More Smalltalk

1
More Smalltalk

• August 30, 2002
• CS 345

2
Objects and Messages

• Everything in Smalltalk is an object.
• You communicate with objects via messages.
• The receiver of a message responds via the
  execution of a method.
• ltobjectgt ltselector1gt arg1 ltselector2gt arg2
• myList at 1 put someObject

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Numbers

• Numbers are instances of class Number
• Respond to messages about arithmetic.
• , -, , /, work as expected.
• Left-to-right evaluation.
• // quotient (integer division)
• \\ remainder (modular division)
• Numbers understand a rich set of methods
• sin, abs, asRandom, asCharacter, bit
  manipulation, looping

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Iteration

• To do a for loop, send a message to a number.
• 1 to 10 do
• index anArray at index put (aValue
  index).
• 10 timesRepeat
• anArray addLast elt.
• code represents a block.
• Treats a chunk of statements as a single object.

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Strings

• Delimited by single quotes.
• s lt- hello there!
• s lt- String new.
• Strings have a rich set of methods
• Byte and character manipulation
• Comparison, matching, search
• Conversion
• Formatting

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Conditionals

• True and False are objects.
• Comparison methods return true or false.
• This can be used to do if and while.
• (a gt 12) ifTrue Transcript show True.
• ifFalse Transcript show False.
• index lt- 1.
• index lt anArray size whileTrue
• anArray at index put (index index).
• (this uses a Block to test the condition)

7
Program design

• Program design consists of constructing a set of
  objects and determining how they relate to each
  other.
• is-A relationship
• has-A relationship
• Typically, youll have an object or set of
  objects (e.g., a GUI) that implement a start or
  run method, triggering other objects.

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OO design

• Objects have attributes and services.
• Attributes instance variables
• Services methods.
• Design For each object, write out its
  responsibilities and its collaborators.
• Think about use cases how will other objects
  want to use this object?

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OO design tools

• Brainstorming
• CRC cards
• UML Unified Modeling Language
• Rational Rose www.rational.com
• Object Management Group www.omg.org/uml
• UML provides a powerful set of tools for
  designing classes and the relationships between
  them.

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OO design example

• Design a calculator
• User interface
• Engine