The Design of the
AP Computer Science
Marine Biology Case Study

Cay S. Horstmann

http://horstmann.com

San Jose State University

Presentation Outline

• Purpose of case study
• Case study history
• Design overview
• Kicking the tires
• Design problems
  
• Conclusion
  

Purpose of Case Study

• Study a significant body of code
• Modify and extend code
• Analysis and design
  
• Shared context for AP exam questions

Case Study History

1. Big integers (Pascal)
2. Big Integers (C++)
   Same case study, new language
   New concept: Class
   
3. Marine Biology (C++)
   New concept: Mutiple Classes
4. Marine Biology (Java)
   Same case study, new language
   New concept: Inheritance
5. ??? (Java)

Marine Biology (C++)

• Few classes:
  Fish
  Position
  Neighborhood
  Environment
  Simulation
  RandGen
  
• No inheritance
  (requires pointers in C++)

Marine Biology (Java)

• Lots of classes
  
• 3 Levels:
• public (10)
• black-box (7)
• secret (about 15)
  
• Interfaces (even for A exam)
• Inheritance (even for A exam)

A First Impression

• Run MBSGUI
• Include mbsbb.jar, mbsgui.jar
  Details depend on the environment!
• File->Open Environment File
• Pick fish.dat from DataFiles directory

A First Impression

Fish

• 20 public methods (C++: 8)
• Implements Locatable interface
• Subclasses DarterFish, SlowFish
  
• Coupled with:
  Location
  Direction
  Environment
  
  

Location

• Encapsulates (row, column)
• Like Position in C++

Locatable 

• An interface
• All objects in an environment must implement it
• In particular, Fish implements it
• One method:
  Location location()
  

Direction

• Encapsulates angle
• Key operations: turn, reverse
• Easy to make 90° turns
• Ok for 45° turns (8 neighbors)
• Ok for other turns (triangular/hexagonal cells)

Environment

• A big interface
• Implemented by BoundedEnv, UnboundedEnv
• Can add, move, remove Locatable objects
• Isotropic: all cell neighborhoods look alike
• Accesses neighbor cells
• Mediates between Location and Direction

BoundedEnv

• Your "starter environment"
  
• Implements Environment
• Extends black-box SquareEnvironment
• Not actually square, but the cells are squares
• 4 neighbors (not diagonal)

SimpleMBSDemo1

• 3 fish in a bounded environment
• loop:
  call act on fish
  display environment
• Uses secret SimpleMBSDisplay
• Suggestion: Demystify by supplying TextDisplay
  

Simulation

• Encapsulates act/display loop
• Only works for Fish! 
• Locatables don't act
  
• Needs EnvDisplay 
• That's a black-box interface with only secret implementations
• Consider TextDisplay
• Demoed by SimpleMBSDemo2
  

Debug

• Used in Fish and subclasses, Simulation
• Like standard Logger
  
• Black-box class
• Call static method turnOn to turn on debug messages

RandNumGenerator

• A singleton random number generator
• Better than instantiating multiple java.util.Random objects
• Can achieve the same effect by calling Math.random

MBSGUI

• Sets types with secret classes
• You need to edit this file to add more fish, environment types
• Reads environment files with secret classes
• Shows secret GUI frame
  

Tire Kicking #1: TextDisplay

• Tip: Bookmark JavaMBS/Documentation/index.html
• One method: showEnv
• Sigh: it doesn't pass the environment
  
  
• Remedy: Supply environment in constructor
• showEnv method loops through all locations
• Prints dot or first letter of toString

Tire Kicking #1: TextDisplay

• Sigh: need mbsgui.jar
• Modify SimpleMBSDemo1
• Replace SimpleMBSDisplay with TextDisplay
• TextDisplay.java
  

• ..........
  ..........
  ..........
  ..........
  ...2......
  ..........
  .........3
  .....1....
  ..........
  ..........
  ----------
  ..........
  ..........
  ..........
  ..........
  ..........
  ...2......
  ..........
  ......1..3
  ..........
  ..........
  ----------

Tire Kicking #2: BlueJ

• Not an IDE
• Great tool for exploring object 
• Follow directions at http://www.bluej.org/help/ap.html
  
• Construct BoundedEnv
• Construct Fish
• Need to supply new Location(2, 2)
• Explore the fish

Tire Kicking #2: BlueJ

Next Steps for Students

• Chapter 1: Understand fish behavior
• Chapter 2: Understand MBS classes
• Chapter 3: Add breeding, dying to act
• Chapter 4: Add DarterFish, SlowFish
• Chapter 5: Data structures for unbounded environments (AB only)
  

Tire Kicking #3: Web Fish

• Model/View: Separate data (model) from presentation (view)
• Upon web hit, compute next step, then return HTML
• Simulation combines step with display
• Must supply dummy display
• index.jsp | SimBean.java
• Environment file parser is secret :-(
  

Tire Kicking #3: Web Fish

Casting Caution to the Wind

• Strong typing is a good thing
• Students should learn to respect types
• Why casting to Fish in Simulation?
  
• Environment holds Locatable
• Why can't locatable objects act?

Spaghetti Relationships

• Each Fish has a Location
• Each Fish has an Environment
• Each Environment maps Locations to Fish
• Why do the fish need the environment and location?
• Do real fish store their location and environment?
  
• act could supply these as parameters
  

Spaghetti Relationships

Protected From Whom?

• Fish has complex protected interface
  

  protected Color randomColor()
  protected void move()
  protected Location nextLocation()
  protected ArrayList emptyNeighbors()
  protected void changeLocation(Location newLoc)
  protected void changeDirection(Direction newDir)

• Dynamic Fish:
  

  protected void breed()
  protected void move()
  protected void generateChild(Location loc)
  

• protected not part of AP Java subset
• UnboundedEnv: protected int indexOf(Location loc)
  

Random Thoughts

• RandNumGenerator just yields a java.util.Random
• Environment makes random directions
• Direction also makes (useless) random directions
  
• Fish makes random colors (!)
  

Too much Class?

• Do we really need Direction and SquareEnvironment?
• Do we really need Environment in A exam?
• Do we really need Debug?
• Do we really need RandNumGenerator?

Nitpicking about Names

• Environment
  
• Locatable[] allObjects()
  Shouldn't that be allLocatables?
• ArrayList neighborsOf(Location loc)
  Shouldn't that be Locatable[]?
• Locatable
• Location location()
  Shouldn't that be getLocation?
• RandNumGenerator, BoundedEnv
  Gratuitous abbrevs?
  

Conclusions

• Big jump from C++ version
• Hurdle on day 1: deal with JAR files
• Lots of classes (like in real life)
• Design not perfect (like in real life)
  Tell your students to do better
• Lots of secret stuff

Conclusions

• A: understand, modify classes
• Much improved over the past:
  closer to real programmer experience
  
• AB: data structures 
• Much improved over the past:
  more interesting context