Lab 8

1 Problem I

A Battery is an object that implements the following interface:

interface BatteryI {
  // The current charge of the battery is a number between 0 
  // and MAX_CHARGE (inclusive).
  final int MAX_CHARGE = 5;
  
  // A call to recharge() sets the current charge to MAX_CHARGE.
  void recharge();
  
  // A call to use() throws the NoPower exception if the current
  // charge is 0; otherwise it decrements the current charge by 1.
  void use () throws NoPower;
}

Write class definitions for NoPower and Battery.

2 Problem II

A Maze is recursively built using two kinds of objects:

• a DeadEnd, or
• a Hall leading to two Mazes: a left one and a right one.

Every Maze object (whether it is a DeadEnd or a Hall):

• has a name of type String that identifies it,
• may have a hidden treasure,
• may have an electric outlet, and
• can be searched by MazeVisitors.

Formally, a Maze object implements the following interface:

interface MazeI {
  String toString ();
  boolean hasTreasure ();
  boolean hasElectricOutlet ();
  void accept (MazeVisitor rv) throws VisitorStuck;
}

Given that the MazeVisitor interface is:

interface MazeVisitor {
  void visitDeadEnd (DeadEnd r) throws VisitorStuck;
  void visitHall (Hall r) throws VisitorStuck;
}

write class definitions for Maze, DeadEnd, and Hall.

3 Problem III

The goal of a TreasureSeeker is to look for a DeadEnd or Hall object that has a treasure. To be able to search the maze, a TreasureSeeker must implement the MazeVisitor interface (defined in Problem II). In addition, every TreasureSeeker will have an instance variable treasureLocation of type String in which it will save the location of the treasure.

The details of visiting a maze are as follows:

• While traversing a maze, a MazeVisitor may fail by throwing a VisitorStuck exception. Failures can be fatal or non-fatal:

  class VisitorStuck extends Exception {
  	boolean fatal;
  	VisitorStuck (boolean f) { fatal = f; }
  }
  

• When visiting a DeadEnd object, the TreasureSeeker will look for the treasure. If it finds it, it saves it in the treasureLocation and returns. Otherwise, it signals failure by throwing a non-fatal VisitorStuck exception.
  
  
• When visiting a Hall object, the TreasureSeeker will first look for the treasure in the current Hall object. If it finds it, it saves it and returns. Otherwise, it searches the left Maze. If a non-fatal exception occurs during that search, it will search the right Maze. If the exception was fatal, it re-throws it.

Write the class TreasureSeeker.

4 Problem IV

A Robot is-a TreasureSeeker operating on batteries. Every Robot has a Battery initially charged to MAX_CHARGE.

When visiting either a DeadEnd or a Hall, the Robot will first try to use its Battery. If there is enough charge, it will perform the same operations as the generic TreasureSeeker. Otherwise, if there is not enough power, the Robot will look for an electricOutlet, and if it finds one, it will recharge its Battery, and try to visit the DeadEnd or Hall again. If the Robot can't find an electricOutlet, it will throw a fatal VisitorStuck exception.

Write the class Robot.

Turn in your code during class on Wednesday, June 2nd.

Last modified: 05/19/99 10:14:25