Csci 490 / Engr 596Csci 490 / Engr 596Special Topics / Special ProjectsSpecial Topics / Special Projects

Software Design and Scala ProgrammingSoftware Design and Scala Programming

Spring Semester 2010Spring Semester 2010Lecture NotesLecture Notes

Understanding InheritanceUnderstanding Inheritance

This set of slides is loosely based on chapter 8 of This set of slides is loosely based on chapter 8 of Timothy Budd's textbook Timothy Budd's textbook

Understanding Object-Oriented Programming with Java, Understanding Object-Oriented Programming with Java, Updated EditionUpdated Edition

(Addison-Wesley, 2000)(Addison-Wesley, 2000)

Created: 14 August 2004, Revised for Scala 9-11 February 2010

Motivation for InheritanceMotivation for Inheritance

Use inheritance to create new software Use inheritance to create new software structures from existing software units to:structures from existing software units to:

improve productivity improve productivity enhance qualityenhance quality

1

Generality and Specialization in Generality and Specialization in Software DevelopmentSoftware Development

Conflict: Conflict: Specific projects usually require very Specific projects usually require very

specialized softwarespecialized softwareReusability usually requires very general Reusability usually requires very general

softwaresoftware

Resolution?Resolution?Inheritance allows general software to be Inheritance allows general software to be

specialized for a projectspecialized for a project

2

Abstract Idea of Inheritance Abstract Idea of Inheritance (is-a)(is-a)

3

Material Object

Non-Living Thing

Rock

Air

Living Thing

Plant

Animal

Reptile

Mammal

Human Being

Dentist Roy

Shopkeeper Flo

Writer John

CatDogPlatypus

Practical Meaning of InheritancePractical Meaning of InheritanceIn programming languages, inheritance means In programming languages, inheritance means

Data and behavior of parent class are part of child Data and behavior of parent class are part of child

Child class may include data and behavior not in parentChild class may include data and behavior not in parent

With respect to the parent class, a child class is, in With respect to the parent class, a child class is, in some sense some sense An extension – larger set of properties An extension – larger set of properties

A contraction – more specialized (restricted) objectsA contraction – more specialized (restricted) objects

4

Idealized Image of InheritanceIdealized Image of Inheritance

Consider Consider Subclass instances must possess all data areas Subclass instances must possess all data areas

of the parent of the parent Subclass instances must implement all Subclass instances must implement all

functionality of the parent functionality of the parent

Thus Thus Subclass instance should be Subclass instance should be indistinguishableindistinguishable

from parent class instance—child can be from parent class instance—child can be substituted for parent substituted for parent

5

Principle of SubstitutabilityPrinciple of Substitutability

If C is a subclass of P, instances of C can If C is a subclass of P, instances of C can be substituted for instances of P in any be substituted for instances of P in any situation with no observable effect.situation with no observable effect.

6

Subclass and SubtypeSubclass and Subtype

SubtypeSubtypeClass that satisfies principle of substitutabilityClass that satisfies principle of substitutabilitySubclass Subclass Something constructed using inheritance, whether Something constructed using inheritance, whether

or not it satisfies the principle of substitutability. or not it satisfies the principle of substitutability. The two concepts are independentThe two concepts are independentNot all subclasses are subtypes Not all subclasses are subtypes Sometimes subtypes are constructed without being Sometimes subtypes are constructed without being

subclasses subclasses

7

Forms of Inheritance Forms of Inheritance SpecializationSpecialization Child class is a special case (subtype) of parent Child class is a special case (subtype) of parent

SpecificationSpecification Parent class defines behavior implemented in the child, but not parent Parent class defines behavior implemented in the child, but not parent

ConstructionConstruction Parent class used only for its behavior -- child class is not subtype -- no Parent class used only for its behavior -- child class is not subtype -- no is-ais-a

relationship to parent relationship to parent GeneralizationGeneralization Child class modifies or overrides some methods of parent, extends the behavior to Child class modifies or overrides some methods of parent, extends the behavior to

more general kind of object more general kind of object Extension Extension Child class adds new functionality to parent, but does not change any inherited Child class adds new functionality to parent, but does not change any inherited

behavior behavior LimitationLimitation Child class limits some of the behavior of parent Child class limits some of the behavior of parent

VarianceVariance Child and parent class are variants of each other -- inheritance to allow code sharing Child and parent class are variants of each other -- inheritance to allow code sharing

-- arbitrary relationship -- arbitrary relationship CombinationCombination Child class inherits features from more than one parent -- multiple inheritance Child class inherits features from more than one parent -- multiple inheritance

8

Forms of Inheritance Forms of Inheritance SpecializationSpecialization

Child class is a special case (subtype) of parent

Most common form of inheritance Most common form of inheritance Example: Example: ProfessorProfessor is specialized form of is specialized form of EmployeeEmployee Child may override behavior of parent to specialize Child may override behavior of parent to specialize Child satisfies specification of parent in all relevant aspects Child satisfies specification of parent in all relevant aspects Preserves substitutabilityPreserves substitutability

9

Forms of InheritanceForms of Inheritance SpecificationSpecification

Parent class defines behaviorbehavior implemented in the child, but not parent Second next most common form of inheritance Second next most common form of inheritance Example: class Example: class StackInArrayStackInArray gives implementations for gives implementations for

method signatures defined in abstract class method signatures defined in abstract class StackStack Java and Scala:Java and Scala: StackInArray extends Stack StackInArray extends Stack

Example: class Example: class ArrayRankedSeqArrayRankedSeq gives implementations for gives implementations for method signatures defined in Java interface or Scala trait method signatures defined in Java interface or Scala trait RankedSequenceRankedSequence Java:Java: ArrayRankedSeq implements RankedSequence ArrayRankedSeq implements RankedSequence

Scala: Scala: ArrayRankedSeq extends RankedSequence ArrayRankedSeq extends RankedSequence

10

Forms of InheritanceForms of Inheritance Specification (continued)Specification (continued)

Parent class defines behaviorbehavior implemented in the child, but not parent …… Subclasses are realizations of incomplete abstract specification Subclasses are realizations of incomplete abstract specification Defines common interface for group of related classes Defines common interface for group of related classes Preserves substitutabilityPreserves substitutability

10

Forms of InheritanceForms of Inheritance ConstructionConstruction

Parent class used only for its behavior — child class is not subtype — no is-a relationship to parent

Sometimes used for convenience, but discouraged Sometimes used for convenience, but discouraged Example: extending Example: extending ListList class to develop class to develop SetSet, without , without

"hiding" unneeded methods "hiding" unneeded methods Example: extending a byte-based I/O stream to a stream for Example: extending a byte-based I/O stream to a stream for

handling other objects handling other objects Often violates substitutability Often violates substitutability More common in dynamically typed languages (e.g., More common in dynamically typed languages (e.g.,

Smalltalk, Ruby) than in statically typed (e.g., Java, Scala) Smalltalk, Ruby) than in statically typed (e.g., Java, Scala) Can sometimes use aggregation (composition) instead Can sometimes use aggregation (composition) instead

11

Forms of InheritanceForms of Inheritance GeneralizationGeneralization

Child class modifies or overrides some methods of parent, extends the behavior to more general kind of object Sometimes used for convenience (or necessity), but Sometimes used for convenience (or necessity), but

discouraged discouraged Example: graphics Example: graphics WindowWindow generalized to generalized to ColorWindowColorWindow

(with background color) (with background color) Opposite of specialization—violates substitutability Opposite of specialization—violates substitutability Used when must build from fixed, difficult-to-modify set of Used when must build from fixed, difficult-to-modify set of

classes classes Where possible, invert class hierarchy or use aggregation Where possible, invert class hierarchy or use aggregation

12

Forms of Inheritance Forms of Inheritance ExtensionExtension

Child class adds new functionality to parent, Child class adds new functionality to parent, but does not change any inherited behaviorbut does not change any inherited behavior Useful technique to give new behaviors to existing base Useful technique to give new behaviors to existing base

class that cannot be modified class that cannot be modified Example: Example: StringSetStringSet extends extends SetSet, adding string-related , adding string-related

methods (e.g, prefix search) methods (e.g, prefix search) Preserves substitutability Preserves substitutability

13

Forms of InheritanceForms of Inheritance LimitationLimitation

Child class limits some of the behavior of Child class limits some of the behavior of parent parent Sometimes used for convenience, but strongly discouraged Sometimes used for convenience, but strongly discouraged ExampleExample:: Stack extends extends DoubleEndedQueue, ,

replacing unneeded methods to give error messages replacing unneeded methods to give error messages Violates substitutability Violates substitutability Used when must build from fixed, difficult-to-modify set Used when must build from fixed, difficult-to-modify set

of classes of classes Avoid when possible, perhaps use aggregationAvoid when possible, perhaps use aggregation

14

Forms of Inheritance Forms of Inheritance Variance Variance

Child and parent class are variants of each other—inheritance to allow code sharing—arbitrary relationship

Sometimes used for convenience, but discouraged Sometimes used for convenience, but discouraged Example: graphics Example: graphics Tablet class extending class extending Mouse to to

share similar control code share similar control code Violates substitutability Violates substitutability Better to define more general parent class like Better to define more general parent class like

PointingDevice 15

Forms of InheritanceForms of Inheritance CombinationCombination

Child class inherits features from more than one parent—multiple inheritance Example: Example: GraduateInstructor might inherit from both might inherit from both

GraduateStudent and and Faculty Often difficult to understand and to implement language Often difficult to understand and to implement language Often use to "mix-in" specification of another role or Often use to "mix-in" specification of another role or

protocol protocol

16

Forms of InheritanceForms of Inheritance Combination (continued)Combination (continued)

Child class inherits features from more than one parent—multiple inheritance …… C++ has multiple inheritance via subclassing, with some C++ has multiple inheritance via subclassing, with some

semantic difficultiessemantic difficulties Java has single inheritance via subclassing (extends), but Java has single inheritance via subclassing (extends), but

multiple inheritance for specification via interface multiple inheritance for specification via interface implementations (implements)implementations (implements)

Scala has single inheritance via subclassing and multiple Scala has single inheritance via subclassing and multiple “mix-in” inheritance of “stackable” traits (avoiding “mix-in” inheritance of “stackable” traits (avoiding semantic issues of C++)semantic issues of C++) 16

Inheritance and Assertions Inheritance and Assertions Suppose C is a subtype of P P and C have interface invariants I and IC, respectively meth() is a public method of P with precondition Q and postcondition

R meth() in C has precondition QC and postcondition RC

Subtype C should not violate I, Q, and R IC implies I – may strengthen invariant – extend interface and data Q implies QC – may weaken precondition – expand valid inputs RC implies R -- may strengthen postcondition – restrict valid outputs

Abstract preconditions can enable controlled "strengthening" of precondition Consider BoundedStack inheriting from an unbounded Stack class Give method push() a "not full" precondition – always true in Stack Refine "not full" in subclass BoundedStack to be true or false

17

Inheritance andInheritance and AssertionsAssertions

Interface Interface invariantsinvariants

meth()meth()

Pre-Pre-conditioncondition

Post-Post-conditioncondition

P (Parent)P (Parent) II QQ RR

C (Child)C (Child) ICIC QCQC RCRC

IC=>IIC=>I Q=>QCQ=>QC RC=>RRC=>R

strengthenstrengthen weakenweaken strengthenstrengthen

18

Trees versus Forests Trees versus Forests Two common views of class hierarchiesTwo common views of class hierarchies

Tree All classes are part of single class hierarchy All classes are part of single class hierarchy Advantage: root's functionality inherited by all objects – all have basic Advantage: root's functionality inherited by all objects – all have basic

functionality functionality Disadvantage: tight coupling of classes, large libraries for an application Disadvantage: tight coupling of classes, large libraries for an application Languages: Java's classes, Scala’s classes, Smalltalk, Objective C, Languages: Java's classes, Scala’s classes, Smalltalk, Objective C,

Delphi Object PascalDelphi Object Pascal

Forest Classes only placed in hierarchies if they have a relationship – many Classes only placed in hierarchies if they have a relationship – many

small hierarchies. small hierarchies. Advantage: smaller libraries of classes for application, less coupling Advantage: smaller libraries of classes for application, less coupling

possible possible Disadvantage: no shared functionality among all objects Disadvantage: no shared functionality among all objects Languages: Java's interfaces, Scala’s traits, C++, Apple Object Pascal Languages: Java's interfaces, Scala’s traits, C++, Apple Object Pascal 19

Tree versus ForestTree versus Forest

A

B C

D E F G

Tree

20

Trees versus ForestsTrees versus Forests

ForestForest A

B C

D E F G

X

Y Z

21

Inheritance in Java Inheritance in Java

Tree-structured class hierarchy (with primitive data types not in hierarchy)

Forest-structured interface hierarchy

Modifiers for classes/interfaces

Access modifiers for class/interface features

22

Inheritance in JavaInheritance in JavaTree-structured Class HierarchyTree-structured Class Hierarchy

Root class is java.lang.Object Other classes extend exactly one other classdefault is Object

Declaration uses keyword extends after class name

23

Object

Material_Object

Non_living_Thing Living_Thing

… …

Inheritance in JavaInheritance in JavaForest-structured Interface HierarchyForest-structured Interface Hierarchy

interface defines an interface specification implements after class name to promise implementation of interface – inheritance for specification An interface extends zero or more other interfaces A class implements zero or more interfaces

public interface Queue public interface Queue { { // signatures of public methods // signatures of public methods

// Queues must provide// Queues must provide } } public class QueueAsLinkedList public class QueueAsLinkedList

implements Queue implements Queue { { // includes implementations// includes implementations // of the Queue methods// of the Queue methods }}

24

Inheritance in JavaInheritance in Java Visibility Modifiers for Class/Interface FeaturesVisibility Modifiers for Class/Interface Features

public features accessible from anywhere in program private features accessible from inside class only Default-access (i.e., "friendly") features accessible from inside the current Java package

protected features accessible in package or inside any child class

26

Inheritance in JavaInheritance in Javapublic abstract class Stack public abstract class Stack { // extends Object by default { // extends Object by default // data definitions plus signatures and // data definitions plus signatures and // possibly implementations of methods // possibly implementations of methods } }

public class StackInArray extends Stack public class StackInArray extends Stack { // extended features plus overridden implementations { // extended features plus overridden implementations } }

public interface Queue public interface Queue { // signatures of public methods Queues must provide { // signatures of public methods Queues must provide } }

public class QueueAsLinkedList implements Queue public class QueueAsLinkedList implements Queue { // includes implementations of the Queue methods { // includes implementations of the Queue methods }} 27

Facilities of Root Class ObjectFacilities of Root Class ObjectMinimum functionality for all objects include

equals(Object obj) is is objobj the same as receiver? the same as receiver?

toString() converts the object to a string value converts the object to a string value

hashCode() return a default hashcode for the objectreturn a default hashcode for the object

getClass() getClass() return an identifier for the class of the objectreturn an identifier for the class of the object

First three above are often overridden in classes.First three above are often overridden in classes. 28

Inheritance in Scala Inheritance in Scala

Tree-structured class hierarchy (with primitives in hierarchy)Classes that extend one class and “mix-in” zero or more traits

Modifiers for classes/traits

Access modifiers for class/interface features (slightly different from Java) (slightly different from Java)

22

Inheritance in ScalaInheritance in ScalaTree-structured Class HierarchyTree-structured Class Hierarchy

Root class is Any Primitive value types extend AnyValAll reference object types extend AnyRef (java.lang.Object)Scala reference objects also mix-in trait ScalaObjectScala traits extend AnyRef

23

Any

AnyVal AnyRef

Java primitive values

Scala classes also have

marker trait ScalaObject

Java classes

Inheritance in ScalaInheritance in Scala

trait Philosphical {trait Philosphical { //method signatures, perhaps implementations//method signatures, perhaps implementations // perhaps data attributes// perhaps data attributes}} class Frog extends Philosphical { . . . }trait HasLegs { . . . }class Animal class Frog extends Animal with Philosphical with HasLegs { … }

27

Inheritance in ScalaInheritance in Scala Visibility Modifiers for Class/Trait FeaturesVisibility Modifiers for Class/Trait Features

private features accessible from inside class only (like Java except for inner classes)protected features accessible only from subclasses (more restricted than Java)No access modifier means “public” access features accessible from anywhereprivate[X] and protected[X] provide qualified access “up to X” – gives capabilities like object private, package protected, etc.

26

Facilities of Root Classes Facilities of Root Classes Any and AnyRefAny and AnyRef

… … Look in the API documentationLook in the API documentation

28

Benefits of InheritanceBenefits of Inheritance

Software reusability (among projects) Software reusability (among projects) Code sharing (within a project) Code sharing (within a project) Increased reliability (resulting from reuse and Increased reliability (resulting from reuse and sharing of well-tested code) sharing of well-tested code) Consistency of interface (among related objects) Consistency of interface (among related objects) Rapid prototyping (quickly assemble from pre-Rapid prototyping (quickly assemble from pre-existing components) existing components) Polymorphism and frameworks (high-level Polymorphism and frameworks (high-level reusable components) reusable components) Information hiding Information hiding

29

Costs of InheritanceCosts of Inheritance

Execution speed Execution speed

Program sizeProgram size

Message-passing overheadMessage-passing overhead

Program complexityProgram complexity

30

AcknowledgementAcknowledgement

31

The development of the original Java-based The development of the original Java-based slides was supported by a grant from slides was supported by a grant from Acxiom Corporation titled “The Acxiom Acxiom Corporation titled “The Acxiom Laboratory for Software Architecture and Laboratory for Software Architecture and Component Engineering (ALSACE).”Component Engineering (ALSACE).”

Students who helped with slides—Jian Li, Yi Students who helped with slides—Jian Li, Yi Liu, Pallavi Tadepalli, etc.Liu, Pallavi Tadepalli, etc.