Assignment 12: Who’s On Deque?
We would like to build a generic list in such a way that we can start either at the front, or at the back, and move through the list in either direction. Here is an example of such a list (shown here with Strings), drawn as an object diagram:
+---------------+
| Deque<String> |
+---------------+
| header ----------+
+---------------+ |
+--------------+
|
v
+------------------+
+---------->| Sentinel<String> |<-------------------------------------------+
| +------------------+ |
| +---- next | |
| | | prev ------------------------------------------------+ |
| | +------------------+ | |
| | | |
| v v |
| +--------------+ +--------------+ +--------------+ +--------------+ |
| | Node<String> | | Node<String> | | Node<String> | | Node<String> | |
| +--------------+ +--------------+ +--------------+ +--------------+ |
| | "abc" | | "bcd" | | "cde" | | "def" | |
| | next ----------->| next ----------->| next ----------->| next ----------+
+-- prev |<--- prev |<--- prev |<--- prev |
+--------------+ +--------------+ +--------------+ +--------------+Every Deque has a header that consists of the Sentinel node. This header field does not change, but the fields within the Sentinel node (that provide the links to the first and the last item in the Deque) can and will change.
Each data Node has two links, one to the next item in the list and one to the previous item in the list.
The Sentinel node is always present. It also has next and prev fields, which are consistently updated to link to the head of the list and to the tail of the list, respectively.
The Sentinel and Node classes both inherit from a common superclass (shown below), because the next or prev item after any given node may be either a data-carrying Node or the Sentinel marking the end of the list.
The list shown above has four data-carrying nodes and the lone sentinel. The empty list has just the Sentinel, and its links to the first and to the last item just reference the Sentinel node itself. So an empty list would have the following object diagram:
+---------------+
| Deque<String> |
+---------------+
| header ---------+
+---------------+ |
|
+--------+
+----+ | +----+
| | | | |
| V V V |
| +------------------+ |
| | Sentinel<String> | |
| +------------------+ |
+--- next | |
| prev ---------------+
+------------------+ +--------------------+
| Deque<T> |
+--------------------+
| Sentinel<T> header |-+
+--------------------+ |
|
+--------------------------+
|
|
| +---------------+
| | ANode<T> |
| +---------------+
| | ANode<T> next |
| | ANode<T> prev |
| +---------------+
| /_\ /_\
| | |
| +----+ |
V | |
+-------------+ +---------+
| Sentinel<T> | | Node<T> |
+-------------+ +---------+
+-------------+ | T data |
+---------+We have an abstract class ANode<T>, which can be either a Sentinel<T> node or an actual Node<T> containing data. We use an abstract class here instead of an interface because we need to share the next and prev fields. Moreover, because all ANodes are contained and hidden within the Deque, no external code needs to know about it: they are implementation details rather than a publicly visibly datatype definition.
12.1 Data definitions and examples
Define the classes ANode<T>, Node<T>, Sentinel<T>, and Deque<T>.
For Sentinel<T>, define a constructor that takes zero arguments, and initializes the next and prev fields of the Sentinel to the Sentinel itself.
For Node<T>, define two constructors: the first one takes just a value of type T, initializes the data field, and then initializes next and prev to null. The second convenience constructor should take a value of type T and two ANode<T> nodes, initialize the data field to the given value, initialize the next and prev fields to the given nodes, and also update the given nodes to refer back to this node. Throw an IllegalArgumentException in this constructor if either of the given nodes is null. (You can use if (theNode == null) { ... } to test for null-ness.) Note carefully the order of the arguments in this constructor!
For Deque<T>, define two constructors: one which takes zero arguments and initializes the header to a new Sentinel<T>, and another convenience constructor which takes a particular Sentinel value to use.
Make examples of at least two lists, including the empty list and a list of Strings with the values ("abc", "bcd", "cde", and "def").
12.2 Methods
Design the method size that counts the number of nodes in a list Deque, not including the header node. (I.e., just count the Nodes and not the Sentinel.)
Design the method addAtHead for the class Deque that consumes a value of type T and inserts it at the front of the list. Be sure to fix up all the links correctly!
Design the method addAtTail for the class Deque that consumes a value of type T and inserts it at the tail of this list. Again, be sure to fix up all the links correctly!
Design the method removeFromHead for the class Deque that removes the first node from this Deque. Throw a RuntimeException if an attempt is made to remove from an empty list. Be sure to fix up all the links correctly! As with ArrayList’s remove method, return the item that’s been removed from the list.
Design the method removeFromTail for the class Deque that removes the last node from this Deque, analogous to removeFromHead above. Again, be sure to fix up all the links correctly!
Design the method find for the class Deque that takes an IPred<T> and produces the first node in this Deque for which the given predicate returns true.
If the predicate never returns true for any value in the Deque, then the find method should return the header node in this Deque. (Hint: think carefully about the return type for find!)
Optional: You may have duplicate code in addAtHead, addAtTail, removeFromHead and removeFromTail. If appropriate, try to abstract out any duplication into helper methods —
most likely, helper methods on ANode<T> and its subclasses. Be sure your tests still pass!