CMPSCI 187: Programming With Data Structures

David Mix Barrington

Fall, 2011

Q&A for Programming Project #4: Searching a Maze With a Queue

Question text in black, answers in blue.

• Question P4.1, posted 9 October: My compiler gave me an error when I tried to have my Maze class implement the Queue interface. It said I didn't have a peek method.

  It is right, and the reason that your Maze class doesn't have a peek method is that it is not trying to be a Queue, just to use a Queue. To use the language I'm trying to emphasize, a Maze has-a Queue but it isn't-a Queue. You will want to define a Queue object as a field of the Maze class, so that every Maze object will have a Queue.

• Question P4.2, posted 15 October: My compiler wouldn't let me create a Queue object with the constructor Queue<QCell>( ).

  That is because in real Java, Queue is an interface, not a class, so there are no Queue objects, only objects from classes that implement the Queue interface. You will need to create an object of one of those classes and put it in a variable of type Queue<QCell>, as in:

  Queue<QCell> q = new ArrayDeque<QCell>( );

  Here you can choose any class that implements the interface -- you can find the list of them in the Java API. If we were to go into your code and replace the ArrayDeque with a LinkedList, for example, it should work just the same.

• Question P4.3, posted 15 October: I have figured out how to put the neighbors of the cell I am looking at onto the queue. But how do I put the neighbors of those neighbors onto the queue as well?

  You don't, at least not yet. Your job when you process a node is to put its neighbors onto the queue. Eventually those neighbors will work their way to the front of the queue, come off, and be processed. At that time, their neighbors will go onto the queue.

Last modified 15 October 2011