23 - Smalltalk

23 - Smalltalk#

Smalltalk Rules#

  1. Everything is an object.

  2. Every object is an instance of a class.

  3. Every class has a superclass.

  4. Everything happens by sending messages.

  5. Method lookup follows the inheritance chain.

  6. Every class is an instance of a metaclass.

  7. The metaclass hierarchy parallels the class hierarchy.

  8. Every metaclass inherits from Class and Behavior.

  9. Every metaclass is an instance of Metaclass.

  10. The metaclass of Metaclass is an instance of Metaclass.

../../_images/class-metaclass.png

OOP in Smalltalk#

Creating a Class#

Object subclass: Account [
    | balance |
    <comment:
        'I represent a place to deposit and withdraw money'>
]

Add a class method#

Account class extend [
    new [
        | r |
        <category: 'instance creation'>
        r := super new.
        r init.
        ^r
    ]
]

Add an instance method#

Account extend [
    init [
        <category: 'initialization'>
        balance := 0
    ]
]

Account extend [
    printOn: stream [
        <category: 'printing'>
        super printOn: stream.
        stream nextPutAll: 'with balance: '.
        balance printOn: stream
    ]
]

Account extend [
    spend: amount [
        <category: 'moving money'>
        balance := balance - amount
    ]
    deposit: amount [
        <category: 'moving money'>
        balance := balance + amount
    ]
]

Put everything together and testing#

See account.st. You can use gst account.st - for testing, or,

gst
st> FileStream fileIn: 'account.st'

Creating a subclass#

Account subclass: Savings [
    | interest |
    init [
        <category: 'initialization'>
        interest := 0
        ^super init
    ]
    interest: amount [
        interest := interest + amount
        self deposit: amount
    ]
    clearInterest [
        | oldinterest |
        oldinterest := interest.
        interest := 0.
        ^oldinterest
    ]
]

Account subclass: Checking [
    | checknum checksleft history |
]

Method with a Block#

checksOver: amount do: aBlock [
    history keysAndValuesDo: [:key :value |
        (value > amount)
            ifTrue: [aBlock value: key]
    ]
]

checksOver: amount do: aBlock [
    | chosen |
    chosen := history select: [:amt | amt > amount].
    chosen keysDo: aBlock
]

Testing#

See test.st.

mycheck := Checking new.
mycheck deposit: 250.
mycheck newChecks: 100 count: 40.
mycheck writeCheck: 10.
mycheck writeCheck: 52.
mycheck writeCheck: 15.
mycheck printChecks.
(mycheck check: 101) printNl. 
mycheck checksOver: 1 do: [:x | x printNl].
mycheck checksOver: 17 do: [:x | x printNl].
mycheck checksOver: 200 do: [:x | x printNl].
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