What is an object?

Here's a really simple object

object countUp {
  private var count = 0         // Data

  def next() = {        // Function
    count += 1
    count
  }

  def hasNext = true    // Function
}

• Object = Map from Names To Properties
• Names = count, next, hasNext
• Properties = Data + Functions

What is an object

Just a fancy value

• Object = Map from Names To Properties
• Properties = Data + Functions

So, how can we USE an Object?

Here's a client function:

type iterable = { def next(): Any }
scala> def tickN(it: iterable  , n: Int) =
         for (i <- 1 to n){ println(it.next()) }

scala> tickN(countUp, 5)

What happens?

A. prints out 1 2 3 4 5

B. prints out 1\n 2\n 3\n 4\n 5\n

C. run-time exception

D. type error at function definition def tickN ...

E. type error at function call tickN(countUp, 5)

So, how can we USE an Object?

Here's a client function:

def tickN(it: Any, n: Int) =
   for (i <- 1 to n){println(it.next())}

Type checker is not happy.

<console>:9: error: value next is not a member of Any
            println(it.next())
                       ^

QUIZ: How to DESCRIBE the Required Properties ?

def tickN(it: Any, n: Int) =
   for (i <- 1 to n){println(it.next())}

<console>:9: error: value next is not a member of Any
            println(it.next())
                       ^

Cannot call tickN with Any value...

Which properties are required by tickN ?

A. count

B. next

C. hasNext

D. next and hasNext

E. all of the above

Describing Objects with "Structural" Types

The mystery type ???

def tickN(it: ???, n: Int) =
   for (i <- 1 to n){println(it.next())}

it must be an object ...

... that maps the name next

... to a function that takes () and returns something.

{ def next(): Any }

• Called a Structural Type

• Describes requirements on structure ...

Need to DESCRIBE the TYPE of an object?

def tickN(it: { def next(): Any}, n: Int) =
   for (i <- 1 to n)
     println(it.next())

Thats it!

Lets take it for a spin.

scala> tickN(countUp, 5)
1
2
3
4
5

Recap

• Object = Names -> Properties

• Type = Names -> Types

What is an object?

Here's another really simple object

object countFib {
  var a = 0             // Data
  var b = 1             // Data

  def next() = {        // Function
    val (a0, b0) = (b, a + b)
    a = a0
    b = b0
    a
  }

  def hasNext = true    // Function
}

• Object = Map from Names To Properties
• Names = a, b, next, hasNext
• Properties = Data + Functions

QUIZ: So, how can we USE an Object?

object countFib {
  var a = 0             // Field
  var b = 1             // Another Field!
  def next() = {        // Method
    val (a1, b1) = (b, a + b)
    a = a1
    b = b1
    a }
  def hasNext = true    // Another Method!
}

What happens if we try tickN(countFib, 5) ?

A. prints out 1 1 2 3 5

B. prints out 1\n 1\n 2\n 3\n 5\n

C. run-time exception

D. type error in def tickN ...

E. type error in tickN(countFib, 5)

So, how can we USE an Object?

We can use it with tickN too!

scala> tickN(countFib, 10)
1
2
3
5
8
13
21
34
55
89

tickN doesn't care about extra names and properties

So, how can we USE an Object?

tickN can be called with any object with suitable next

• A structural subtype of {def next():Any}

scala> tickN("yellowmattercustard".iterator, 5)
y
e
l
l
o

So, how can we USE an Object?

tickN can be called with any object with suitable next

scala> tickN(List("cat", "dog").iterator, 5)

cat
dog

java.util.NoSuchElementException: next on empty iterator

Whoops.

So, how can we USE an Object?

We can add more required methods to tickN

def tickN(it: {def next(): Any; def hasNext: Boolean}, n: Int) =
  for (i <- 1 to n){
    if (it.hasNext) println(it.next())
  }

So, how can we USE an Object?

We can add more required methods to tickN

Better to give the new required type a name

type ticker = { def next(): Any; def hasNext: Boolean }

and then ...

def tickN(it: ticker, n: Int) =
  for (i <- 1 to n){
    if (it.hasNext) {
      println(it.next())
    } else {
      println("Out of Stuff!")
    }
  }

So, how can we USE an Object?

Which works quite nicely...

scala> tickN(List("cat", "dog").iterator, 5)
cat
dog
Out of Stuff!
Out of Stuff!
Out of Stuff!

Recap

Objects

• Maps from names to properties

• Properties = Data + Functions (+ Objects)

Types

• Short descriptions of Objects

• Maps from names to types

• Structural Types

So thats OOP! (?)

What else do we need?

What IS a class Anyway?

Seem to be doing just fine without them!

Many OO languages are Class-less

• Google's Go

• JavaScript (also type-less ...)

• Java-- (only interfaces)

Why do we need Classes?

You tell me!

• ???

• ???

• ???

• ???

What is a Class?

Unified mechanism for two tasks

Specification

A name for describing contents

Implementation

A template for creating new objects

What is a Class?

Spectrum from Specification to Implementation

1. Interfaces

• Only Specification of Types

2. Abstract Class

• Specification + Some Concrete Implementations

3. Class

• Specification + All Concrete Implementations

The Class Spectrum in Scala: Interfaces

Interfaces: Only Specify Type

type ticker = { def next(): Any; def hasNext: Boolean }

Some other ways too (later.)

The Class Spectrum in Scala: Abstract Classes

Lets make a ScreenSaver!

Here's the basic loop for the graphics:

def run(box: JPanel, points: List[Point]) {
  ...
  while (true) {
    for (p <- points) {
      p.draw(g)             
      p.move(d, delta)
      p.collide(d)
    }
  }
  ...
}

Need a type Point describing objects that we can:

• draw, move, collide

The Class Spectrum in Scala: Abstract Classes

Need a type describing draw, move, collide

First, some essential fields...

abstract class Point {

 // Must be filled in by (concrete) sub-classes
  val name : String     // immutable
  var x    : Int        // mutable
  var y    : Int        // mutable

 // Defined, but may be "overridden" by sub-classes
  var xVel = Rand(1, 5)
  var yVel = Rand(1, 5)
  ...
}

The Class Spectrum in Scala: Abstract Classes

Need a type describing draw, move, collide

Next, we specify the methods ...

abstract class Point {
  // Must be filled in by (concrete) sub-classes
  def xDim : Int
  def yDim : Int
  def render(g: Graphics): Unit
  def move(d: Dimension, delta: Int)
  ...
}

... these are pure specification

... which must be filled in an actual Point object

The Class Spectrum in Scala: Abstract Classes

Finally, we implement some common methods ...

abstract class Point {

  // Can be overridden by concrete Points
  def collide(d: Dimension) {
    hitWall(x, 0, d.width - xDim) map {(v:Int) =>
      x    = v
      xVel = -xVel
    } ...
  }
  ...
}

The Class Spectrum in Scala: Abstract Classes

Finally, we implement some common methods ...

abstract class Point {

  // Cannot be overridden by sub-classes
  final def draw(g: Graphics) {
    val c = g.getColor()    // save old color
    render(g)               // render object
    g.setColor(c)           // restore old color
  }
}

Screensaver: Point only needs collide, draw and move.

Which of the following are type-safe modifications to Point?

A. Delete the specification of x and y

B. Delete the specification of collide

C. Delete the implementation of collide

D. (A) and (C)

E. (A), (B) and (C)

The Class Spectrum in Scala: (Concrete) Classes

Like an abstract class but with everything defined.

class Dot(val name: String, x0: Int, y0: Int) extends Point {
    ...
}

extends Point says Dot is a concrete implementation of Point.

The Class Spectrum in Scala: Classes

Parameters define several things "for free"

class Dot(val name: String, x0: Int, y0: Int) extends Point {
  // Filled in by "constructor"
  var x     = x0
  var y     = y0
  def xDim  = 20
  def yDim  = 20
  ...
}

Generated Constructor

• Initializes fields from parameters x0, y0

• Named parameters automatically become fields

The Class Spectrum in Scala: Classes

We define required methods

class Dot(val name: String, x0: Int, y0: Int) extends Point {
  def render(g: Graphics) {
    g.fillOval(x, y, xDim, yDim)
  }

  override def move(d: Dimension, delta: Int) {
    x += (delta * xVel)
    y += (delta * yVel)
  }
}

• Remainder inherited from Point

The Class Spectrum in Scala: Classes

Also automatically defined new type ... with name Dot

class Dot(val name: String, x0: Int, y0: Int) extends Point {
  def render(g: Graphics) {
    g.fillOval(x, y, xDim, yDim)
  }

  override def move(d: Dimension, delta: Int) {
    x += (delta * xVel)
    y += (delta * yVel)
  }
}

• Called Nominal (by-name) subtyping
• Can use Dot where Point is expected

The Class Spectrum in Scala: Classes

Note the auxiliary constructor this

• Feeling lazy about supplying x0, y0?

// Auxiliary constructor, invokes "real" constructor
def this(n: String) =
  this(n, Rand(0, 100), Rand(0, 100))

The Class Spectrum in Scala: Classes

Lets create an instance

With the default constructor ...

new Dot("p1", 50, 50) // Default constructor

... and with the auxiliary one

new Dot("p2")       // Auxiliary constructor

QUIZ: Mutation Revisited

class Dot(val name: String, x0: Int, y0: Int) extends Point {
  // Filled in by "constructor"
  var x     = x0
  var y     = y0
  def xDim  = 20
  def yDim  = 20
  ...
}

Are instances of Dot

A. Mutable

B. Immutable

C. None of the above

Recap: What is a Class?

Spectrum from Specification to Implementation

1. Interfaces

Only Specification of Types

e.g. ticker

2. Abstract Class

Specification + Some Concrete Implementations

e.g. Point

3. Class

Specification + All Concrete Implementations

e.g. Dot

OK! Lets Run The Screensaver!

So far: Objects + Classes + (Some) Inheritance

How to get colored dots?

Inheritance

Extend Dot with a notion of color

class ColorDotV1(name: String, color: Color)
extends Dot(name) {
  override def render(g: Graphics) {
    g.setColor(color) //"color" in scope from params
    super.render(g)
  }
}

Note: extends Dot(name)

• Pithy way of calling the (old) super-class constructor

• Passing it the parameter(s) of the (new) sub-class

QUIZ: Inheritance

class ColorDotV1(name: String, color: Color)
extends Dot(name) {
  override def render(g: Graphics) {
    g.setColor(color) //"color" in scope from params
    super.render(g)
  }
}

What has ColorDotV1 inherited ?

A. render from Point

B. move from Point

C. render from Dot

D. move from Dot

E. All of the above

QUIZ: Inheritance

Lets extend Dot with a notion of color

class ColorDotV1(name: String, color: Color)
extends Dot(name) {
  override def render(g: Graphics) {
    g.setColor(color) //"color" in scope from params
    super.render(g)
  }
}

What does super.render refer to?

A. render in Point

B. render in Dot

C. render in Surrender

QUIZ: Inheritance

class ColorDotV1(name: String, color: Color)
extends Dot(name) {
  override def render(g: Graphics) {
    g.setColor(color) //"color" in scope from params
    super.render(g)
  }
}

The new type ColorDotV1 is a subtype of

A. ticker

B. Point

C. Dot

D. Both Point and Dot

E. Neither Point nor Dot

Inheritance Enables Reuse

Get lots of properties from without any work!

• Only need to override or extend bits that are different...

Lets make another shape

How about a Box ?

class Box(name: String, h: Int, w: Int) extends Dot(name) {
  override def yDim = w
  override def xDim = h

  override def render(g: Graphics) {
    g.fillRect(x, y, h, w)
  }
}

Get lots of stuff without any work!

• Get move (from Dot) for free!

• Get collide (from Point) for free!

Lets make another shape

How about a Box ?

Lets check it out ...

val points = List( ...
                 , new Box("p4", 20, 40)
                 )

But wait a minute...

Wait a minute ... beware of things you get for free

• Good Angel: Is a Box really a Dot ?
• Evil Angel: But we're getting so much reuse !!
• Good Angel: ...

Lets make (yet) another shape

How about we color them Boxes?

• But how?

• Inheritance?

• Lets see...

Lets make (yet) another shape: ColorBox

Lets try extending Box ...

class ColorBoxV1(name: String, color: Color, h: Int, w: Int)
extends Box(name, h: Int, w: Int) {
  override def render(g: Graphics) {
    g.setColor(color)
    super.render(g)
  }
}

... Yikes! We're repeating ourselves!

• render is just like ColorDot.render

• How can we reuse the above render ?

Lets make (yet) another shape: ColorBox

No problem, just make ColorBox inherit from (extend) ColorDot.

Whats the problem?

A. Cannot extend ColorDot

B. Would have to duplicate render method from Dot

C. Don't fuss, THERE IS NO PROBLEM!

D. Would have to duplicate render method from Box

E. Would have to duplicate constructor from Box

Problem

We bundled Colored-ness into ColorDot ...

... but color is totally independent of Shape!

How can we free up colored-ness from its chains?

Traits

Describe object behaviors independent particular classes ...

... Can be mixed into any class to add that behavior to the class!

Traits describe Object Behaviors Independently

trait Colored extends Point {

  val color: Color  // Will be defined by mixin-target

  abstract override def render(g: Graphics) {
    g.setColor(color)     
    super.render(g) // Call render of mixin-target
  }
}

This exactly specifies and implements colored-ness ...

... without bundling it inside a particular class!

Traits describe Object Behaviors Independently

trait Colored extends Point {

  val color: Color  // Will be defined by mixin-target

  abstract override def render(g: Graphics) {
    g.setColor(color)     
    super.render(g) // Call render of mixin-target
  }
}

Wait! Aren't traits just like Java interfaces?

QUIZ: Traits describe Object Behaviors Independently

trait Colored extends Point {
  val color: Color  // Will be defined by mixin-target
  abstract override def render(g: Graphics) {
    g.setColor(color)     
    super.render(g) // Call render of mixin-target
  }
}

Why do we write extends Point?

A. Good documentation, we don't have to write it

B. Only Point objects can have a color field

C. Only Point objects can have a setColor method

D. Both B and C

E. Only Point objects can have a render method

Traits Can Be Mixed-Into Appropriate (Host) Classes

To use a trait, just mixin to an appropriate host class.

(e.g. subtype of Point)

class ColorDot(name: String, val color: Color)
  extends Dot(name) with Colored

class ColorBox(name: String, val color: Color, h: Int, w: Int)
  extends Box(name, h, w) with Colored

Lets take the new classes for a spin!

Traits: Recap

Describe object behaviors independent particular classes ...

... Can be mixed into any class to add that behavior to the class!

Are traits different from Multiple Inheritance

Why can't I do something like this:

abstract class Point
class Dot extends Point {...}
class Colored extends Point {...}

and then

class ColoredDot extends Dot, Colored {...}

Wouldn't this solve the problem of reusing Color and Dot ?

• Problem 1: Which parent do I pick method from?
• Problem 2: Need to compose render from both

Are traits different from Multiple Inheritance

The cunning thing about traits:

trait Colored extends Point {

  val color: Color  // Will be defined by mixin-target

  abstract override def render(g: Graphics) {
    g.setColor(color)     
    super.render(g) // Call render of mixin-target
  }
}

• stacks the new behavior

• on top of existing behavior! (super.render)

• independent of the host class!

• So, color-a-box, color-a-dot, color-anything!

Another Trait: Bouncing

Lets add a notion of gravity to the objects...

trait Bouncing extends Point {
  ...
  abstract override def move(d: Dimension, delta: Int) {
    yVel += delta       
    shootUp()               // if ROTFL
    super.move(d, delta)    // preserve
  }
}

Another Trait: Bouncing

Lets add a gravity behavior to the objects...

• independent of shape, color, etc.

trait Bouncing extends Point {
  ...
  abstract override def move(d: Dimension, delta: Int) {
    yVel += delta       
    shootUp()               // if ROTFL
    super.move(d, delta)    // preserve
  }
}

Note how new behavior stacked on top of old!

• Reuse old (horizontal) motion via super.move

Another Trait: Bouncing

We can add bouncing behavior to any Point object

class BouncingColorDot(name: String, val color: Color)
  extends Dot(name) with Colored with Bouncing

class BouncingColorBox(name: String, val color: Color, h: Int, w: Int)
  extends Box(name, h, w) with Colored with Bouncing

Or even directly retro-actively to the instance object

new Dot("p2") with Bouncing

Moral of the story

Inheritance enables reuse

BUT

Class(ical) inheritance locks reusable behaviors

Into unrelated classes (e.g. ColorDotV1)

In fact ...

QUIZ: Our ScreenSaver can be cleaned up more...

There remains another accidental intertwining...

...behavior locked inside unrelated class.

Where?

A. collide

B. move

C. color

D. Dot

E. Box

Moral of the story

Inheritance enables reuse

BUT

Class(ical) inheritance locks reusable behaviors

Into unrelated classes (e.g. ColorDotV1)

SO

Keep a sharp eye out for independent behaviors,

and set them free using traits.