This week in lecture we are introducing queues, both as presented
in L&C with the QueueADT interface and as they exist in
java.util with the Queue interface. A queue
is a linear data structure that supports adding at the rear and
removing
from the front -- the QueueADT interface has methods
enqueue, dequeue, and first
(look
at first element without removing it), along with isEmpty
and size.
Here we define a related linear data structure called a deque for "double-ended queue" -- the word is pronounced like "deck" and should not be confused with the "dequeue" operation which is pronounced like the letters "DQ". In a deque we can add, remove, and look at elements at either end -- its methods are:
public interface Deque {
public void addToFront (T element);
public T removeFront ( ) throws EmptyCollectionException;
public T first ( ) throws EmptyCollectionException;
public void addToRear (T element);
public T removeRear ( ) throws EmptyCollectionException;
public T last ( ) throws EmptyCollectionException;
public boolean isEmpty ( );
public int size ( );}
We'll only start to think about implementing deques this week if we have time -- this afternoon we will show how to implement queues with either circular arrays of linked data structures. Today we will be primarily just getting familiar with deques.
Question 1: One again assume that we have the
Dog
class defined and that we have declared and created Dog
objects named ace, biscuit,
cardie,
and duncan. Trace a deque through the following sequence
of operations, starting with an empty deque:
addToRear (cardie); addtoFront (duncan); addToFront (biscuit);
removeFront ( ); addToRear (ace); removeFront ( ); addToRear (cardie);
addToRear (duncan); removeFront ( ); removeRear ( ); addToFront
(biscuit);
Question 2: Write complete code for two new classes:
DequeStack and DequeQueue, each of which
will extend a class DequeClass that implements
Deque. The class DequeStack should have the
methods push, pop, and peek,
and
the class DequeQueue should have the methods
enqueue. dequeue, and first.
Implement these methods by calling on the public methods of
DequeClass.
You may leave out the constructors. (Why?)
Question 3: Write a method public void
switchLastTwo
to go into DequeClass, using Deque methods
to duplicate the functionality of last week's method with the same
name
in the DogTeam class. Remember that last week's method
never threw an exception, and did nothing when called by a collection
with zero or one element.
Question 4: (only if time) Do you think it will be easy to implement a deque with a circular array like that you used in Project 3? Why or why not? Ignore for now what happens if the queue exceeds the array's capacity.
Question 5: (only if time) Do you think it will be easy
to implement a deque with (singly-linked) linear nodes like those we used for
ArrayStack and DogTeam?
Why or why not?
Last modified 13 October 2011