Don Slater

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Helping Students See Polymorphism

• Don Slater
• Lecturer, Carnegie Mellon University
• dslater_at_andrew.cmu.edu
• http//www.cs.cmu.edu/djslater

2
Polymorphism

• Means many shapes
• Sending the same message to different objects,
  and having them behave differently

3
Spiral Down Pedagogy and Karel

• Develop mental models with multiple passes at
  the material
• Originally emphasized program decomposition, top
  down design and control structures
• Dynamic polymorphism is the key idea to Karel
  and Karel J Robot
• Introducing Objects with Karel J. Robot
• Joe Bergin, ECOOP 2000

4
Using Visualization in Teaching CS

• So much in programming happens under the hood
• Experienced programmers have developed abstract
  models for understanding what is going on
• Inexperienced programmers need help in developing
  these models

5
Karel the Robot and me

• Published by Richard Pattis in 1981
• Pascal
• Joe Bergin, et. al in 1995 - 97 published Karel
• C
• Joe Bergin, 1999 (?), developed KarelJRobot
• Java

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The Robot World
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Robot Capabilities

• A Robot can move
• Forward in the direction it is facing,from corner
  to corner
• Turn in place
• Turn itself off
• A Robot can detect
• Walls that are 1/2 block in front of them
• Beepers on the same corner as the robot
• Other robots on the same corner with the robot
• A Robot can navigate by detecting the direction
  it is facing (north, south, east, west)
• A Robot can manipulate beepers
• by carrying them,
• picking them up, and putting them down,
• knowing if it is carrying any

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A Robot Task
Initial Situation
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A Robot Program

• If polymorphism is the key idea, then subclassing
  has to start right away
• The Robot base class is ur_Robot
• move() turnLeft()
• pickBeeper() putBeeper()
• turnOff()
• All Robots are subclasses of ur_Robot

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Robot Predicates

• The Karel J. Robot package contains a Robot class
• a sub-class of ur_Robot
• with predicate methods (return boolean)
• frontIsClear()
• nextToABeeper() nextToARobot()
  anyBeepersInBeeperBag()
• facingNorth() facingSouth() facingEast()
  facingWest()

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Two Step Robot

• Demonstrates polymorphism through inheritance and
  overwritten methods (move)
• First pass at overwriting an inherited method
• Use of the super reserved word
• illustration of the IS-A relationship
• TwoStepRobot is a Robot

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RacerRobot

• Demonstration of polymorphism with abstract
  classes

SteepleChase
HighHurdles
Hurdles
Sprint
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Similar, but not Identical Tasks

• Each robot races to the finish line, a beeper
• runRace()
• But different obstacles may be encountered along
  the way
• raceStride()

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Sprinter Class Relationships
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RacerRobot Abstract Class

• Implement the runRace() method
• Specify the abstract method raceStride() method

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Sprinter v. Hurdler

• A Sprinter race stride is substantially different
  than a Hurdler race stride
• raceStride is simply a move()
• subclass HurdleRobot realizes raceStride and then
  specifies up(), over(), and down() as behaviors a
  Hurdler robot must implement

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RacerInterface

• I have also generated an interface for RacerRobot
• RacerRobot implements raceStride in terms of
  running to the beeper
• If we wanted to build RacerRobots that ran for a
  specified distance, instead of the finish line
• then raceStride could be implemented differently

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Turtle and Hare Robots

• Let us work with a collection of racing Robots
• We will use the Racer Interface but now create a
  Turtle (slow and steady) a Hare, (move when the
  spirit moves it) when we realize raceStride()

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Ready to Race
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Turtle and Hare Class Relationships
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An array of Racers

• We have built an array of Racers, holding both
  HareRobots and TurtleRobots
• We cycle through the array, sending each Racer
  the raceStride() message
• Each will behave according to the nature of its
  class
• builds on the IS-A relationship we saw at the
  beginning

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The RandomWalker

• A random walk is a model built on mathematical
  and physical concepts that is used to explain how
  molecules move in a closed space, or as the basis
  for several mathematical models that predict
  stock market pricesOwen AstrachanA Computer
  Science Tapestry, Chapter 7McGraw Hill, New
  York, 1997
• We can also use random walks as a way to further
  our understanding of the use and development of
  classes and polymorphism

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RandomWalker

• The robot randomly moves East and West around its
  starting point
• The robot flips a coin to determine which
  direction to move
• It keeps track of how many steps it has taken
• It can report its distance from origin

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RandomWalker
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2-D Random Walker

• Now the robot may move North and South as well as
  East and West
• Still counts steps and reports distance

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2D Random Walker
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2-D RandomWalker

• We want to track multiple robots in
  two-dimensional space
• How to differentiate between multiple robots
• We can give a robot different colors
• We can change the visibility of a robot

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Multiple Robots in Karels World
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2-D RandomWalker

• A better Environment for tracking randomly moving
  objects exists in the Marine Biology Simulation
  of the Advanced Placement Computer Science Exam

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Marine Biology Simulation

• Developed by Alyce Brady, Chris Nevison, and
  Julie Zelinski
• A simulation using multiple interacting classes
  facilitating the study of a marine environment

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Multiple Fish in MBS Environment
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2-D RandomWalker

• We want to turn our Walker Robot into a Fishbot
  and move it the Marine Biology Simulation

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Walker in The MBS Environment
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Robot 2D Class Relationships
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An MBS Overview
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Extending from Fish

• What does inheritance get us?
• DarterFish will be a model for FishBot

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Karel to MBS

• By having our Robot extend Fish instead of
  MyRobot we can use our Robots in the MBS
  Environment with the flexibility and display
  advantages it provides

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Building a FishBot

• The mechanism for dropping our FishBot into the
  Environment of the MBS
• By becoming a subclass of Fish we inherit an
  Environment object
• Instead of keeping track of streets and avenues
  we need to have a Location object (also 2-D)
• We still have a Direction and Color (fortuitous)
• No beepers (at present)

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the move() method

• It is also fortuitous that Robot classes and the
  MBS use move() to advance the objects from one
  location to the next
• How hard will it be to take advantage of this to
  use inheritance and polymorphism to create our
  FishBot?

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Walker move()

• The robot is entirely responsible for navigating
  through its world
• The robot gets a random value
• The random value is used to rotate the robot to
  the appropriate direction
• If the robots path is clear, it will then change
  its position

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Fish move()

• A fish interacts with its environment in order
  to
• Find out what neighboring positions are available
  (the fish does get to select one of these
  neighboring positions)
• What direction it is facing
• Be displayed

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Why this difference?

• Robots are always going to work in a 2-D world
• In the MBS, an Environment can be implemented in
  many different ways
• hexagonal?
• A fish should not make assumptions about the
  world it lives in

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However...

• For our purposes, (demonstrating polymorphism,
  you thought I forgot) we will assume a 2D world
• The Environment provides us methods we can work
  with

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FishBot ctor

• Will need three different ctors to deal with
  different ways MBS builds Fish
• essentially cut and paste
• For parameters, change street and avenue to
  Location, numBeepers to Color, and add an
  environment
• The actual body of the ctor looks virtually the
  same

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FishBot nextLocation()

• In the Fish class, not in the Walker class
• A useful way to think about the behavior
• Most of the Walker move is here but
• instead of turning to face a different direction
  we build a new location
• Check for validity with Environment

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Fishbot move()

• Gets its own next location (assuming a 2D world)
• If not valid, will remain unchanged
• Tell the environment the new location
• Have the environment tell it what direction it is
  facing
• Updates number of steps

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Finally

• Some changes must be made to the MBSGUI class
• drives the simulation
• add a FishBot to the list of fish in the
  simulation

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Whats the Point?

• A demonstration of loose coupling (?) of classes
• A demonstration of the power of inheritance
• A steppingstone into the MBS
• identifies different components of MBS
• can compare and contrast different ways to
  implement a RandomWalk / MBS

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Credits

• Graphics - screen shots from
• KarelJRobot simulator
• BlueJ
• DarterFish class diagram copied from Alyce
  Bradys web site
• Sprinter classes derived from Karel examples
  and problems

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Documentation and References - Karel

• Joe Bergins Karel website
• http//csis.pace.edu/bergin/KarelJava2ed/KarelJ
  avaEdition.html
• http//www.cafepress.com/kareljrobot
• There are other excellent Karel implementations,
  simulators, and resources
• A Google search on Karel the Robot returns
  about 10,200 hits

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Documentation and References

• AP Central The official site for all Advanced
  Placement course information
• http//apcentral.collegeboard.com
• After registration, you have access to official
  MBS materials
• manuals, documentation, software
• There is access to a growing collection of
  resources, many related to the MBS

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Documentation and References - MBS

• Alyce Bradys MBS web site
• http//max.cs.kzoo.edu/AP/MBS/index.html
• The object diagrams are useful, source code for
  black box classes also available here
• Chris Nevison - Former APCS Chief Reader - the
  Unofficial APCS website
• http//cs.colgate.edu/APCS/Java/MBScasestudy/MBSca
  sestudy.htm
• Materials from training programs he has conducted