CS 49J - Lecture 6

Cay S. Horstmann

Homework Show and Tell

GridWorld

• Used in AP CS between 2008-2013
• Teaches inheritance
• Grid holds instances of Actor subclasses
• Sample actors: Rock, Bug, Critter

BoxBug

• Moves in a box shape.
• Override act.
• Sometimes, act calls move. Sometimes it calls turn.
• Runner class:

  public class BoxBugRunner
  {
      public static void main(String[] args)
      {
          ActorWorld world = new ActorWorld();
          BoxBug bob = new BoxBug(3);
          world.add(new Location(5, 5), bob);
          world.show();
      }
  }
  

• Let's figure out the BoxBug class.

Critters

• Fixed act method.
• Five (!) methods to override:

  ArrayList<Actor> getActors()
  void processActors(ArrayList<Actor> actors)
  ArrayList<Location> getMoveLocations()
  Location selectMoveLocation(ArrayList<Location> locs)
  void makeMove(Location loc)
  

• Default implementations:
  • getActors gets neighbors
  • processActors eats any actors that aren't rocks or critters.
  • getMoveLocations gets empty neighbor locations
  • selectMoveLocation selects location at random
  • makeMove moves to the given location

Crab Critters

• A crab critter is a critter with this behavior:
  • A crab critter eats whatever is found in front, right-front, or left-front.
  • It will not eat rocks or critters.
  • It can only move to the right or left.
  • If both locations are available, it randomly chooses one.
  • If neither are available, it turns 90 degrees, randomly right or left.
• Which methods can be inherited?
• How do the remaining methods need to be overridden?

Account Puzzle

What is wrong?

public class Account
{
   private double balance;
   public Account() { balance = 0; }
   public double getBalance() { return balance; }
   public void deposit(double amount) { balance += amount; }
   public void withdraw(double amount) { balance -= amount; }
}

public class CheckingAccount extends Account // Charges $1 per withdrawal
{
   private double balance;
   public void withdraw(double amount)
   {
      balance = balance - amount - 1;
   }
}

Account Puzzle 2

public class Account
{
   private double balance;
   public Account() { balance = 0; }
   public double getBalance() { return balance; }
   public void deposit(double amount) { balance += amount; }
   public void withdraw(double amount) { balance -= amount; }
}

public class CheckingAccount extends Account // Charges $1 per withdrawal
{
   private double balance;
   public void withdraw(double amount)
   {
      balance = balance - amount - 1;
   }
}

Account Puzzle 3

What is wrong?

public class Account
{
   private double balance;
   public Account() { balance = 0; }
   public double getBalance() { return balance; }
   public void deposit(double amount) { balance += amount; }
   public void withdraw(double amount) { balance -= amount; }
}

public class CheckingAccount extends Account // Charges $1 per withdrawal
{
   private double balance;
   public void withdraw(double amount)
   {
      withdraw(amount + 1);
   }
}

Account Puzzle 4

class Account
{
   private double balance;
   public String type = "Generic";
   public Account() { balance = 0; }
   public double getBalance() { return balance; }
   public void deposit(double amount) { balance += amount; }
   public void withdraw(double amount) { balance -= amount; }
}

class CheckingAccount extends Account // Charges $1 per withdrawal
{
   public String type = "Checking";
   public void withdraw(double amount)
   {
      super.withdraw(amount + 1);
   }
}

• What does this program print?

  public class Demo
  {
     public static void main(String[] args)
     {
        Account acct = new CheckingAccount();
        System.out.println(acct.type);
     }
  }
  

• What happens if you turn both type into methods?

     public String type() { return  "Generic" ;};
     public String type() { return  "Checking" ;};