Linear Recursive Data Structures Using ObjectOriented State Patterns

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Linear Recursive Data Structures Using
Object-Oriented State Patterns

• or
• Just how smart is a a null node anyway?

Stephen Wong, Oberlin College Dung Zung Nguyen,
Pepperdine University
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Squares and Rectangles

• Is a square a rectangle?
• YesNo..Aieeeeeee!..
• Two states of a rectangle?
• Square
• Non-Square

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Recursive Definition of Linear Structures (Lists)

• What is a LRS/list?
• Two different types of LRSs
• Empty
• Non-Empty
• Are the different types really the same?
• Do you care?

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Abstraction, a la Matisse
A Process through which superfluous details are
systematically removed to reveal the intrinsic
nature.
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Recursive Definition of Linear Structures (Lists)

• Abstract the structure!
• For non-empty LRS
• The data or First ( car)
• The tail or Rest ( cdr), which is a LRS!
• Abstract the behavior!
• Separate the Invariant vs. Variant Behaviors
• Get/Set methods for First and Rest.
• State change methods (insertAsFirst, remove).
• Other variant methods (queries, display, etc.)

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The Power of Abstraction

• Closest match between the reality of the
  situation and the software model.
• Helps control and understand the complexity of a
  large system.
• Creates robust, flexible and extensible code

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State Diagram for a LRS
Empty List
Non-Empty List
AddItem
Remove Item
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Rectangles and Lists?

• Systems that change states dynamically.
• Dynamic reclassification an object that behaves
  differently in different states as if it changes
  classes.
• Design solution for languages that do not support
  this feature?

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State Design Pattern

• Encapsulate states as concrete classes.
• Abstract them into an abstract class.
• The system contains a reference to an abstract
  state class, which can be an of the concrete
  state classes dynamically.

How can we express this?
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Unified Modeling Language

• A way of focusing on the the structure of a
  system of objects.
• Separates the interfaces from the
  implementations.
• Shows classes, methods, and attributes.
• Shows inheritance relationships and associations.

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UML Diagram of the State Design Pattern
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Invariant LRS Behaviors

• Get/Set First
• Get/Set Rest
• InsertAsFirst
• RemoveFirst

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Intelligent States

• The LRSs user doesnt necessarily care which
  state the LRS is in.
• Each case determines its own course.
• No conditional statements are necessary.
• The null (empty) case can handle itself.
• Emptiness has behavior and intelligence!

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Object-Oriented Data Structures
Represent the pure behavior of the structure of
the data, independent of the data itself.
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Whats wrong with this picture?

• Why are the variant behaviors coded together with
  the invariant behaviors?
• But arent the variant behaviors still the domain
  of the data structure object?
• But who can predict future behavioral needs?
• We need extensibility.
• We need to de-couple the variant algorithms from
  the data structure.

Stay tuned!!
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Venn Diagram of Rectangles?
Non Square
Square
Nahhh.
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The Yin-Yang of Rectangles
Square
Non Square
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Just how smart is a null node?
Just because you dont have any information
doesnt mean you arent smart enough to take
care of yourself!