We need a class hierarchy
Animal
Mammal
Bird
Cat
Dog
4
Figure 6.1 Animal Kingdom in Java

abstract class Animal abstract public void
speak()
class Bird extends Animal public void speak()
System.out.println("twitter")
class Mammal extends Animal public void speak()
System.out.println("can't speak") public
void bark() System.out.println("can't bark")
class Cat extends Mammal public void speak()
System.out.println("meow") public void purr()
System.out.println("purrrrr")
class Dog extends Mammal public void speak()
System.out.println("wouf") public void bark()
System.out.println("wouf")

5
Figure 6.2 Animal Kingdom in C

class Animal public virtual void speak() 0
class Bird public Animal public virtual
void speak() printf("twitter")
class Mammal public Animal public virtual
void speak() printf("can't speak") void
bark() printf("can't bark")
class Cat public Mammal public void speak()
printf("meow") virtual void purr()
printf("purrrrr")
class Dog public Mammal public virtual void
speak() printf("wouf") void bark()
printf("wouf")

6
Virtual Non-virtual Overriding

Overriding a method in a parent class is
replaced in a child class by a method having
exact same type signature.
In C, overriding uses the keyword virtual.

7
Example of overriding

Dog d new Dog() Mammal m d d-gtbark()
// wouf m-gtbark() // can't bark
d-gtspeak() // wouf m-gtspeak() // also
wouf Animal a d a-gtspeak() // and more
wouf
d-gtbark() // wouf a-gtbark() // compile error,
not allowed
Mammal mm mm d mm.speak() // can't
speak d-gtspeak() // although dog will wouf

8
Impact of Virtual on Size

When a class contains a virtual method, an
internal table called the virtual method table is
created.
This table is used in the implementation of the
dynamic binding of message to method required by
the virtual method.
Each instance of a class will contain an
additional hidden pointer value, which references
the virtual method table.

9
Obtaining Type Information from a Dynamic Value

getClass ( ) in Java
Animal a new Dog() // following will print
class Dog System.out.println("class is "
a.getClass().getName())
typeid ( ) in C
Animal a new Dog() // will print the
class Dog println("class is s\n",
typeid(a).name())

10
Abstract Classes

A pure virtual method must be overridden in
subclasses.
An interface can be simulated by pure virtual
methods.

11
Example of Abstract Classes

class KeyPressHandler // specification for key
press event handlerpublic virtual void keyDown
(char c) 0
class MouseDownHandler // specification for
mouse down event handlerpublic virtual void
mouseDown (int x, int y) 0 virtual void
mouseUp (int x, int y) 0
class EventHandler public KeyPressHandler,
public MouseDownHandler public void keyDown
(char c) ... void mouseDown (int x, int y)
... void mouseUp (int x, int y)( ...

12
Downcasting (Reverse Polymorphism)

Downcasting reverses the assignment to a
polymorphic variable.
Used to undo the effect of a polymorphic
assignment (go from a higher class to a lower one)

13
Downcasting (Reverse Polymorphism)

C does not perform a run-time check to ensure
the validity of cast conversion.
To resolve the problem, dynamic cast, a part of
run-time type information system (RTTI).

14
Example of Downcasting

Cat c dynamic_cast ltCat gt (a) if
(c) printf("variable was a cat") else printf
("variable was not a cat")
void v ...
// we know, from elsewhere, that v is really a
cat
Cat c static_castltCat gt(v)
Whenever possible, use the RTTI instead of
standard unchecked cast conversions.

15
Name Resolution

Name resolution is matching a function body to a
function name.
class Parent public void test (int i)
printf("parent test") class Child
public Parent public void test (int i, int
i) printf("child two arg test") void test
(Parent p) printf("child object test")

16
Different techniques for NR

Java Find the method with the best fit to the
arguments, regardless of scope
C Find the closest name scope that contains
the name, then find the best fit for the
arguments within that name scope

17
Example of error

Child c new Child()
c-gttest(3) // will generate compiler error
Will generate a compiler error in C, similar
code would not generate an error in Java
Because the closest scope is not the one with the
correct function argument signatures

18
Getting around this problem

Redefine any inherited names that are overloaded
with different type signatures.
class Child public Parent public void test
(int i) Parenttest(i) // redefine
inherited method void test (int i, int i)
printf("child two arg test") void test
(Parent p) printf("child object test")

19
A Forest, Not a Tree

Classes in C are not part of a single
hierarchy.
If a class is not defined as inheriting from
another class, it is the root of its own
hierarchy and provides only the behavior defined
by the class description.
A typical C program contains a number of
different class hierarchies, each independent of
the others.

20
Virtual Destructors

A destructor is a method that is invoked
immediately before a variable it to be deleted.
Declare a virtual destructor if a class has any
virtual methods.

21
Example of Destructors

class Animal virtual Animal ()
printf("goodbye animal") ... ...
class Cat public Mammal Cat ()
printf("goodbye cat") ...

22
Private Inheritance

If inheritance is protected, fields declared as
public in the parent class become protected in
the child class.
If inheritance is private, fields declared either
as public or protected in the parent become
private in the child class.

23
Private Inheritance
Parent
Parent
public
public
Child
Child
Private inheritance
Public inheritance
24
Example of Private Inheritance

class Stack public List // assume elements
are integerspublic push (int val)
addToFront(val) pop () removeFirstElement()
top () return firstElement()
class Stack private List public push (int
val) addToFront(val) pop ()
removeFirstElement() top () return
firstElement()
class Stack private List public push (int
val) addToFront(val) pop ()
removeFirstElement() top () return
firstElement() using isEmpty() using int
size()

25
Inheritance and Arrays

Java permits array to be assigned to a variable
that is declared as an array of the parent class
Dog dogs new Dog10 // an array of dog
values
Animal pets dogs // legal
pets2 aCat // is this legal?
To prevent, Java actually performs a run-time
check on assignments to arrays of objects.

26
C Gets this right

This is one place where C seems to do the more
intelligent thing than does Java
Dog dogs new Dog10 // an array of dog
values
Animal pets dogs // Not legal

27
Overloading

Overloaded when two or more function bodies are
associated with a single function name.
class Child public Parent public void test
(int i) Parenttest(i) // redefine
inherited method void test (int i, int j)
printf("child two arg test") void test
(Parent p) printf("child object test")

28
Overloading