Algorithmics 3
Algorithmics 3
Inheritance
September 27, 2006
Algorithmics 3
Program
Program
Classes
About classes
Object fields and methods
Class methods and fields
Constructors
Inheritance
Polymorphism
Summary
Algorithmics 3
Program
What we’ve seen so far
I
I
A specification is a description in English (or in French).
Object-oriented programming is all about
I
I
I
I
I
objects
the state of these objects (i.e. their properties)
what the objects can do
what messages the objects can answer to
the contracts objects enter.
Algorithmics 3
Program
What we’ve seen so far
I
I
A specification is a description in English (or in French).
Object-oriented programming is all about
I
I
I
I
I
I
objects
the state of these objects (i.e. their properties)
what the objects can do
what messages the objects can answer to
the contracts objects enter.
we’re still short of concrete relations between objects !
Algorithmics 3
Program
Today
I
Classes.
I
Constructors.
I
Object methods and fields
I
Class methods and fields.
I
Inheritance.
I
Polymorphism.
Algorithmics 3
Program
Today
I
Classes.
I
Constructors.
I
Object methods and fields
I
Class methods and fields.
I
Inheritance.
I
Polymorphism.
Most of these things you know already. Don’t worry, there’ll be new
things, too.
I
Algorithmics 3
Classes
About classes
Program
Classes
About classes
Object fields and methods
Class methods and fields
Constructors
Inheritance
Polymorphism
Summary
Algorithmics 3
Classes
About classes
What’s a class ?
Q What’s a class ?
Algorithmics 3
Classes
About classes
What’s a class ?
Q What’s a class ?
A A class describes a set of objects.
Algorithmics 3
Classes
About classes
What’s a class ?
Q What’s a class ?
A A class describes a set of objects.
A A class is a manner of building objects sharing some traits
(properties, messages, etc.).
Algorithmics 3
Classes
About classes
What’s a class ?
Q What’s a class ?
A A class describes a set of objects.
A A class is a manner of building objects sharing some traits
(properties, messages, etc.).
A In Java, a class is the only manner of building objects.
Not in OCaml or JavaScript.
Algorithmics 3
Classes
About classes
What’s a class ?
Q What’s a class ?
A A class describes a set of objects.
A A class is a manner of building objects sharing some traits
(properties, messages, etc.).
A In Java, a class is the only manner of building objects.
Not in OCaml or JavaScript.
Two objects of the same class will share all the traits
defined by that class.
Algorithmics 3
Classes
About classes
Defining a class
Each class must be defined in its own file.
// I n f i l e MyClass . j a v a
p u b l i c c l a s s MyClass
{
// T h i s i s an empty c l a s s
}
Algorithmics 3
Classes
Object fields and methods
Object fields and object methods
Classes can contain object fields and object methods.
// I n f i l e M y S e c o n d C l a s s . j a v a
p u b l i c c l a s s MySecondClass
{
p r i v a t e i n t someField = 0;
p r i v a t e i n t s o m e O t h e r F i e l d ; // S h o u l d be i n i t i a l i z e d !
p u b l i c i n t getSomeValue ( )
{
r e t u r n t h i s . someField ∗2;
}
p u b l i c i n t getOtherField ()
{
r e t u r n t h i s . s o m e O t h e r F i e l d +1;
}
p u b l i c boolean areValuesEqual ()
{
r e t u r n t h i s . g e t S o m e V a l u e ()== t h i s . g e t O t h e r F i e l d ( ) ;
}
}
Algorithmics 3
Classes
Object fields and methods
What’s this ?
Algorithmics 3
Classes
Object fields and methods
What’s this ?
The special variable this represents the current object.
Algorithmics 3
Classes
Object fields and methods
What’s this ?
The special variable this represents the current object.
An object field is part of all the objects of the corresponding class. An
object method is part of all the objects of the corresponding class.
Both fields and methods can be accessed with this..
Algorithmics 3
Classes
Object fields and methods
What’s this ?
The special variable this represents the current object.
An object field is part of all the objects of the corresponding class. An
object method is part of all the objects of the corresponding class.
Both fields and methods can be accessed with this..
That’s the only kind of field you have seen so far.
That’s not the only kind of method you have seen so far.
Algorithmics 3
Classes
Object fields and methods
What’s this ?
The special variable this represents the current object.
An object field is part of all the objects of the corresponding class. An
object method is part of all the objects of the corresponding class.
Both fields and methods can be accessed with this..
That’s the only kind of field you have seen so far.
That’s not the only kind of method you have seen so far.
Until now, you probably haven’t used this. to access fields and
methods. It’s not compulsory but it often makes your code much more
readable.
Algorithmics 3
Classes
Class methods and fields
Applications
A class can define method public static void main(String[]
arguments). This defines a potential entry point to the program.
// I n f i l e M y A p p l i c a t i o n . j a v a
public class MyApplication
{
p u b l i c s t a t i c v o i d main ( S t r i n g [ ] a r g u m e n t s )
{
// . . .
}
}
Algorithmics 3
Classes
Class methods and fields
Applications
A class can define method public static void main(String[]
arguments). This defines a potential entry point to the program.
// I n f i l e M y A p p l i c a t i o n . j a v a
public class MyApplication
{
p u b l i c s t a t i c v o i d main ( S t r i n g [ ] a r g u m e n t s )
{
// . . .
}
}
Note that there can be several entry points to the program (usually for
testing purposes).
Parameter arguments defines command-line arguments you can pass to
the program.
Algorithmics 3
Classes
Class methods and fields
Staticity ?
// I n f i l e M y A p p l i c a t i o n . j a v a
public class MyApplication
{
p u b l i c s t a t i c v o i d main ( S t r i n g [ ] a r g u m e n t s )
{
// . . .
}
}
Keyword static specifies that the method is a class method. In other
words, the method belongs to the class, not to its instances.
Algorithmics 3
Classes
Class methods and fields
Staticity ?
// I n f i l e M y A p p l i c a t i o n . j a v a
public class MyApplication
{
p u b l i c s t a t i c v o i d main ( S t r i n g [ ] a r g u m e n t s )
{
// . . .
}
}
Keyword static specifies that the method is a class method. In other
words, the method belongs to the class, not to its instances.
main is a class method of class MyApplication, which means that you
can use it without having to instanciate MyApplication.
System.out is a class field of class System, which means that you can
use it without having to instanciate System.
Algorithmics 3
Classes
Class methods and fields
Uses and limitations
// I n f i l e M y S t a t i c C l a s s . j a v a
public class MyStaticClass
{
public static f i n a l String blue = " blue " ;
public static f i n a l red = " red " ;
public s t a t i c void t e s t ( )
{
System . o u t . p r i n t l n ( " T h i s ␣ i s ␣ a ␣ t e s t " ) ;
}
}
Class fields act as global variables, while class methods act as global
procedures and functions.
Algorithmics 3
Classes
Class methods and fields
Uses and limitations
// I n f i l e M y S t a t i c C l a s s . j a v a
public class MyStaticClass
{
public static f i n a l String blue = " blue " ;
public static f i n a l red = " red " ;
public s t a t i c void t e s t ( )
{
System . o u t . p r i n t l n ( " T h i s ␣ i s ␣ a ␣ t e s t " ) ;
}
}
Class fields act as global variables, while class methods act as global
procedures and functions.
In other words, MyStaticClass.red = "yellow"; changes the value of
MyStaticClass.red for the whole universe.
Algorithmics 3
Classes
Class methods and fields
Uses and limitations
// I n f i l e M y S t a t i c C l a s s . j a v a
public class MyStaticClass
{
public static f i n a l String blue = " blue " ;
public static f i n a l red = " red " ;
public s t a t i c void t e s t ( )
{
System . o u t . p r i n t l n ( " T h i s ␣ i s ␣ a ␣ t e s t " ) ;
}
}
Class fields act as global variables, while class methods act as global
procedures and functions.
In other words, MyStaticClass.red = "yellow"; changes the value of
MyStaticClass.red for the whole universe.
Usually, if you don’t specifically know that you need to define a class
field/method, don’t.
Algorithmics 3
Classes
Constructors
Program
Classes
About classes
Object fields and methods
Class methods and fields
Constructors
Inheritance
Polymorphism
Summary
Algorithmics 3
Classes
Constructors
Class construction
Object o = new Object()
Algorithmics 3
Classes
Constructors
Class construction
Object o = new Object()
Construction
I
is what happens when you call new
initializes the memory
I
creates the actual object (this)
I
must initalize final fields.
I
Algorithmics 3
Classes
Constructors
Class construction
Object o = new Object()
Construction
I
is what happens when you call new
initializes the memory
I
creates the actual object (this)
I
must initalize final fields.
I
Until construction is complete, you cannot reliably call methods, access
fields, or use this.
Algorithmics 3
Classes
Constructors
Class construction
Object o = new Object()
Construction
I
is what happens when you call new
initializes the memory
I
creates the actual object (this)
I
must initalize final fields.
I
Until construction is complete, you cannot reliably call methods, access
fields, or use this.
Java will not check that !
Algorithmics 3
Classes
Constructors
Declaring a constructor
In Java, a constructor is declared almost like a method – but it’s not a
method !
Algorithmics 3
Classes
Constructors
Declaring a constructor
In Java, a constructor is declared almost like a method – but it’s not a
method !
I
you can think of the constructor as of a function returning the
object
Algorithmics 3
Classes
Constructors
Declaring a constructor
In Java, a constructor is declared almost like a method – but it’s not a
method !
I
you can think of the constructor as of a function returning the
object – indeed, that’s what it is in OCaml or JavaScript
I
a constructor can be public, private, protected or
package-protected
a constructor doesn’t have a return value
I
I
a constructor cannot return
the name of the constructor must be exactly the name of the class
I
the constructor accepts parameters just like a method
I
you can have several different constructors, provided they accept
different parameter types
I
at the end of the constructor, all object fields should be initialized
I
at the end of the constructor, all final object fields must be
initialized.
I
Algorithmics 3
Classes
Constructors
Syntactically
public
{
class
MyClassWithConstructor
f i n a l i n t number ;
p u b l i c MyClassWithConstructor ()
{
number = 0 ;
}
p u b l i c MyClassWithConstructor ( f i n a l
{
number = s o m e V a l u e ;
}
}
int
someValue )
Algorithmics 3
Classes
Constructors
What about destruction ?
Just like constructors, you can write destructors (actually, finalizers).
Algorithmics 3
Classes
Constructors
What about destruction ?
Just like constructors, you can write destructors (actually, finalizers).
We’re just not going to detail them today.
Algorithmics 3
Classes
Inheritance
And the meek...
You already know that every class inherits from class Object.
When class Child inherits from class Parent,
I every public, protected or package-protected object field of
Parent automatically appears as an object field of Child –
private fields cannot be manipulated
I every public, protected or package-protected object method of
Parent automatically appears as an object method of Child –
private methods cannot be manipulated
I
every instance of Child can be used as if it were a Parent.
Algorithmics 3
Classes
Inheritance
And the meek...
You already know that every class inherits from class Object.
When class Child inherits from class Parent,
I every public, protected or package-protected object field of
Parent automatically appears as an object field of Child –
private fields cannot be manipulated
I every public, protected or package-protected object method of
Parent automatically appears as an object method of Child –
private methods cannot be manipulated
I
every instance of Child can be used as if it were a Parent.
In other words, class Child specializes class Parent.
Algorithmics 3
Classes
Inheritance
Declaring inheritance
public
{
class
Parent
// D e f i n e
// D e f i n e
methods
fields
}
public
{
class
Child
extends
Parent
// E v e r y p u b l i c / p r o t e c t e d / p a c k a g e−p r o t e c t e d method and f i e l d
// o f P a r e n t c a n be u s e d h e r e , a s i f i t had b e e n d e f i n e d i n
// C h i l d
}
Inheritance is declared with extends.
Algorithmics 3
Classes
Inheritance
Inheritance and construction
public
{
class
Rectangle
p r i v a t e f i n a l i n t width ;
p r i v a t e f i n a l i n t height ;
p u b l i c Rectangle ( i n t width , i n t h e i g h t )
{
t h i s . width = width ;
t h i s . h e i g h t= h e i g h t ;
}
p u b l i c i n t getWidth ( )
{
r e t u r n width ;
}
p u b l i c i n t getHeight ()
{
return height ;
}
p u b l i c i n t getArea ()
{
r e t u r n getWidth ()∗ g e t H e i g h t ( ) ;
}
}
public
{
class
Square
public
{
inherits
Square ( i n t
Rectangle
side )
super ( side ,
side );
}
}
Descendants must invoke one of the constructors of their parent, using
keyword super.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
3. A constructor of Object is called.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
3. A constructor of Object is called.
4. Some magic creates the object.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
3. A constructor of Object is called.
4. Some magic creates the object.
5. The constructor of Object proceeds and terminates.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
3. A constructor of Object is called.
4. Some magic creates the object.
5. The constructor of Object proceeds and terminates.
6. this is now a fully-fledged Object.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
3. A constructor of Object is called.
4. Some magic creates the object.
5. The constructor of Object proceeds and terminates.
6. this is now a fully-fledged Object.
7. The constructor of Rectangle proceeds and terminates.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
3. A constructor of Object is called.
4. Some magic creates the object.
5. The constructor of Object proceeds and terminates.
6. this is now a fully-fledged Object.
7. The constructor of Rectangle proceeds and terminates.
8. this is now also a fully-fledged Rectangle.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
3. A constructor of Object is called.
4. Some magic creates the object.
5. The constructor of Object proceeds and terminates.
6. this is now a fully-fledged Object.
7. The constructor of Rectangle proceeds and terminates.
8. this is now also a fully-fledged Rectangle.
9. The constructor of Square proceeds and terminates.
Algorithmics 3
Classes
Inheritance
More about construction
Let’s instanciate Square with new Square(10).
1. A constructor of Square is called.
2. A constructor of Rectangle is called.
3. A constructor of Object is called.
4. Some magic creates the object.
5. The constructor of Object proceeds and terminates.
6. this is now a fully-fledged Object.
7. The constructor of Rectangle proceeds and terminates.
8. this is now also a fully-fledged Rectangle.
9. The constructor of Square proceeds and terminates.
10. this is now also a fully-fledged Square.
Algorithmics 3
Classes
Polymorphism
Towards polymorphism
Let’s refine our definition of squares
public
{
class
OtherSquare
public
{
extends
OtherSquare ( i n t
Square
size )
super ( size );
}
public
{
int
getWidth ( )
System . out . p r i n t l n ( " C a l l i n g ␣ OtherSquare . getWidth ( ) " ) ;
s u p e r . getWidth ( ) ;
}
}
Algorithmics 3
Classes
Polymorphism
What have we done ?
O t h e r S q u a r e a = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n a . getArea ( ) ;
I
Algorithmics 3
Classes
Polymorphism
What have we done ?
O t h e r S q u a r e a = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n a . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
S q u a r e b = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n b . getArea ( ) ;
I
Algorithmics 3
Classes
Polymorphism
What have we done ?
O t h e r S q u a r e a = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n a . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
S q u a r e b = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n b . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
R e c t a n g l e c = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n c . getArea ( ) ;
I
Algorithmics 3
Classes
Polymorphism
What have we done ?
O t h e r S q u a r e a = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n a . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
S q u a r e b = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n b . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
R e c t a n g l e c = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n c . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
O b j e c t d = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n d . getArea ( ) ;
I
Algorithmics 3
Classes
Polymorphism
What have we done ?
O t h e r S q u a r e a = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n a . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
S q u a r e b = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n b . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
R e c t a n g l e c = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n c . getArea ( ) ;
I
"Calling OtherSquare.getWidth()"
O b j e c t d = new O t h e r S q u a r e ( 1 0 ) ;
r e t u r n d . getArea ( ) ;
I
Doesn’t compile
Algorithmics 3
Classes
Polymorphism
That’s polymorphism
Any object method can be redefined (or “overloaded”) in its subclasses.
This is called polymorphism.
This is a powerful mechanism but it also makes object-oriented programs
more fragile.
Algorithmics 3
Classes
Polymorphism
That’s polymorphism
Any object method can be redefined (or “overloaded”) in its subclasses.
This is called polymorphism.
This is a powerful mechanism but it also makes object-oriented programs
more fragile.
That’s the reason why we define actions as protected: so that they can
be overloaded by subclasses.
Algorithmics 3
Classes
Polymorphism
That’s polymorphism
Any object method can be redefined (or “overloaded”) in its subclasses.
This is called polymorphism.
This is a powerful mechanism but it also makes object-oriented programs
more fragile.
That’s the reason why we define actions as protected: so that they can
be overloaded by subclasses.
The alternative would be to declare actions as
I either private – hence unusable and unmodifiable by subclasses
I
or final – hence unmodifiable by subclasses.
Algorithmics 3
Summary
Summary
I
I
A specification is a description in English (or in French).
Object-oriented programming is all about
I
I
I
I
I
I
objects
the state of these objects (i.e. their properties)
what the objects can do
what messages the objects can answer to
the contracts objects enter
building specialized objects from more generic objects.
Algorithmics 3
Summary
Next week
Swing !