CSE 130 : Fall 2008

Programming Languages

Lecture 16:St ti T f Obj tStatic Types for Objects

Ranjit JhalaUC San Diego

Quiz

Last timeLast time

• Tricks with namespaces: decorators • Tricks with namespaces: decorators …

TodayToday

• Inheritance• Inheritance

S i T f Obj • Static Types for Objects

What is a type?What is a type?

What is a type?What is a type?

• A description of the actions that one can A description of the actions that one can successfully perform on an object

• A description of an object: – fields it containsfields it contains– methods it contains

• Type = attributes an object contains– “attribute” recursively includes its typeattribute recursively includes its type

Types for Objects: InterfacesTypes for Objects: Interfaces

Interface: List of attributes (with types)

interface Point {

Interface: List of attributes (with types)

{double getX();double getY();void move(double dx double dy)void move(double dx, double dy)void jump(double x, double y)void draw(Screen s)

}

ExampleExample

interface Point { set of objects with attrs:

The following type: Corresponds to:

interface Point {double getX();double getY();void move(double dx, double dy)

set of objects with attrs: getX getY move

void jump(double x, double y)void draw(Screen s)

}

jump draw

Object may have many Object may have many other attributes!

void screenSaver(Point [] p) {Compiler checks that screenSaver only uses known attributes of Point

}known attributes of Point

Clients of screenSaverClients of screenSaver

What objects can be passed to screenSaver?What objects can be passed to screenSaver?

Any object w/ Point’s attributes • Any object w/ Point s attributes

N bl if bj t h tt ib t• No problem if object has more attributes

• This idea is called subtyping

Subtype Polymorphism Subtype Polymorphism

“T1 is a subtype of type T2 ”T1 is a subtype of type T2

If h bj f T b • If wherever an object of type T2 can be used, so can an object of type T1

• If T1 <: T2 then wherever an object of 1 2 jtype T2 is required, you can safely pass in an object of type T1j yp 1

SubtypingSubtyping

When can we say T1 <: T2 ?When can we say T1 <: T2 ?

Wh bj f T h When an object of type T1 has all the attributes of an object of type T2

Subtyping with types-as-setsSubtyping with types as sets

If: If: 1. T1 <: T2

2 objs(T ) = set of objects of type T2. objs(T1) = set of objects of type T1

3. objs(T2) = set of objects of type T2

th h bj (T ) d bj (T ) l t d?then how are objs(T1) and objs(T2) related?

T

Ans: obs(T1) ⊆ objs(T2)T2

T1

Example of subtypingExample of subtypinginterface Point {

double getX();Recall:

double getX();double getY();void move(double dx, double dy)void jump(double x, double y)

id d (S )void draw(Screen s)}

interface ColorPoint { interface TextPoint {interface ColorPoint {double getX();double getY();void move(double dx, double dy)

interface TextPoint {double getX();double getY();void move(double dx, double dy)

void jump(double x, double y)void draw(Screen s)void setColor(Color c)Color getColor()

void jump(double x, double y)void draw(Screen s)void setText(string s)string getText()Color getColor()

}string getText()

}

Example of subtypingExample of subtyping

objects

Point

ColorPoint TextPoint

Another exampleAnother example

interface ColorTriangle {interface Triangle { void foo(Triangle t)g {ColorPoint p1;ColorPoint p2;ColorPoint p3;

}

g {Point p1;Point p2;Point p3;

}

( g )

I C l T i l T i l ?

}}

Is ColorTriangle <: Triangle ?• Is it the case that wherever a Triangle is

expected it is safe to pass in a ColorTriangle ?expected, it is safe to pass in a ColorTriangle ?• Is it safe to pass ColorTriangle to foo ?

Another exampleAnother example

void foo(Triangle t)interface ColorTriangle {interface Triangle { ( g )g {ColorPoint p1;ColorPoint p2;ColorPoint p3;

}

g {Point p1;Point p2;Point p3;

}

S f t C l T i l t f ?

}}

• Safe to pass ColorTriangle to foo?• Suppose foo only reads fields of t

– Then it’s safe.

• What if foo writes fields of t ?

Another exampleAnother example

void foo(Triangle t) {interface ColorTriangle {interface Triangle { ( g ) {t.p1 := new Point()

}

g {ColorPoint p1;ColorPoint p2;ColorPoint p3;

}

g {Point p1;Point p2;Point p3;

} t C l T i l ()}} t := new ColorTriangle();foo(t);t.p1.color // yikes!

SSummary:• interface A is a subtype of interface B• if attributes of A and B match

…but its tricky! details in CSE 230

What happens in Java ?What happens in Java ?

How does Java figure out if How does Java figure out if one interface is a subtype of another?

• Looks at the subclass relationship!

Structural vs. Nominal subtypingStructural vs. Nominal subtyping• Structural subtyping

– subtyping based on type structure (attributes)

• Nominal subtyping– subtyping “by name”– subtyping relation given by programmer

• Java: – Interface = Type– Class = Type + Implementation– i.e. Class automatically defines an interface

Programmer declares SubtypingProgrammer declares Subtyping

ColoredPoint <: Pointclass ColoredPoint implements Point {

}

ColoredPoint <: Pointclass ColoredPoint extends Point {

}

Whats the difference ?Whats the difference ?

ColoredPoint <: Pointclass ColoredPoint implements Point {

Compiler checks ll b f

}

all attributes of Point are defined in ColorPoint

Subtyping

ColoredPoint <: Pointclass ColoredPoint extends Point {

Compiler includes all attributes of Point

}

I h it all attributes of Point in ColorPoint

Inheritance

Subtype Polymorphism in JavaSubtype Polymorphism in Java• How to get polymorphic lists in Java ?

interface List {void add(Object o);Object get(int i);

• But

Object get(int i);}

• But …List l = … ;l.add(“persnickety”);MString s = l.get(0); Compiler

Grumbles

Lost information with supertype “object”

A Hack : CastingA Hack : CastingList l = … ;l.add(“persnickety”);MString s = (string) l.get(0);String s (string) l.get(0);Int i = (Int) l.get(0); Compiler

happy but…

Unsafe: runtime error!

Generics: ML-style Polymorphism Generics: ML style Polymorphism

• How to get polymorphic lists in Java ?interface List <T> {void add(T o);T get(int i);

Generalize

• Now

T get(int i);}

• Now …List <string> l = … ;l.add(“persnickety”);

Instantiate( p y )

MString s = l.get(0);l add(223);

Safel.add(223);

Compile time error

Bounded Polymorphism in JavaBounded Polymorphism in Java• How to get polymorphic “drawable” lists in Java ?

interface Drawable {void draw();

}

BoundedGeneralize∀ T<: Drawable

interface DList <T extends Drawable>{void add(T o);T get(int i);

DList <circle> l = … ; Instantiate Check :

T get(int i);}

;l.add(/* circle obj */);MCircle c = l.get(0);

Safe

Instantiate Check : <: Drawable

CSE 230, Winter 07

Square s = l.get(0); SafeCompile time error

That’s all for objectsThat s all for objects

Good luck with PA 6Good luck with PA 6

N t k P lNext week: Prolog

Happy Thanksgiving!