AP "Case Studies"

1
AP "Case Studies"

• A big program that all AP students are expected
  to understand
• Approximately 20 of the possible points on the
  AP exam will come from case study questions
• First Case Study introduced in 1994
• Directory Manager (Pascal)
• Big Int (C)
• Marine Biology (C)
• Marine Biology Simulation (Java)

2
The Story

• A CS student, Pat, gets a summer job working for
  marine biologists.
• Hired to enhance an existing program that
  simulates fish movement in a bounded environment.
• Needs to understand existing program
• Designs, codes, and tests modifications
• Occasionally Pat turns to an experienced
  programmer, Jamie, for help.
• Narrative is Pats report of summer job.

3
The Program
4
The Chapters

• Experiment with existing program (run it)
• Guided tour of the code by Jamie
• Add breeding and dying
• Add two new kinds of fish (inheritance)
• Provide alternative representations (unbounded
  environment, others)

5
Marine Biology Simulation

• Data for the simulation is stored in text files
• oneFish.dat
• bounded 7 5
• Fish 3 2 North
• "Bounded" environment with 7 rows and 5 columns
• Fish starts from Row 3 Column 2 (zero based)
  facing "North" (up)

6

• bounded 7 5
• Fish 3 2 North

7
manyFish.dat

• bounded 46 46
• Fish 0 0 North
• Fish 0 1 South
• Fish 0 3 East
• Fish 0 6 West
• Fish 0 10 North
• Fish 0 15 South
• Fish 0 21 East
• Fish 0 28 North
• Fish 0 36 South
• Fish 0 45 East
• Fish 1 0 North
• Fish 1 1 South
• and so on

8
There are two charts you will need to fill out
that keep track of fish movement
9
SimpleMBSDemo1.java

• public class SimpleMBSDemo1
• . . . .
• public static void main(String args)
• // Construct an empty environment and
  several fish in the context
• // of that environment.
• BoundedEnv env new BoundedEnv(ENV_ROWS,
  ENV_COLS)
• Fish f1 new Fish(env, new Location(2,
  2))
• Fish f2 new Fish(env, new Location(2,
  3))
• Fish f3 new Fish(env, new Location(5,
  8))
• // Construct an object that knows how to
  draw the environment with
• // a delay display the initial
  configuration of the environment.
• SimpleMBSDisplay display new
  SimpleMBSDisplay(env, DELAY)
• display.showEnv()
• // Run the simulation for the specified
  number of steps.
• for ( int i 0 i lt NUM_STEPS i )

10
SimpleMBSDemo2.java

• public class SimpleMBSDemo2
• private static final int ENV_ROWS 10
  // rows in environment
• private static final int ENV_COLS 10
  // columns in environment
• private static final int NUM_STEPS 15
  // number of timesteps
• private static final int DELAY 1000
  // delay in milliseconds
• public static void main(String args)
• // Construct an empty environment and
  several fish in the context
• // of that environment.
• BoundedEnv env new BoundedEnv(ENV_ROWS,
  ENV_COLS)
• Fish f1 new Fish(env, new Location(2,
  2))
• Fish f2 new Fish(env, new Location(2,
  3))
• Fish f3 new Fish(env, new Location(5,
  8))
• // Construct an object that knows how to
  draw the environment with
• // a delay.
• SimpleMBSDisplay display new
  SimpleMBSDisplay(env, DELAY)

11
Marine Biology Simulation

• The biologists think of the environment as a
  rectangular grid, with fish moving from cell to
  cell in the grid. Each cell contains zero or one
  fish.
• To model fish swimming in a bounded environment,
  the program has Fish objects and an Environment
  object.
• The purpose of the program is to simulate fish
  moving in the environment, so the program also
  has a Simulation object.
• There are other useful, but less important
  "utility classes."

12
The Simulation class

• Has a step method that executes a single timestep
• A timestep is where each fish has an opportunity
  to move once
• To have the fish continue to move, we would use a
  loop to drive the simulation
• //code from SimpleMBSDemo2.java
• for(int i 0 i lt NUM_STEPS i)
• sim.step()

13
The Simulation class is pretty simple

• public class Simulation
• private Environment theEnv
• private EnvDisplay theDisplay
• public Simulation(Environment env, EnvDisplay
  display)
• theEnv env
• theDisplay display
• public void step()
• Locatable theFishes theEnv.allObjects()
• for ( int index 0 index lt theFishes.length
  index )
• ((Fish)theFishesindex).act()
• theDisplay.showEnv()

14
One step in the simulation
15
The Environment interface

• public interface Environment
• Direction randomDirection()
• Direction getDirection(Location fromLoc,
  Location toLoc)
• Location getNeighbor(Location fromLoc,
  Direction compassDir)
• ArrayList neighborsOf(Location ofLoc)
• int numObjects()
• Locatable allObjects()
• boolean isEmpty(Location loc)
• Locatable objectAt(Location loc)
• void add(Locatable obj)
• void remove(Locatable obj)
• void recordMove(Locatable obj, Location
  oldLoc)

16
The BoundedEnv class

• a class that implements the Environment interface
• BoundedEnv env
• new BoundedEnv(ENV_ROWS, ENV_COLS)

17
The Direction class

• Contains constants for the directions
• Direction.NORTH
• Direction.SOUTH
• //and so on. . .
• and functions to find relative directions
• Direction toRight(int degrees)
• Direction toLeft(int degrees)
• Direction reverse()
• boolean equals(Object other)
• String toString()
• What would this be?
• Direction.NORTH.toRight(90)

18
The Location class

• Stores a location with a row and column
• Location(int row, int col)
• int row()
• int col()
• boolean equals(Object other)
• String toString()
• What would this display?
• Location one new Location(4,5)
• Location two new Location(3,5)
• System.out.println(one)
• System.out.println(two.col()one.col())
• System.out.println(two.equals(one))

19
What is the output of this program fragment?

• BoundedEnv env new BoundedEnv(10, 10)
• Location loc1 new Location(2,3)
• Location loc2 new Location(5,3)
• Direction dir1 env.getDirection(loc1,loc2)
• System.out.println(dir1)
• Direction dir2 dir1.toLeft(90)
• System.out.println(dir2)
• Direction dir3 dir2.reverse()
• System.out.println(dir3)
• Location loc3 env.getNeighbor(loc1,dir1)
• System.out.println(loc3)
• Location loc4 env.getNeighbor(
• new
  Location(5,2),dir3)
• System.out.println(loc4)

20
The Fish class

• Fish f1 new Fish(env, new Location(2, 2))
• Fish f2 new Fish(env, new Location(2, 3))
• Fish f3 new Fish(env, new Location(5, 8))
• Each fish has a Location
• All fish share a reference (handle) to their
  environment

21
The Fish class constructors

• public Fish(Environment env, Location loc)
• public Fish(Environment env, Location loc,
  Direction dir)
• public Fish(Environment env, Location loc,
  Direction dir, Color col)
• //helper methods
• private void initialize(Environment env, Location
  loc, Direction dir, Color col)
• protected Color randomColor()

22
The Fish class "accessors"

• public int id()
• public Environment environment()
• public Color color()
• public Location location()
• public Direction direction()
• public boolean toString()

23
The Fish class "modifiers"

• public void act()
• //helper methods
• protected void move()
• protected Location nextLocation()
• protected ArrayList emptyNeighbors()
• protected void changeLocation(Location newLoc)
• protected void changeDirection(Direction newDir)

24
What is the output of this program?

• public class WhatOutput
• public static void main(String args)
• BoundedEnv env new BoundedEnv(5, 4)
• Fish nemo new Fish(env,new
  Location(3,2))
• Fish dory new Fish(env,new
  Location(2,3))
• int nRow nemo.location().row()
• int nCol nemo.location().col()
• nCol
• nemo.changeLocation(new
  Location(nRow,nCol))
• nRow dory.location().row()
• nCol dory.location().col()
• nRow--
• dory.changeLocation(new
  Location(nRow,nCol))
• System.out.println(nemo.location())
• System.out.println(dory.location())

25
What is the output of this program?

• public class WhatOutput
• public static void main(String args)
• BoundedEnv env new BoundedEnv(10, 10)
• Location loc1 new Location(2,3)
• Location loc2 new Location(5,3)
• Location loc3 new Location(3,3)
• Direction dir1 env.getDirection(loc1,lo
  c2)
• Location loc4 env.getNeighbor(loc1,dir1
  )
• System.out.println(loc4)
• System.out.println(loc3 loc4)
• System.out.println(loc3.equals(loc4))
• System.out.println(env.neighborsOf(loc1))
  
• System.out.println(env.neighborsOf(loc1)
• 
  .contains(loc4))

26
Reading the MBSCS documentation

• The previous program uses three methods from the
  Environment interface
• getDirection()
• getNeighbor()
• neighborsOf()
• One method from the Location class
• equals()
• One method from the ArrayList class
• contains()

27
toString()

• There are "invisible" calls to the toString()
  method for the Location class
• System.out.println(loc4)
• //is the same as
• System.out.println(loc4.toString())

28
toString()

• Appendix C "Black Box Classes" has the
  documentation for the Location class
• public String toString()
• Returns a string indicating the row and column of
  the location in (row,col) format

29
The Environment documentation

• You can find the documentation for the
  Environment interface in the Java code, but it's
  a little hard to read
• Direction getDirection(Location fromLoc, Location
  toLoc)
• / Returns the adjacent neighbor (whether valid
  or invalid) of a location in the specified
  direction.
• _at_param fromLoc starting location for
  search
• _at_param compassDir direction in which to
  look for adjacent neighbor
• _at_return neighbor of ltcodegtfromLoclt/codegt in
  given direction /

30
The Environment documentation

• You can create documentation that is easier to
  read using Java's "JavaDoc" feature. In RealJ
• With your MarineBiologySimulation project open
  choose ToolsJavaDoc Project Files
• Select the folder where you want the
  documentation to be stored
• Open the documentation in a browser like Netscape
  or Explorer

31
The Environment documentation
32
The Fish class

• Three constructors
• public Fish(Environment env, Location loc)
• public Fish(Environment env, Location loc,
  Direction dir)
• public Fish(Environment env, Location lo,
  Direction dir, Color col)
• Rather than duplicating code, each constructor
  calls a private method
• private void initialize(Environment env, Location
  loc, Direction dir, Color col)

33
The Fish class

• public Fish(Environment env, Location loc)
• initialize(env, loc, env.randomDirection(),
  randomColor())
• If the direction and color aren't specified,
  methods are used to generate random values

34
The Fish class IDs

• Every Fish has a unique ID
• private static int nextAvailableID 1
• Since it is static, all fish share a single ID
  "mailbox"
• private void initialize(Environment env, Location
  loc, Direction dir, Color col)
• theEnv env
• myId nextAvailableID
• nextAvailableID
• myLoc loc
• myDir dir
• myColor col
• theEnv.add(this)
• // object is at location myLoc in environment
  

35
The Fish class

• When a Fish is constructed, it is immediately
  added to the Environment (no fish out of water!)
• private void initialize(Environment env, Location
  loc, Direction dir, Color col)
• theEnv env
• myId nextAvailableID
• nextAvailableID
• myLoc loc
• myDir dir
• myColor col
• theEnv.add(this)
• // object is at location myLoc in environment
  

36
super()

• Refers to the Parent class
• Here, Person is the parent of ProWrestler
• class Person
• private String myName
• public Person(String sName)myName sName
• public String toString()return myName
• class ProWrestler extends Person

37
super()

• What do ProWrestler's have that normal people
  don't? A gimmick!
• class ProWrestler extends Person
• private String myGimmick
• public ProWrestler(String sName,
• String sGimmick)
• super(sName)
• myGimmick sGimmick
• public String getGimmick()return myGimmick
• public String toString()
• return super.toString() ", " myGimmick

38
super()

• class ProWrestler extends Person
• private String myGimmick
• public ProWrestler(String sName,
• String sGimmick)
• super(sName)
• //calls parent constructor
• myGimmick sGimmick
• public String getGimmick()return myGimmick
• public String toString()
• return super.toString() ", " myGimmick

39
super()

• class ProWrestler extends Person
• private String myGimmick
• public ProWrestler(String sName,
• String sGimmick)
• super(sName)
• //calls parent constructor
• myGimmick sGimmick
• public String getGimmick()return myGimmick
• public String toString()
• return super.toString() ", " myGimmick
• //calls the toString method of the
• //parent class

40
super()

• public class SuperDemo
• public static void main(String args)
• ProWrestler theWorst new ProWrestler
• ("Yeti", "Giant guy covered in toilet
  paper")
• System.out.println(theWorst)

41

• public class superWhatOutput
• public static void main(String args)
• Thingy aBunch new Thingy5
• for(int nI 0 nI lt aBunch.length nI)
• if(nI 2 0)
• aBunchnI new Thingy(nI)
• else
• aBunchnI new Widget(nI,'x')
• for(int nI 0 nI lt aBunch.length nI)
• aBunchnI.write()
• class Thingy
• private int myNum
• public Thingy(int nNum)myNum nNum

• class Widget extends Thingy
• private char myLetter
• public Widget(int nNum, char cLetter)
• super(nNum)
• myLetter cLetter
• public String toString()
• return myLetter super.toString()

42
Instance variables vs. Class variables

• class SomeClass
• double myDouble
• static String myString
• myDouble is an "instance variable" every
  instance of SomeClass has one
• myString is a "class variable" all instances of
  SomeClass share the same String "mailbox"