Sample Free-Response Solution (AB Exam)

(a)

/** Returns all the objects in this environment.
 *  @return an array of all the environment objects
 **/
public Locatable[] allObjects()
{
  Locatable[] theObjects = new Locatable[numObjects()];
  int tempObjectCount = 0;

  // Look at all grid locations.

  // insert code here

  ListNode curNode;
  for (int r = 0; r < numRows(); r++)
  {
    curNode = theGrid[r];
    while (curNode != null)
    {
      theObjects[tempObjectCount] = (Locatable)curNode.getValue();
      curNode = curNode.getNext();
      tempObjectCount++;
    }
  }

  // end of inserted code here

  return theObjects;
}

(b)

/** Adds a new object to this environment at the location
 *  it specifies.
 *  (Precondition: <code>obj.location()</code> is a valid empty
 *  location.)
 *  @param obj the new object to be added
 *  @throws IllegalArgumentException if precondition is not met
 **/
public void add(Locatable obj)
{
  // Check precondition. Location should be empty.
  Location loc = obj.location();
  if ( ! isEmpty(loc) )
    throw new IllegalArgumentException("Location " + loc +
                                   " is not a valid empty location");

  // Add object to the environment.

  // insert code here

  if (theGrid[loc.row()] == null ||
      ((Locatable)theGrid[loc.row()].getValue()).location().col() > loc.col())
  {
    theGrid[loc.row()] = new ListNode(obj, theGrid[loc.row()]);
  }
  else
  {
    ListNode curNode = theGrid[loc.row()];
    while (curNode.getNext() != null &&
           ((Locatable)curNode.getNext().getValue()).location().col() < loc.col())
    {
      curNode = curNode.getNext();
    } 
    ListNode temp = new ListNode(obj, curNode.getNext());
    curNode.setNext(temp);
  }

  objectCount++;
}