Object Identity

Object-oriented programming and design

1

Object Identity

= vs. ==

copying objects

Value Object

ValueWithHistory


2

Identity“I’ll take the same thing he is having.”

Objects have identity. Their state changes, but their identity doesn’t change.

You can change the value of an object, but a 3 is always 3.


3

Object Protocol

Operations understood by all objects:

== anObject identity - same object

= anObjectequality - same value

~~ anObject different objects

~= anObject different values


4

Equality vs. Identity

Equality is user defined.

= anObject

^self == anObject

Identity is system defined.

== anObject

<primitive: 110>


5

Equality

(Date newDay: 40 year: 1995) ==

(Date newDay: 40 year: 1995)

is false, because they are physically two distinct objects. However, they represent the same value, so they are equal.


6

Implementation of Identity

Each object has its own region of memory.

“Object ID” is essentially a pointer.

Variable contains object ID, not the space of an object.

“Passing an object as an argument” means passing the object ID.

-- tests for pointers being equal.


7

New ObjectsA new object is different from any existing object.

X new == X new

is almost always false.

“Rectangle new = Rectangle new” is true.


8

Sharing

Invoice

Invoice

Invoice

Invoice

name “Ann Jones”

Oct. 3, 2005

Oct. 3, 2005

Oct. 4, 2005

“Ann Jones”

“Joe Smith”

name

name

date

name

date

date

date


9

Sharing

Sharing saves space.

Sharing makes changes to one object visible to another.

Sharing is always safe when objects are immutable.


10

What is the value of?

(Point x: 3 y: 17) == (Point x: 3 y: 17)

(Point x: 3 y: 17) = (Point x: 3 y: 17)

'this is a string' == 'this is a string'

#aSymbol == #aSymbol


11

Value Objects

Some objects (numbers, symbols, Military ranks) never change state.

A value object is an object that is used like a value. Initialize variables when it is created and

never change them afterwards. Define =


12

Value Objects

Classes that represent special values in your domain (SocialSecurityIdentifier, Address, MusicalNote, DayOfWeek, Money)

The only methods that assign to instance variables are initialization methods.

Only instance creation methods should call initialization methods.


13

Why Value Objects?

Reveal intention

Check consistency (dynamic type check)

Better printing / input

Disadvantages

More classes


14

Information Hiding

An employee’s transactions are entirely hidden from clients.

To add a transaction, use postTransaction:

There is no way to access transactions outside Employee.


15

Information Hiding

Suppose you want to iterate over transactions of an employee.

Alternative 1

1) Add #transactions method

2) anEmployee transactions do:


16

Violating Information Hiding

An accessing method discloses information.

anEmployee transactions add: (Paycheck new)

anEmployee transactions remove

Very dangerous!


17

Information Hiding

Alternative 2

transactionsDo: aBlock

transaction do: aBlock

Alternative 3

transactions

^transactions copy


18

Copying

"shallow copy" -- copy object, but not contents of object

"deep copy" -- copy object and contents of object, recursively

(usually VERY selectively)

Copying is usually shallow copying.


19

CopyingIn class Object:

copy

^self shallowCopy postCopy

Template Method pattern!

Redefine postCopy to change the way to copy variables, not copy.


20

OCollectionOCollection

transactionstransactions

OCollectionOCollection

TimecardTimecard

EmployeeEmployee

PaycheckPaycheckTimecardTimecard

transactionstransactions


21

| t s |

t := Point x: 1 y: 17.

s := t copy.

s x: 5.

s = t

| t s |

t := Point x: 1 y: 17.

s := t.

s x: 5.

s = t| t s |

t := Point x: 1 y: 17.

s := t copy.

s == t

2

3

1


22

Classic bug

aSet do: [:each | each isBold ifTrue: [aSet remove: each]]

Avoid modifying a collection when you are iterating over it.

aSet copy do: [:each | each isBold ifTrue: [aSet remove: each]]


23

Keeping Track of Time

Bug:

1. Forget to enter timecard

2. Raise salary

3. Notice that timecard is missing and enter it.


24


Problem: how do you ensure that each transaction is processed with rules in effect at the time it took place?


25


Things that are hard with current payroll design:

• Make a graph of salary paid per month for each person.

• Make a graph of vacation time taken per month for the entire company.


26

ValueWithHistory

I represent a (probably numerical) value that changes over time. I can answer my value at any point in time (usually a date). I can update my value at any point in time, and also add a number to my value from any point in time on into the future. My value does not change continuously, but changes at discrete points in time.


27

ValueWithHistory

Instead of storing a value in a variable, store it in a ValueWithHistory.

earnings := ValueWithHistory zero.

earnings at: today add: 50.

earnings starting: endOfYear become: 0.


28

ValueWithHistory protocol

at: aDate - return value at aDate

at: aDate add: anAmount - add anAmount to value at aDate and

at all times in the future

starting: aDate become: anAmount - set value from aDate to the next

specificed time to anAmount


29

ValueWithHistory variablesdate <SortedCollection of: Date>

value <OrderedCollection of: Number>

The i'th element of value matches the i'th element of date. The date collection indicates when the value takes on the next element of the value collection.


30

ValueWithHistory

initialize

date := SortedSequence new.

value := OrderedCollection new.


31

ValueWithHistory

at: aDate

"Return value at the date"

| index |

index := date indexOfStartOfIntervalContaining: aDate.

index = 0 ifTrue: [self error: 'date is too early'].

^value at: index


32

SortedSequence

SortedSequence is a subclass of SortedCollection with one method:

indexOfStartOfIntervalContaining: anElement

"Return the index of anElement or, if it is not present, of the index of the largest element smaller than it."


33

SortedSequence

indexOfStartOfIntervalContaining: anElement

self isEmpty ifTrue: [^0].

^(self indexForInserting: anElement) - firstIndex


34

ValueWithHistory

at: aDate add: anAmount | start |

start := (self indexForAccessing: aDate).

start

to: value size

do: [:each | value at: each put: (value at: each) + anAmount]


35

ValueWithHistoryindexForAccessing: aDate

"Return index of slot for aDate, creating it if necessary."

| index | index := date indexOfStartOfIntervalContaining: aDate.

index = 0 ifTrue: [date add: aDate. value addFirst: 0. ^1].

(date at: index) = aDate ifTrue: [^index].


36

(continued)

date add: aDate.

value add: (value at: index) beforeIndex: index + 1.

^index + 1


37

Collection methods

add: anElement

adds to end of OrderedCollection, to position in sort-order of SortedCollection, to random position in Set


38

OrderedCollection methods

addFirst: anElement

adds to beginning of OrderedCollection

add: anElement beforeIndex: anInteger

insert element at location, moving what is there


39

ValueWithHistory

starting: aDate become: aValue "Change value so that it becomes aValue at aDate."

| index |

index := self indexForAccessing: aDate.

value at: index put: aValue


40

Moral

It is often best not to make instance variable be simple object like number or string.

1) Make it be a domain object. (Money instead of Number) This usually is a “value object”.

2) Make it be a holder. (ValueWithHistory instead of Number or Money)

Next time

"A Laboratory For Teaching Object-Oriented Thinking"-by Beck, K, Cunningham, W.

http://c2.com/doc/oopsla89/paper.html


41

Documents

Object Identity