Problem Solving with Data Structures using Java: A Multimedia Approach

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Problem Solving with Data Structures using Java
A Multimedia Approach

• Chapter 14 Using an Existing Simulation Package

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Chapter Objectives
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Simulations

• A simulation is a representation of a system of
  objects in a real or fantasy world.The purpose
  of creating a computer simulation is to provide a
  framework in which to understand the simulated
  situation, for example, to understand the
  behavior of a waiting line, the workload of
  clerks, or the timeliness of service to
  customers.A computer simulation makes it
  possible to collect statistics about these
  situations, and to test out new ideas about their
  organization.
• Adele Goldberg David Robson, Smalltalk-80 The
  Language and Its Implementation (Addison-Wesley,
  1989)

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Simulations and Objects

• Object-oriented programming was invented, in
  part, to make simulations easier to build!
• The characteristics of objects make them more
  like real world objects, e.g.,
• Each thing knows some stuff and knows how to do
  some stuff.
• Objects get things done by asking each other to
  do things.
• Your internals are private, unless you want to
  make them otherwise.

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Continuous vs. Discrete Simulations

• Two main kinds of simulations in the world.
• Continuous Each moment of time is simulated.
• When every moment counts.
• Discrete Skip to the important moments.
• Want to simulate 100 years?

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Actors or Agents Those that act in the
simulations

• Actors or Agents do things, take time, and
  request and use resources.
• In continuous simulations, actors are told to
  act().
• In discrete event simulations, actors do
  something, then reschedule themselves in the
  simulation.

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Resources

• Resources are points of coordination in a
  simulation.
• Examples A cashier, a library book, a parking
  space on a ferry, a jelly bean.
• Some resources are fixed and others are produced
  and consumed.
• Some resources are renewable and shared.
• Others are coordinated.
• Example For a surgeon to do a surgery, the
  patient must meet the surgeon at the operating
  table (the resource)

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When an object has to wait

• What happens if you (or your proxy object) need a
  resource and its not available?
• You wait in a queue
• A list that is first-in-first-out (FIFO)

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A simulation is an executed model

• Setting up a simulation is a process of modeling
  the world (real or fantasy) to be simulated.
• That model is realized in terms of objects.
• We want our model to
• Reflect the world.
• Be easy to extend and change.
• Some of our modeling techniques
• Aggregation
• Generalization and specialization

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Aggregation

• Some objects are made up of other objects.
• Cars have engines
• People have livers and lungs
• These internal things are objects, too!
• Livers dont directly mess with the innards of
  lungs!
• We call this aggregation
• Putting references to some objects inside of
  other objects.

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Generalization and Specialization

• There are general and specialized forms of real
  world objects.
• Cells are biological objects that have membranes
  and a nucleus and mitochondria and
• Blood, lung, and liver cells are all cells but
  have specialized functions.
• The superclass-subclass relationship is a way of
  modeling general forms of objects and specialized
  forms of objects

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Start Greenfoot

• Click on the Greenfoot icon to start it
• The first time it will ask if you want to do the
  tutorial
• After that it will open the last scenario
• Click on Scenario and open to open another one
• Like balloons
• Like wombat

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Objects and Classes

• Classes define what objects know and can do
• There is one Balloon class
• Objects do the action
• There can be many objects of the same class
• There are many balloon objects

Classes
Objects
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Worlds and Actors

• Greenfoot has two main classes
• World
• Place to hold and display actors
• Has a width and height and cell size
• Of type integer (int)
• Has a background image
• Actor
• Actors know how to act
• Actors have a x and y location in the world

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Balloon Code and Doc

• Double-click on the Balloon class
• Select Documentation on the right
• You can switch between the source code and
  documentation

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Documentation

• Starts with the class name
• Should be an uppercase letter
• Then the superclasses
• Then the class definition
• And a short description
• Author
• Constructors
• Methods

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Methods

• Provide behavior for objects
• Balloons can act and pop
• public is the visibility
• Who can see and execute this method
• void is the return type
• Nothing is returned from the method
• The () means that the methods take no values

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Open the Wombat Scenario
You may need to click the Compile button to
update everything if classes looked hashed
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Make a Wombat, and some Leaves
Right click on Wombat class, Choose New Wombat()
Now, have the wombat act()
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Current Wombat act() method
Can you see why the Wombat wont necessarily find
all leaves?
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Create a new method for random walk
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New Wombat act() method
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Greenfoot worlds

• World class
• Abstract
• Subclass it to create a new class
• Actor class
• Subclass it to create Wombats and Balloons

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WombatWorld Class
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Making a Wall class
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Use the randomLeaves method as starting place
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New randomWombats
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New randomWalls
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New WombatWorld Constructor
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New Wombat act() so random turns
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Teach wombats to avoid walls
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Questions

• Does at least one wombat find the leaf?
• How long does it take a wombat to find a leaf?
• Can you make a smarter wombat?
• Can you modify the scenario so that wombats can
  smell leaves that are within a certain radius of
  their current position?

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MakingBreakout
http//www.greenfoot.org/scenarios/files/breakout.
zip
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Making the BreakoutWorld

• import greenfoot. // (World, Actor,
  GreenfootImage, and Greenfoot)
• import java.awt.Color
• import java.util.List
• public class BreakoutWorld extends World
• /////////////////// constants
  //////////////////////////
• / the width of the bricks in pixels (cells)
  /
• public static final int BRICK_WIDTH 36
• / the height of the bricks in pixels (cells)
  /
• public static final int BRICK_HEIGHT 8
• / the distance between bricks in pixels
  (cells) /
• public static final int BRICK_SEP 4
• / the number of bricks per row /
• public static final int NUM_BRICKS_PER_ROW
  10
• / distance from the top edge in pixels
  (cells) /
• public static final int BRICK_Y_OFFSET 70
  
• / the number of pixels per cell /

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• public static final int RESOLUTION 1
  
• / world width in pixels (cells) /
• public static final int WIDTH (BRICK_WIDTH
  BRICK_SEP) NUM_BRICKS_PER_ROW BRICK_SEP
• / world height in pixels (cells) /
• public static final int HEIGHT 600
• / number of rows of bricks /
• public static final int NUM_ROWS 10
• / the colors to use for each row of bricks
  /
• public static final Color colorArray
  Color.RED, Color.RED,
• Color.ORANGE, Color.ORANGE,
  Color.GREEN,Color.GREEN,
• Color.YELLOW,Color.YELLOW, Color.CYAN,
  Color.CYAN
• ////////////////// instance fields
  ///////////////////////////////
• / the number of balls created in the game so
  far /
• private int numBalls 0
• / a message displayed for the user /
• private Message message null

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Constructor
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setUpBreakout()
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Create and set background image
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setUpBricks()
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Creating the brick

• public class Brick extends Actor
• / the width of the brick /
• private int width 36
• / the height of the brick /
• private int height 8
• / the color of the brick /
• private Color color

• /////////// constructor ////////////////
• /
• Constructor that takes the color for the
  brick
• _at_param theColor the color to use for this
  brick
• /
• public Brick(Color theColor)
• color theColor
• updateImage()

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Constructor for Brick

• /
• Constructor that takes the width, height,
  and color for this
• brick.
• /
• public Brick(int theWidth, int theHeight,
  Color theColor)
• width theWidth
• height theHeight
• color theColor
• updateImage()

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What Bricks do

• /
• Method to act during a time step
• /
• public void act()
• /
• Method to create the image and set it for
  this brick.
• If you change the width, height, or color
  invoke this
• method.
• /
• public void updateImage()
• GreenfootImage image new
  GreenfootImage(width,height)
• image.setColor(this.color)
• image.fillRect(0,0,width,height)
• setImage(image)

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Ball class

• public class Ball extends Actor
• //////////// fields //////////////////////////
  ///
• / the radius of this ball /
• private int radius 10
• / the width of this ball (diameter) /
• private int width radius 2
• / the color of the ball /
• private Color color Color.BLACK
• / the amount of change in x during each act
  /
• private int velX
• / the amount of change in y during each act
  /
• private int velY 3

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End of Ball class
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Making newBall in BreakoutWorld
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checkIfWon()

• /
• Method to check if the game is over and if
  so tell the user
• and stop the simulation
• /
• public void checkIfWon()
• List brickList this.getObjects(Brick.cla
  ss)
• if (brickList.size() 0)
• message.setText("You Won!!!!!!")
• Greenfoot.stop()

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Foxes and Rabbits

• An example predator and prey simulation

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Interactive Question

• In what method is it defined how far away a fox
  looks for a rabbit?

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To-Do

• Make the fox look two cells away instead of one.
• What happens?
• How do you restore balance to the ecology?
• Make the fox look three cells away instead of
  one.
• What happens?
• How do you restore balance to the ecology?