Recitation 7

1
Recitation 7

• Godfrey Tan
• March 21, 2002

2
Administrivia

• PS6 due Tuesday right after break
• Final Project descriptions will be handed out
  Monday after break
• Each group must agree on the common project
• If you are not satisfied with your group
  assignment, you should complain to me by the end
  of recitation.

3
Final Project

• Choice of two projects Gizmoball, Anti-chess
• Equal difficulty/amount of work
• Each TA will provide help on one of two projects
• A spec amendment will be introduced halfway
  through your project

4
Gizmoball

• Ball bounced around by gizmos (demo)
• Physics package will be provided
• can be extended (e.g. magnetic field, relativity)
• Open-ended goal
• pin-ball, visual effect, physics simulation, etc.
• GUI/graphics intensive
• Competition?

5
Antichess

• Like Chess but the winning objective is to lose
  all your pieces
• Involves chess board design, AI player, referee
• Algorithmically intensive
• Anti-chess tournament

6
Choosing Your Project

• Preliminary descriptions will be posted on April
  1
• People without a group should indicate their
  preference by this afternoon to help us assign
  groups
• Email will be sent to the class soon

7
True Subtyping

• Substitution priniciple
• Subtypes must be substitutable for supertypes
• The rep invariants and specification constraints
  of the supertypes must not be violated by the
  subtypes
• Subtypes may have more methods and properties so
  long as they abide by the substitution principle.
  

8
Subclassing in Java

• Pure implementation inheritance
• Does not worry about the behavioral constraints

9
Problems with subclassing

• Break encapsulation
• May lead to abuse sub-classing things that are
  not subtypes (e.g. Stack extends Vector)
• Should only be used when a true sub-type
  relationship exists

10
Example

• class IHashSet extends HashSet
• private int addCount
• public void add(Object o)
• addCount
• super.add(o)
• public void addAll(Collection c)
• addCount c.size()
• super.addAll(c)

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Example

• class IHashSet extends HashSet
• private int addCount
• // _at_modifies addCount
• public void add(Object o)
• addCount
• super.add(o)

12
Composition and Forwarding

• Composition A class that contains another class
  (a wrapper)
• Forwarding A class that calls methods of the
  other class that it contains
• No longer a subtype rather, it is a different
  implementation of the same interface
• IHashSet extends Set vs IHashSet implements Set
• Gives programmer more control with method
  dispatching (vs Javas default)
• Drawback more code less efficient (added level
  of indirection)

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Example

• class IHashSet implements Set
• private HashSet s
• private int addCount
• public void add(Object o)
• addCount
• s.add(o)
• public void addAll(Collection c)
• addCount c.size()
• s.addAll(c)

14
pset 6

• Design, implement GUI and then integrate it with
  your backend
• Testing GUI
• Use state machine to model GUI events
• Decouple GUI testing from backend testing
• Insert println statements before and after you
  call backend API method
• or, create a dumb backend stub

15
State Machine

• Directed graph nodes represent a state and arcs
  represent a transition (action) caused by an
  input (event)

Portfolio Name Input
Add PortfolioButton Pressed
Initial
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Testing

• Excite all transitions with all boundary inputs
  (e.g., adding a Portfolio with new name, with an
  existing name, etc.)
• GUI should handle exceptional inputs (e.g.
  entering alphabets instead of numbers for number
  of stocks to purchase)

17
Integration Testing

• Repeat your GUI test (using the same input values
  from your backend tests if possible), and check
  if your program still works!
• Hopefully, you will have repChk() in your backend
  to catch your bugs.