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1Ohjelmistoarkkitehtuurit Syksy 2006 TTY Ohjelmistotekniikka

3.1 The component vision: rationalizing software development

3.2 The component concept

3.3 Components as software units

3.4 Interfaces

3.5 Specifying interfaces

3.6 Customizing components

3. Software components and interfaces3. Software components and interfaces


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3.1 The component vision: rationalizing software3.1 The component vision: rationalizing software

productionproduction

Developing software products from existing components

products become more realiable

easier to construct products easier to train people

component competition reduces prices

More or less standard practice in other engineering areas


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Exercise: Write down 5 characteristicExercise: Write down 5 characteristic

properties of a good software componentproperties of a good software component


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Composing applications from componentsComposing applications from components

Composing new applications from existing componentsVisual component integration tools


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Example: BeanBoxExample: BeanBox


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Component = independent unit of software providingservices through a well-defined interface

Szyperski:A software component is a unit of composition with contractuallyspecified interfaces and explicit context dependencies only.

A software component can be deployed independently and issubject to composition by third parties.

3.2 The component concept3.2 The component concept


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Degree of independence

Deployment

Size

Standardization

Technology-specific properties

Issues related to componentsIssues related to components


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3.3 Components as software units3.3 Components as software units

Units of architecture

Units of functionality who is responsible of functionalites?

Units of reuse what remains common?

Units of product configuration what is included in a product?

Units of deployment what can be upgraded in isolation?

Units of variance what can be changed, how?

Units of third-party software what can be obtained from outside?

Units of work division what can be separated as tasks?


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3.4 Interfaces3.4 Interfaces

Subroutine library

Module library

Class library

M

A

B

C

A

B

C

A

C

B

A

C

B

X

Y Z

History:

Abstract servicesignature

Service

implementation

module

class

interface

subroutine

data

datadata


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Interfaces as firstInterfaces as first--class entitiesclass entities

ComponentA

ComponentB

Interface1

Interface2

Interface3

Interface4


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Provided and required interfaces in UML 2.0Provided and required interfaces in UML 2.0

Car

Engine

Car Engine

PowerSource

PowerSource

PowerSource

required interface

provided interface


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Possible implementation in Java:Possible implementation in Java:

interface PowerSource {

void start();

int temperature();

void stop();

}

class Engine implements PowerSource ... {...

public void start() {...}

public int temperature() {...}

public void stop() {...}

}

class Car ... {

...

private PowerSource eng;

...

public void setEng(PowerSource e) {

eng = e;

}

public void run() {...

eng.start();

...

eng.stop();...

}

}


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Ports in UML 2.0Ports in UML 2.0

Point of interaction between an object (component) and

its environment

Engine

PowerSource

Warnings

Engine

PowerSource

Warnings

host

Port


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Client Serviceprovider

service interface

postcondition: must hold,when the service has been delivered,assuming that the precondition heldwhen the service was called

precondition: must hold,when the service is called

3.5 Specifying interfaces:3.5 Specifying interfaces: contractcontractss

defines the abstract semantics of the service allows exchange of participants,

as long as the interface and its pre-and postconditions hold

defines the responsibilities of checking(no multiple checking, clear separation of

concerns)

design-by-contract(Eiffel)


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Class/component invariantsClass/component invariants

Person

op1(...)

op2(...)op3(...)

invariantage > 0 &age < toberetired

age: inttoberetired: int


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Inheritance & contractsInheritance & contracts

public oper(...)prepost

public oper(...)pre'post'

pre post

pre' post'

pre'

pre post

post'

pre post'

pre' post

pre'

pre post'

post

A

B

C

D

Which is correct (A,B,C,D)?


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Design by contract: example (1)Design by contract: example (1)

class Stack {private Object [] storage;private int top;

public Stack(int size) {storage = new Object[size];top = -1;

}

public int size() {return top+1}public boolean full() {return top == storage.length;}public void push(Object x) {

if (top < storage.length-1) {

storage[++top] = x;

}else {

System.out.println("Overflow");}

}


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public Object pop() {

if (top >= 0) {return storage[top--];

}else {

System.out.println("Underflow");return null;

}}

}

client: ... if s.size() > 0 then {s.pop();} else {... error handling ...} ...

Problems?


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interface Stack {Object pop()

//pre size() > 0;//post size = old size 1;

}

class ArrayStack implements Stack {public Object pop() {

return storage[top--];}

}

Prove that the pre- and postconditions hold (?)Produce run-time checks that the pre- and postconditions hold (?)

Idealisticsolution:


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Pragmatic approach: exceptionsPragmatic approach: exceptions

public Object pop() throws Underflow {// pre: stack non-empty// post: stack size reduced by one// Underflow: stack is empty upon entry

if (top >= 0) {return storage[top--];}else {

throw new Underflow(this);}

}

exception = contractor is unable to fulfill its part of the contract method either fulfills the contract or throws an exception implies that the service provider is able to detect the unableness

exceptions become part of the interface pre- and postconditions can be given informally


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3.6 Customizing components3.6 Customizing components

Changing the initial state of a component

Implementing required interfaces

Subclassing


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Changing the initial state of a componentChanging the initial state of a component

Comp

ServicesIsetProperty(...)

:CompClient

create

setProperty(...)

use


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In Java:In Java:

class Button extends Component ... {

...

String label;

public Button() {...

}

public Button(String arg) {

label = arg;...

}

public setLabel(String arg) {

label = arg;

}

...

}


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Implementing required interfacesImplementing required interfaces

Button AppLogic

ActionListener


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SubclassingSubclassing

Button

CompanyButton


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In Java:In Java:

class CompanyButton extends Button {

public CompanyButton(String arg) {super(arg);

label = arg.toUpperCase();

}

public void setLabel(String arg) {

label = arg.toUpperCase();

}}


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Customizedcomponent

Fragile base class problemFragile base class problem

"Innocent" modification ofthe original component

can make the customizedcomponent invalid

Comp

Custom

Library

component

E l (1)E l (1)


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Example (1)Example (1)

List

CountedList

addContainer:

{ ...addElement(...); ... }

addElement:

{ super.addElement(...);counter++; }

addElement(Object)

addContainer(Container)

Container

getCount()

CountedContainer


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Example (2)Example (2)

List

CountedList

addContainer:

{ ...}

addElement:

{ super.addElement(...);counter++; }

addElement(Object)

addContainer(Container)

Container

getCount()

CountedContainer

addElementno more called!

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