programming in scala – mixin inheritance summer 2008
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Programming in Scala – Mixin inheritance Summer 2008
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The very short history of multiple inheritance
• C++ had multiple inheritance
• Java removed it • Mostly due to diamond-
inheritance problem.
• Should alice behave like Women are defined to behave, or like Directors are defined to behave?
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Person
Director Woman
alice
«instanceOf» «instanceOf»
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Mixin-based inheritance
• Scala allows mixin-type inheritance to be used as a mean to improve reusability and modularization
• Contents of this talk: 1. Introduction 2. Sparse interfaces versus rich interfaces 3. Stackable modifications via traits 4. Improving modularization of JSF beans
• Themes 1-3 are rephrased from the chapter 12 of ’Programming in Scala’
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Theme 1: Introduction
• Inheritance between two classes can be defined as R = P ⊕ ΔR
where R is the newly defined subclass, P denotes the properties inherited from an existing class,
ΔR denotes the incrementally added new properties that differentate R from P and
⊕ stands for operation of combining P and ΔR
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Traditional (single-)inheritance
• Traditional inheritance defines classes as Class P is [..properties..]
Endclass
Class R inherits P and [.. modifies P by ΔR .. ]
Endclass
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Mixin-inheritance
• In mixin-inheritance, the ΔR part is written into its own mixin-class, as:
Class P is [… properties … ]
Endclass Class Rmixin is [… modifications (ΔR) …] Endclass
Class R inherits P, Rmixin.
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Properties of mixins
• One never instantiates a mixin; but a mixin is combined with a concrete class
• Many mixins can be combined with a concrete class at a time
• The order of the mixins gives a resolution to the diamond-inheritance problem
• Both abstract methods and concrete methods can be contained in a mixin-class – This is different than in Java, which allows
multiple inheritance via interfaces only
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Scala-examples
• Defining a mixin-class Philosophical: trait Philosophical {
def philosophize() { println("I consume memory, therefore I am!”) }
} • The trait does not declare a superclass, so similar to
classes, it has the default superclass of AnyRef. • It defines one method, named philosophize, which
is concrete.
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Defining a philosophical frog
abstract class Animal { def color: String override def toString = ”a ” + color + ” animal.” } class Frog extends Animal with Philosophical {
def color = "green" } • We’ve defined three methods for class Frog:
color(), toString() and philosophize()
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Traits are more than interfaces with implementations
• Traits do about all the same than classes, e.g. – Traits can declare fields and contain state – Traits can be defined in inheritance hierarchies – Traits can be declared abstract
• There are two cases in which trait classes differ from regular classes: 1. It is not possible to define class parameters for
traits – it is the class, which defines how it is constructed
2. The ’super’-calls are dynamically bound when the trait is mixed with a class
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Theme 2: Sparse and Rich interfaces
• Sparse interfaces define just a few functions, which need to be implemented in the implementing class
• A tradeoff between caller convenience and implementor convenience – In a rich interface, it is more work to implement
all of the required methods • The Java API favours sparse interfaces, due to
single-inheritance • Traits allow to reduce the tradeoff’s effect
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Example: a rectangle
• Rectangles are used in graphical user interfaces in various contexts: – Windows have a rectangular bounding box – Bitmap images are rectangular – Regions selected with a mouse are rectangular
• We could implement the rectangulariness in the base class of all rectangular things – But then, other aspects of these things would be
missing – A.k.a tyranny of dominant decomposition
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An enrichment trait - Rectangular
class Point trait Rectangular { def topLeft: Point def bottomRight: Point
def left = topLeft.x def right = bottomRight.x def width = right - left def height = bottomRight.y – topLeft.y; // and many more geometric methods... }
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A selection event (hypothetical)
class SelectionEvent(val topLeft: Point, val bottomRight: Point) extends MouseEvent with Rectangular { // structure from MouseEvent is // mixed with Rectangular’s methods and // attributes }
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A bitmap image (hypothetical)
class Bitmap(val topLeft: Point, val bottomRight: Point) extends Rectangular {
// It is also possible to directly to extend a trait class
def setImage(img: Array[int]) = { if(img.length != width * length) { throw new IllegalArgumentException(”!!”); } }
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Another trait-example: rational numbers
class Rational(val numerator: Int, val denominator: Int) { // ... def < (that: Rational) = this.numerator * that.denominator > that.numerator * this.denominator def > (that: Rational) = that < this def <= (that: Rational) = (this < that) || (this == that) def >= (that: Rational) = (this > that) || (this == that) }
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The ordered trait
• Rational’s rich interface is cumbersome to write, as it would be duplicated in similar classes
• While the order can be defined with a single function, ”compare”, it is inconvenient to need to write
object1 compare object2 <= 0 when actually meaning to say object1 <= object2.
• Ordering of objects is another area where rich interfaces can be useful
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Ordered.scala
trait Ordered[A] { def compare(that: A): Int
def < (that: A): Boolean = (this compare that) < 0 def > (that: A): Boolean = (this compare that) > 0 def <= (that: A): Boolean = (this compare that) <= 0 def >= (that: A): Boolean = (this compare that) >= 0
def compareTo(that: A): Int = compare(that) }
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MixinRational.scala
class MixinRational(val numer: Int, val denom: Int) extends Ordered[MixinRational] { // ... def compare(that: MixinRational) = (this.numer * that.denom) - (that.numer * this.denom)
// todo: define equals, hashcode }
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Theme 3: Stackable modifications
• Due to a trait’s super-calls being bound dynamically, multiple traits can be stacked
• Let’s think about a Queue, where you can put values into and get values from:
abstract class IntQueue { def get(): Int def put(x: Int) }
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BasicIntQueue.scala
import scala.collection.mutable.ArrayBuffer class BasicIntQueue extends IntQueue { private val buf = new ArrayBuffer[Int]
def get() = buf.remove(0) def put(x: Int) { buf += x } }
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Defining three behaviour modifying traits
1. Doubling: – double all integers that are put in the queue
2. Incrementing: – increment all integers that are put in the queue
3. Filtering: – filter out negative integers from a queue
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Doubling.scala
trait Doubling extends IntQueue { abstract override def put(x: Int) { super.put(2 * x) }
}
• abstract override is not possible for regular classes • For traits, it means that this trait can only be mixed
in with a class that already has the corresponding method concretely defined
• Usage: – class MyQueue extends BasicIntQueue with
Doubling
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Incrementing.scala, Filtering.scala
trait Incrementing extends IntQueue { abstract override def put(x: Int) { super.put(x + 1) } }
trait Filtering extends IntQueue { abstract override def put(x: Int) { if (x >= 0) super.put(x) } }
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Ordering of mixins is significant
• Basically, the list of mixin-modifications is traversed from right to left
• Each new mixin is put ”on stack” of the previous ones
• new BasicIntQueue with Incrementing with Doubling – First double, then increment
• new BasicInQueue with Doubling with Incrementing – First increment, then double
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Theme 4: JSF-example
• Java Server Faces, JSF, is a JSR-127, thus a ’standard’ way of constructing web applications in Java
• Facelets are a common technique for defining reuseable layouts in XML
• User interface state is contained in Managed beans, whose getters and setters follow the JavaBeans conventions
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JSF data table
• Two components: a data table and a scroller
• Data table’s state – List of cars – First row number
• Scroller’s state – Number of pages – Current page
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Car listing’s backing bean
• A managed bean in Java, which contains accessors for both the data table and the scroller’s state
class CarListingManagedBean { private int startingRow; private Object carListing; // an array or an iterator private int currentPage; private int pageCount;
public void setStartingRow(int i) { … } public int getStartingRow() { …. } ….. } 28
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Single-point of non-modularization
• While the facelet UI pages can be modularized to a reasonable degree, the managed bean part tends to become a mold of all corresponding state variables
• E.g. both the data table’s and scroller’s variables are contained in the one UI bean class
• When reusing the same UI components, the state variables and accessors are duplicated to multiple managed beans
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Traits to the rescue
• With traits, we can isolate each of the UI component’s state variables into their own modules
• For a given UI page’s backing bean, mix in the corresponding traits
• E.g.
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trait DataTableBacker { var startingRow: int; var listing: Object; }
trait DataScrollerBacker { var currentPage: int; var lastPage: int;
}
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References
• [Bracha & Cook 1990]: Mixin-based inheritance • [Taivalsaari 1996]: On the notion of inheritance
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