implementation [a&n 23 + supplement]

Implementation[A&N 23 + supplement]

2

Converting UML classes to Java

• Map association to attribute• Issues, how to map:

– association name– multiplicity– navigation direction– aggregation & composition– inheritance

Item-name : String+ getPrice() : int

Price+ getPrice () : int

has price

class Item { private name : String private price : Price public getPrice () …}

class Price { public getPrice () …}

0..1

p = new Price() a = new Item(“Ale”,p) p : Price

a : Itemname = “Ale”price =

Item(n,p) { name = n price = p}

3


Item-name : String+ getPrice() : int+ add(d : Discount)

Discount- val: int+ getValue() : int+ attach(x : Item)

has*

ordered, nonunique

[0..1]

discounts

class Item { private name : String private discounts : List<Discount> public Item(…) … public getPrice () … public add(d:Discount) …}

a = new Item(“Ale”) a.add(D) a.add(D)

a : Itemname = “Ale”discounts =

D : Discount

Q: how do you map e.g. [0..10] multiplicity ?

: List

4


Item-name : String+ getPrice() : int+ add(d : Discount)

Discount- val: int+ getValue() : int+ attach(x : Item)

has*

ordered, nonunique

[0..1]

discountsitem

Java has no direct support for bidirectional navigation.

class Discount { private val : int private item : Item public Discount(…) … public getValue() { return val } public attach(x:Item) …}

class Item { private name : String private discounts : List<Discount> public Item(…) … public getPrice () … public add(d:Discount) …}

attach(x) { item = x ; x.add(this) ; }

Q: so, how to implement if we change the multiplicity of “item” from [0..1] to 1 ?

5

Mapping aggregation & composition

• Make sure you maintain asymmetry• For composition, make sure you don’t leak containment

A B

A B

A

a1 : A

b : B

a2 : A

X

6

Mapping inheritance

• Java has single inheritance• Multiple inh. has been discussed• But inheritance is “static” in Java (if you make a

parent, you can’t turn it to a coordinator at the run time, vice versa) will be discussed more in Design Pattern

ParentnameaddChild(c)

CoordinatorreportMiss()

7

Converting use-case / sequence diagram

• Mapping use case mapping seq. diagram• How to map interactions sequence• “Interaction” a send a message m to b• Synchronous : sender waits for the receiver’s answer.

• Else asynchronous

• Synchronous send method call• Async more complicated

charge(p)

return arrow can also be implicit

charge(p)

ok

8

Mapping “sequencing” (of interactions)

: System b : Basket

getTotalPrice()

c : Customer

totprice

charge(totprice)

reset()

{ totprice > 0 }

{ b.items isEmpty() }

opt [ b.items notEmpty() ]

class System { buy() { basket = … customer = … if (basket.items.isEmpty()) throw … totprice = basket.getTotalPrice() assert totprice > 0 customer.charge(totprice) basket.reset() assert basket.items.isEmpty() }}

buy()

Notice the sync. sends.

: User

9

It is “design” … expect to fill details.

: System b : Basket

getTotalPrice()

i : Item

getPrice()

totprice

class Basket { items : List<Item> … getTotalPrice() { int s = 0 for (Item i : items) s += i.getPrice() return s }}

10

Asynchronous send

: System

register(email)

• use unique nr. to identify matching reply/send.• you need to implement a matching mechanism

mail(email)

Sometimes, it is not realistic to impose on synchronous msg exchanges; e.g. the response time is long, e.g. via email. Then you will have to implement differently...

: User

c : Customer<<create>>Cust(email)

{ registered }

class Customer { String email boolean registered

Customer(email) { this.email = email ; … unique = …. mail(email, unique) }

static mailReply(unique) { c = … find matching cust. c.activate() }

activate() { registered = true ; sys.add(this) }}

add(c)

11

Persistence

• Your business data is very important.• Persistence saving your business data– because your system can crash– because the data is too big to fit in memory

• You want to rely on dedicated persistence technology database– reliable– performance– support for query, rollback, concurrency

12

Typical architecture

Presentation(User Interface)

Business logic

database

persistence layer /data access management

Persistence

Customer Item*

buys

save, load, queryPersistence layer provides an abstract interface to save, load, and query objects to/from the database, that is independent from the specific database technology being used.

Name SureName Age

Bob Sponge 4

Patrick Star 4

Bob Star 40

13

Relational db

• Most popular, well proven technology• In mathematics, a relation R : A×B×C is a set of

“tuples” from A,B,C, e.g. :

{ (a1,b1,c1), (a2,b2,c2) }

• A “table” is a relation: A B Ca1 b1 c1

a2 b2 c2

14

Some db terms

• attribute• row, column• key, primary key• foreign key• query• joint

Prod ProdName Price

1 Mac 1000

2 Toshiba 800

3 HP 1000

Name SureName Age

Bob Sponge 4

Patrick Star 4

Bob Star 40

Customer

Product

TransactionName SureName TrNr Prod

Bob Sponge 0 1

Patrick Star 1 1

Bob Sponge 2 2

Name SureName TrNr Prod ProdName Price

Bob Sponge 0 1 Mac 1000

Bob Sponge 2 2 Toshiba 800

15

Mapping class to table

• You may need to add “shadow” attributes to support persistence– Object ID – Creation time– Update counter

CustomerNameEmail

OID Name Email creationTime UpdateCtr

10 Bob [email protected] … …11 Patrick [email protected] … …

Customer

OIDcreationTimeUpdateCtr

16

Mapping 1-to-many association

CustomerNameEmail

CityCodeName

* 1lives in

OID Name Email City

10 Bob [email protected] AMS

11 Patrick [email protected] UTR

OID Code Name

0 AMS Amsterdam

1 UTR Utrecht

Customer City

Via a foreign key on the “many”-side

17

Mapping many-to-many association

CustomerNameEmail

ItemNamePrice

* *buys

OID Name Email

10 Bob [email protected]

11 Patrick [email protected]

OID Name Price

0 Mac 1000

1 Toshiba 800

Customer Item

TransactionDate

Date Customer Item

… 10 0

… 10 1

Transaction

With an association table..

18

Mapping inheritance

ItemNamePrice

PCOpSys

OID OpSys

10 MacOS11 Vista12 Vista

OID Name Price1 USB 10 1010 iMac 100011 Toshiba 80012 HP 800

• map the entire inheritance hierarchy to a single table, or :• map each class to its own table

Item PC

19

Saving and loading objects

• When you “load” a customer x, do you also expect that all objects navigable from x to be loaded as well?

• Similarly, when you save x, do you need to also save all objects navigable from x?

• From OO perspective: yes.• Additional implementation work

“Bob”[email protected]

“Patrick”[email protected]

“Octo”[email protected]

ID Name Email

10 Bob [email protected] Patrick [email protected] Octo [email protected]

ID1 ID2

10 115 105 1111 5

20

Impedance mismatch

• Between your OO application and its relational db back-end.

• Objects’ “graph” structure do not map directly to tables.

• Issues:– how to save/load/query such a structure to/from tables?– what to do with concurrent access ? (you can potentially

pull a large graph of objects)• Available solutions:

– Semi-automating it with a mapping framework– Use an object-relational DB instead– Use an OO DB

21

Not used, future backup

22

Some guide lines..

• Maintenance can be quite expensive! Important contributing aspects:– Complexity hard to understand, buggy, cost to fix bugs, testing cost– Flexibility if bad, then changing or adding features can be a real pain

• Strategically towards those, important “variables” to balance:

• Tactically: design patterns, refactoring, coding style,…

• Encapsulation• Coupling• Cohesiveness• Inheritance

23

“Encapsulation”

• Meaning ….– Hiding some part of your data/functionalities– packing related functionalities into one unit, and exposing

only the needed one for public use.• Software built from encapsulated units is less

complex.• OO gives you powerful mechanism for encapsulation

24

Balancing…

• A cohesive class is easier to understand• Low coupling between classes contribute to less overall

complexity• But often you have to balance them…

Product - name : String - price : int // e.g. in euro+ getPrice(c : Currency) : Int

Currency- code : String+ covert(c : Currency)

1

will have to do the currency conversion himself, which is not really a job for a Product.

delegation…

25

Balancing…

• Inheritance let you reuse code (less maintenance), but you get implicit coupling with superclasses.

• Beware of deep inheritance trees

Programmer may overlook influence of a superclass that is far up in the tree.

Duration- start- end+ isValid()

MemberDiscount RegisteredCustomer

Discount- val+ getValue ()

26

Balancing…

• When coding it is convenient to be able to access an object’s operations at will, but this introduces coupling.

• Demeter “Law” (Holland 87): an object should only talk with its neighbors.

• More structural: you open up many attributes/operations as package-private. But then all classes in the package are coupled together! You may pay the price when you want to modify some of them.

Basket ItemgetPrice()

CustomergetPrice()

BasketgetPrice()

ItemgetPrice()

CustomergetPrice()

*

*

Meet Demeter, but in a way we trade off with Basket’s cohesiveness

27

Software metric

• Quantitative indicator of a certain aspect of your program, e.g. its complexity. – Can be calculated with a tool– Help you to decide whether or not to refactor– Useful as a strategic instrument

• Examples:– Line numbers– Counting coupling, inheritance depth etc– Halstead– Mc Cabe

28

Halstead

x = x + x ; x++ x = x + y ; z++

(alleen ter info)

Halstead approximates complexity in terms of effort to “read” the program:

Just for info

29

Mc Cabe

int P() { if (...) return 100 else return 0}

0

12

Control Flow Graph (CFG)

0

13

2

4

5

Mc Cabe approximates complexity in the numbers of “linearly independent” execution paths through the program.

Just for info

30

Metric for encapsulation etc for OO

• Chen & Lu, 1993; simple, to “count”:– Encapsulation– Coupling– Cohesiveness– Inheritance

• CL approximate encapsulation”of a class C by the sum of signature complexity of its public method.– more parameters more complex– complex typed parameter

increases complexity

Just for info

Type complexityvalue

boolean, int 0

double 2

object 6-9

31

Inheritance and cohesion

• CL approximate complexity due inheritance on a class C by the sum of:– # C’s methods (own + inh)– C’s inheritance distance– # C’s direct superclasses– # C’s subclasses

• CL approx. cohesion by counting the number of groups of “related” methods in C.– Put method with “similar” param-types in the same group:

add(discount), setName(name), remove(discount)

32

Tool support

32

33

Issues (in persistence)

• Mapping: how to map your objects to the back-end DB

• When you anticipate multiple users:– Performance• Optimizing your logical & physical data model

– Concurrency– Security

• We’ll discuss some of these• Related courses: db, distributed programming,

cryptography

34

Some db terms

• attribute• row, column• key, primary key• foreign key• query• joint

Prod ProdName Price

1 Mac 1000

2 Toshiba 800

3 HP 1000

Name SureName Age

Bob Sponge 4

Patrick Star 4

Bob Star 40

Customer

Product

TransactionName SureName TrNr Prod

Bob Sponge 0 1

Patrick Star 1 1

Bob Sponge 2 2

Name SureName TrNr Prod ProdName Price

Bob Sponge 0 1 Mac 1000

Patrick Star 1 1 Mac 1000

Bob Sponge 2 2 Toshiba 800

3 HP 1000

35

Map to normalized tables…

• What if we store our data like this?

• Several “normal forms” to eliminate data redundancy• sometimes “de-normalize” to make certain queries faster

ID Name Email TrNr Prod ProdName Price

1035 Bob [email protected] 1 1 Mac 1000

1123 Patrick [email protected] 2 2 Toshiba 800

1123 Patrick [email protected] 3 1 Mac 1000

3 HP 1000

we only need 1x table data redundancy wasting space + consistency problem have to deal with “anomalies”, e.g. what to do if we want to delete Patrick ? certain queries may be faster

36

Look at attributes dependencies

• Let A,B,C be attributes in a table T• A,B C = C “depends on” A,B = if the values of AB uniquely

determines the value of C• Let K = {A,B} be a “candidate” key of T ; C partially depends K

if it only depends on part of K redundancy.



1 1 Mac 1000



0 dummy 0 3 HP 1000

TrNr Prod is a candidate key here

not 2NF

37

So-called 2nd Normal Form

• Table T is 2NF if it contains no partial dependency wrt any candidate key.

• Transform the previous scheme:

• Make sure you don’t lose information!

TrNr Prod

1 1

2 2

2 1

0 3

Prod ProdName Price

1 Mac 1000

2 Toshiba 800

3 HP 1000


ID Name Email TrNr

1035 Bob [email protected] 1

1123 Patrick [email protected] 2

0 dummy 0

38

3rd Normal Form

• T is 3NF if it is 2NF, and every attribute A which is not part of a candidate key, must depend only on a candidate key.

ID Name Email TrNr




0 dummy 0

not 3NF

ID TrNr

1035 1

1123 2

1123 3

0 0

ID Name Email



0 dummy

3NF

39

Denormalization

• But what if you frequently query the customers names and emails of given transactions? you will need to join often

• To favor query speed : denormalize.

ID Name Email TrNr




0 dummy 0

not 3NF

ID TrNr

1035 1

1123 2

1123 3

0 0

ID Name Email



0 dummy

3NF

40

You may have seen this: entity Relationship (ER) model …

CustomerID {PK}NameEmail

ProductID {PK}NamePrice

*

ID Name Email



CustomerID Name Price

1 Mac 1000

2 Toshiba 800

Product

nr date CustID ItemID

1 ... 10 1

2 ... 11 1

2 ... 11 2

Purchase

purchasedate

nr

*

41

Typical architecture

Presentation(User Interface)

Business logic

database

persistence layer /data access management

Persistence

Customer Item*

buys

save, load, query


1035 Bob [email protected] 1 1 Mac 1000



3 HP 1000

Persistence layer provides an abstract interface to save, load, and query objects to/from the database, that is independent from the specific database technology being used.

42

Concurrency

• Each functionality invoked by a client may require data from certain tables, and update to some rows.

• Danger: collision. you need to lock your data!• Pessimistic locking safe, but does not perform• Optimistic locking

– Blow up the complexity of your persistence layer– Error prone

Appdb

A

clients

B

C

implementation [a&n 23 + supplement]

Documents