Inheritance

What Is Subclassing?

Subclassing is a process in which a new class is derived from an old one. It
is a distinct concept from
(subtype) polymorphism,
generalization/specialization and
composition.

Michal P´se (CTU in Prague)
ıˇ Object Programming Lect. 4: Inheritance October 19, 2010 2 / 12

Subclassing = Subtyping

Subtypes are not required to share any code with their supertypes.



On the other hand, subtypes are required to stick to the contract of
their supertype (the Liskov substition principle).



On the other hand, subtypes are required to stick to the contract of
their supertype (the Liskov substition principle).
Subtyping only concerns behavior, subclassing only concerns code.


The Fallacy of Current Industrial Programming Languages

In Java (and many other languages)
subclassing implies subtyping, in other words,
every subclass has to stick to the contract of its parent class.
That severly limits possible code reuse!


Example

void DFS(Node n) {
addToStack( n);
while ( stackNotEmpty()) {
n = removeFromStack();
processNode( n);
for ( Node c: n.children())
addToStack( c);
}
}


Example

Obviously, addToStack
void DFS(Node n) {
inserts a node at the
addToStack( n);
beginning of a list and
removeFromStack removes
a node from the beginning
processNode( n);
of that list.
addToStack( c);
}
}


Example

Obviously, addToStack
void DFS(Node n) {
inserts a node at the
addToStack( n);
beginning of a list and
removeFromStack removes
a node from the beginning
processNode( n);
of that list.
addToStack( c); Is it correct to override
} addToStack to insert a
} node at the end of the list?


Example (II)

Not in Java—the new subtype would break the contract of the
supertype.


Example (II)

Not in Java—the new subtype would break the contract of the
supertype.
However, it is perfectly legitimate in languages where subclassing
does not imply subtyping—the new class simply isn’t type-compatible
with the old one.


Subclassing = Specialization

Subclassing does not imply specialization—subclass does not have to
be a specialized version of superclass.



Specialization does not imply subclassing—a specialized version of
object does not have to share code with the base case.



Specialization does not imply subclassing—a specialized version of
object does not have to share code with the base case.
In fact, subtyping and specialization are distinct notions as well.


Subclassing = Composition

Subclassing and composition both facilitate code reuse.
Is there anything that can be achieved by subclassing and not
achieved by composition or vice versa?


Poor Man’s Inheritance

class Foo { class Qux {
void bar() {...} Foo foo = new Foo();
int baz() {...} void bar() {
} foo.bar();
}

int baz() {
return foo.baz();
}
}


Poor Man’s Inheritance

class Foo { class Qux {
void bar() {...} Foo foo = new Foo();
int baz() {...} void bar() {
} foo.bar();
}

int baz() {
return foo.baz();
}
}

Emulation of inheritance using composition is far from perfect—method
overriding can be emulated only in the simplest cases.


The Essence of Inheritance

So what exactly is inheritance?



Inheritance is a mechanism facilitating code reuse.



It can be thought about as the C-style include with overriding.



It can be thought about as the C-style include with overriding.
It enables reuse even in ways the original code creator never imagined.


Implementation of Inheritance

C++: virtual methods table.



In Smalltalk: method dictionary.



In Smalltalk: method dictionary.
In systems with runtime: inline cache.


See

Taivalsaari, A. On the notion of inheritance. ACM Computing Surveys
28, 3 (Sep. 1996), 438–479.
http://doi.acm.org/10.1145/243439.243441


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