Designing with inheritance (c) Eraj Basnayake 1 Applying Inheritance to Solve problems.

Designing with inheritance (c) Eraj Basnayake

1

Applying Inheritance to Solve problems

Designing with inheritance (c) Eraj Basnayake

2

Applied Inheritance

bullDecide on the classes involved from the problem description

bullGroup classes into conceptually equivalentconceptually equivalent groups

bullPer group of classes the generalizing concept will be a Parent to those classes

bullDistribute responsibilities amongst allall classes based on what is appropriateclasses based on what is appropriateand customary for that class

Example

A program to teach geometry needs to model some basic shapes All shapes

have surface area could have volume and location The shapes that

need to be modeled are Circle Rectangle Sphere Cylinder and a Square

What classes will you need Draw an UML diagram showing class relationships

if any

Designing with inheritance (c) Eraj Basnayake

3

ExampleClasses

Circle Rectangle Sphere Cylinder Square Coordinate

All ldquoShapesrdquo

Square is a Rectangle

Coordinate

3D Shapes

2D Shapes

Inheritance hierarchy

Group classes

Shape

2D Shapes 3D Shapes

Circle Rectangle

Square

Cylinder Sphere

Circle

Rectangle

SquareCylinder

Sphere

Designing with inheritance (c) Eraj Basnayake

4

Shape

Shapes2D Shapes3D

Circle Rectangle

Square

Cylinder Sphere

Examplehellip

Distribute Responsibilities

Shape

+getSurfaceArea()

+clone()

+equals()

+toString()

+Object getLocation()

Shape2D

+

Shape3D

+getVolume()

Is Shape2D needed

Designing with inheritance (c) Eraj Basnayake

5

public abstractabstract class Shape

public abstract double getSurfaceArea()

public abstract Object getLocation()

public abstract Object clone()

public abstract String toString()

public abstract boolean equals()

Example hellip Abstract classes

Observe the syntax of Shape

The methods are not empty methods

Abstract ClassAbstract Class A class with at least oneat least one unimplemented method

InterfaceInterface A class with onlyonly unimplemented methods

Shape above is an example of an Abstract class

public interfaceinterface Shape

public double getSurfaceArea()

public Object getLocation()

public Object clone()

public String toString()

public boolean equals()

Designing with inheritance (c) Eraj Basnayake

6

Abstract Classesbull An abstract class is a placeholder

ndash It is too generic to be of use by itself ndash It is used in a class hierarchy to organize common features at appropriate

levels bull Abstract class cannot be instantiated

ndash However an abstract class variable can reference any concrete derived object

bull An abstract method has no implementation just a signaturendash It is never intended to be called directly (a placeholder)ndash Abstract methods can appear only in classes that themselves been

declared abstract bull The modifier abstract is used to define abstract classes amp methods

ndash The children of the abstract class are expected to define implementations for the abstract methods in ways appropriate for them

bull 1048715If a child class does not define all abstract methods of the parent then the child is also abstract

Designing with inheritance (c) Eraj Basnayake

7

Interfaces vs Abstract classes how to decide

Not all programming languages have Interfacersquos

1 unim

plem

ente

d

met

hod2

unimple

men

ted

met

hodAll

met

hods

unimple

men

ted

All m

ethods

imple

men

ted

Regular

Class

Abstract

classInterface

hellip

So why have abstract classes and interfaces

bullTo enforce design conformitybullClients can expect a uniform set of methods for related classesbullInterfacersquos are used to ldquotagrdquo a class with a property While interfaces

are classesare classes they represent ldquoaction ideasrdquo ldquoverbsrdquo or ldquopropertiesrdquo

about class

Designing with inheritance (c) Eraj Basnayake

8

InterfacesSample Interfacersquos

Cloneable Drawable Colorable Runable Comparable hellip

Q How do you inherit from an interface

A By implementrsquoing the interface

public interface Shape implements Cloneable

hellip

Q How do you inherit from an Abstract class

A The usual way by extendrsquoing

public abstract class Shape2D implements Shape

hellip

public class Circle extendsextends Shape2D

hellip

extends

Designing with inheritance (c) Eraj Basnayake

9

Its all still inheritance

public interface Shape implements Clonable

hellip

public abstract class Shape2D implements Shape

hellip

public class Circle extends Shape2D

Circle has to implement all unimplemented methods ofbull 2DShapebull Shapebull Clonable

Client Code

hellip

Circle c = new Circle()

hellip

Shape s = c

Cloneable cln = s

Shape2D s2d = (Shape2D)s

extends

Designing with inheritance (c) Eraj Basnayake

10

Its all still inheritance

Client Code

hellip

Circle c = new Circle()

hellip

Shape s = c

Cloneable cln = s

Shape2D s2d = (Shape2D) s

through c you can access ALL methods

of Circle AND its parents

Through s you can access ONLY methods

of Shape and Cloneable implemented in Circle

Through cln you can access ONLY methods

of Cloneable implemented in Circle

Through s2d you can access ONLY methods

of Shape Cloneable and Shape2D implemented

in Circle

Designing with inheritance (c) Eraj Basnayake

11

Multiple Inheritance

Some programming languages allow for multiple inheritance

In Java

Can have multiple interface implementation

Single inheritance

Types of possible inheritance

bullAn interface can inherit from multiple interfaces by implementing

bullA class (abstract or otherwise) can inherit from multiple interfaces by

implementing

bullA class (abstract or otherwise) can inherit from a single other (abstract or

normal) class

Designing with inheritance (c) Eraj Basnayake

12

A harder ExampleA Stack Machine is a device that lets you write simple programs that

use a Stack Items (in our example numbers) are placed on and removed

from the stack using two commands ldquopushrdquo to place on and ldquopoprdquo to take off

Items are always placed on top of the stack and taken off the top of the stack

A stack is a last-in-first-out structure

Arithmetic operations can be performed by assuming that the two operands

needed for the operation will be the top two items on the stack and the answer

will be placed back on the stack

push 5

5

Stack

push 10

5

Stack

1

0

add

15

Stack

Any mathematical expression can be converted into a series of machine instructions

5 + (6 (5 - 4))

push 5

push 4

subtract

push 6

multiply

push 5

add

print

Designing with inheritance (c) Eraj Basnayake

13

Examplehellip What are the classes

Clients viewpublic static void main(String[] args)

StackMachine sm = new StackMachine()

smload(ldquoprogramFileNamerdquo)

smrun()

StackMachineCoordinate

all operations

void load(String fileName)

void run()

ProgramLoaderIts purpose is to read the

program in

void load(String filename)

void String getInstruction(int i)

int numInstructions()

StackIts purpose is to

model the Stack

idea

void push(double value)

double pop()

Designing with inheritance (c) Eraj Basnayake

14

Examplehellip gaining an understanding

public class StackMachine

ProgramLoader pl

Stack stack

public StackMachine()

pl = new ProgramLoader()

stack = new Stack()

public void load(String fileName)

plload(fileName)

public void run()

String instruction

for(int i=0 iltplnumInstructions() i++)

instruction = plget(i)

Systemoutprintln(instruction)

examplestk

push 5

push 4

subtract

push 6

multiply

push 5

add

print

Client code

public static void main(String[] args)

StackMachine sm = new StackMachine()

smload(ldquoexamplestkrdquo)

smrun()

What happens here

Designing with inheritance (c) Eraj Basnayake

15

Examplehellip implementing the instructions - one solution

public class StackMachine

hellip

public void run()

String instruction

for(int i=0 iltplnumInstructions() i++)

instruction = plget(i)

StringTokenizer st = new StringTokenizer(instruction)

String command = stnextToken()

if (commandtoUpperCase(ldquoPUSHrdquo)

String parameter = stnextToken()

stackpush(DoubleparseDouble(parameter))

else if (commandtoUpperCase(ldquoPRINTrdquo)

double d = stackpop()

Systemoutprintln(d)

hellip

Messy long yuck hellip

Designing with inheritance (c) Eraj Basnayake

16

Examplehellip another way

Think of instruction as ldquoclassesrdquo and group theminstructions

push

pop

addsubtract

divide

multiply printone parameter

instruction no parameter

instruction

push

pop

add

subtract

dividemultiply

print

instructions

arithmetic instructions

stack

instructions

utility

instructions

Organized by

number of parameters

Organized by function

Designing with inheritance (c) Eraj Basnayake

17

Examplehellip another way Instruction

OneParameterInstruction OneParameterInstruction

push pop add multiply divide subtractprint

Instruction

StackInstruction ArithmeticInstruction UtilityInstruction

push pop add subtract divide multiply print

bull Each instruction will know how to ldquoexecuterdquo itselfbull Each instruction will know how to initialize itself

Designing with inheritance (c) Eraj Basnayake

18

Examplehellip another way

public interface Instruction

public void load(StringTokenizer)

public void execute(Stack)

public abstract class ArithmeticInstruction implements Instruction

public void load(StringTokenizer st)

public class Add extends ArithmeticInstruction

public void execute(Stack stack)

double operand1 = stackpop()

double operand2 = stackpop()

stackpush(operand1+operand2)

Quick Review

Whatrsquos going on

hellip

Add a = new Add()

Instruction i = a

iload(st)

iexecute(stack)

Designing with inheritance (c) Eraj Basnayake

19

Examplehellip another way public interface Instruction

public void load(StringTokenizer)

public void execute(Stack)

public abstract class StackInstruction implements Instruction

only here for design completeness

public class Pop extends StackInstruction

public void load(StringTokenizer st)

public void execute(Stack stack)

stackpop()

public class Push extends StackInstruction

double value

public void load(StringTokenizer st)

String parameter = stnextToken()

value = DoubleparseDouble(parameter)

public void execute(Stack stack)

stackpush(value)

Designing with inheritance (c) Eraj Basnayake

20

Examplehellip another way

Will this workpublic class StackMachine

hellip

public void run()

String instruction

for(int i=0 iltplnumInstructions() i++)

instruction = plget(i)

StringTokenizer st = new StringTokenizer(instruction)

String command = stnextToken()

if (commandtoUpperCase()equals(ldquoPUSHrdquo))

Push p = new Push()

pload(st)

pexecute(stack)

else if (commandtoUpperCase()equals(ldquoSUBTRACTrdquo))

Subtract p = new Subtract()

pload(st)

pexecute(stack)

hellip

common

code

Is it any better

public class StackMachine

hellip

public void run()

String instruction

for(int i=0 iltplnumInstructions() i++)

instruction = plget(i)

StringTokenizer st = new StringTokenizer(instruction)

String command = stnextToken()

if (commandtoUpperCase()equals(ldquoPUSHrdquo))

Push p = new Push()

pload(st)

pexecute(stack)

else if (commandtoUpperCase()equals(ldquoSUBTRACTrdquo))

Subtract p = new Subtract()

pload(st)

pexecute(stack)

hellip

public class StackMachine

hellip

public void run()

String instruction

for(int i=0 iltplnumInstructions() i++)

instruction = plget(i)

StringTokenizer st = new StringTokenizer(instruction)

String command = stnextToken()

if (commandtoUpperCase()equals(ldquoPUSHrdquo))

Push p = new Push()

pload(st)

pexecute(stack)

else if (commandtoUpperCase()equals(ldquoSUBTRACTrdquo))

Subtract p = new Subtract()

pload(st)

pexecute(stack)

hellip

Designing with inheritance (c) Eraj Basnayake

21

Examplehellip another way Will this work

public class StackMachine

hellip

public void run()

String instruction

for(int i=0 iltplnumInstructions() i++)

instruction = plget(i)

StringTokenizer st = new StringTokenizer(instruction)

String command = stnextToken()

if (commandtoUpperCase()equals(ldquoPUSHrdquo))

Push p = new Push()

else if (commandtoUpperCase()equals(ldquoSUBTRACTrdquo))

Subtract p = new Subtract()

else if hellip

hellip

pload(st)

pexecute(stack)

for

Common code

factored outWill this work

Why or why not

Designing with inheritance (c) Eraj Basnayake

22

Examplehellip another way Will this work

public class StackMachine hellip public void run() String instruction for(int i=0 iltplnumInstructions() i++)

instruction = plget(i)StringTokenizer st = new StringTokenizer(instruction)String command = stnextToken()

Instruction p if (commandtoUpperCase()equals(ldquoPUSHrdquo))

p = new Push() upcastelse if (commandtoUpperCase()equals(ldquoSUBTRACTrdquo))

p = new Subtract() upcasthellip

pload(st) method overriding

pexecute(stack) method overriding for

Designing with inheritance (c) Eraj Basnayake

23

Examplehellip another way public class StackMachine

hellip

public void run()

String instruction

for(int i=0 iltplnumInstructions() i++)

instruction = plget(i)

StringTokenizer st = new StringTokenizer(instruction)

String command = stnextToken()

try

Instruction instRef = (Instruction) (ClassforName(command))newInstance() instRefload(st)

instRefexecute(stack)

catch(Exception e)

Systemoutprintln(ldquoSyntax error - bad instructionrdquo)

Systemexit(0)

Client code

public static void main(String[] args)

StackMachine sm = new StackMachine()

smload(ldquoexamplestkrdquo)

smrun()

A really cool way

to do all that