CS 570: Homework Assignment 3
Due: October 14, 11:59pm
Collaboration Policy. Homework will be done individually: each student must hand in
their own answers. It is acceptable for students to collaborate in understanding the material
but not in solving the problems or programming. Use of the Internet is allowed, but should
not include searching for existing solutions.
Under absolutely no circumstances code can be exchanged between students.
Excerpts of code presented in class can be used.
Assignments from previous oﬀerings of the course must not be re-used. Viola-
tions will be penalized appropriately.
This assignment consists in implementing a double-linked list with fast accessing. Fast
accessing is provided by an internal index. An index is just an array-based list that stores
references to nodes. Before going further, let’s take a step back and recall some basic notions
regarding double-linked lists.
As explained in the lectures, a double-linked list (DLL) is a list in which each node has a
reference to the next one and also a reference to the previous one. The corresponding Java
class therefore has three data ﬁelds or attributes:
Accessing the elements of the list is therefore realized through the references head and
tail. For example, the i-th element is obtained by starting from head and then jumping
through i − 1 nodes. Indeed, just like single-linked lists, accessing an element in a DLL
is of time complexity O(n). In order to alleviate this situation this assignment asks you
to implement an enhanced DLL, Indexed DLL or IDLL. An IDLL includes an additional
attribute, namely an index. An index is simply a list based array that stores the references
to each node in the DLL. Since the access to an element in an array-based list is O(1),
this will allow the users of IDLL to enjoy the beneﬁts of fast access, and at the same time,
use a list implementation which does not waste memory given that it may shrink or grow
dynamically, a property which is known to be one of the advantages of linked-lists in general.
The way faster access is achieved is that the get(int i) operation, in its implementation,
rather than starting from the head of the list and traversing each node until the i-th node is
reached, it simply uses the get(int i) operation of an array-based list or index called indices
which it maintains, together with the other data ﬁelds.
This does come at a price though. We need more memory to store the array-based list
indices for one thing. Another is that all the operations of IDLL will have to maintain
the indices up to date. For example, whenever a new element is added to the DLL, the
array-based indices will have to be updated by inserting the new reference.
You are requested to implement a class IDLList<E> that encodes Indexed DLLs, following
the guidelines presented in the next section.
.1 Design of the Class IDLList<E>
2.1.1 The Inner Class Node<E>
First of all, an inner class Node<E> should be declared. This class should include three data
It should also include the following operations:
Node (E elem), a constructor that creates a node holding elem.
Node (E elem, Node<E> prev, Node<E> next), a constructor that creates a node holding
elem, with next as next and prev as prev.
2.1.2 The Class IDLList<E>
The class IDLList<E> should include the declaration of this inner private class Node<E>. Apart
from that, it should have four data ﬁelds:
Note that indices is an array-based list of references to nodes. A reference to the ﬁrst
element of list is therefore available as the ﬁrst element of indices. A reference to the second
element of the list is therefore the second element in indices. And so on.
You are requested to implement the following operations (a summary is provided at the
end of this assignment, in a UML diagram) for IDLList<E>:
public IDList (), that creates an empty double-linked list.
public boolean add (int index, E elem) that adds elem at position index (counting from
wherever head is). It uses the index for fast access.
public boolean add (E elem) that adds elem at the head (i.e. it becomes the ﬁrst element
of the list).
public boolean append (E elem) that adds elem as the new last element of the list (i.e. at
public E get (int index) that returns the object at position index from the head. It uses
the index for fast access. Indexing starts from 0, thus get(0) returns the head element
of the list.
public E getHead () that returns the object at the head.
public E getLast () that returns the object at the tail.
public int size() that returns the list size.
public E remove() that removes and returns the element at the head.
public E removeLast () that removes and returns the element at the tail.
public E removeAt (int index) that removes and returns the element at the index index.
Use the index for fast access.
public boolean remove (E elem) that removes the ﬁrst occurrence of elem in the list and
returns true. Return false if elem was not in the list.
public String toString(). That presents a string representation of the list.
The following operations require index maintenance (i.e. they have to assign or modify
public IDLList ().
public boolean add (int index, E elem).
public boolean add (E elem).
public boolean append (E elem).
public E remove().
public E removeLast ().
public E removeAt (int index).
public boolean remove (E elem).
Submit a single ﬁle named IDLList.zip through Canvas that includes IDLList.java and
IDLListTest.java with your test cases. No report is required. Your grade will be deter-
mined as follows:
You will get 0 if your code does not compile.
The code must implement the following UML diagram precisely.
We will try to feed erroneous and inconsistent inputs to all methods. All arguments
should be checked.
Partial credit may be given for style, comments and readability.
The private inner class Node<E> should follow the UML diagram:
Node (E elem)
Node (E elem, Node[E] prev, Node[E] next)
The class IDLList<E> should include the following operations:
private Node[E] head
private Node[E] tail
private int size
private ArrayList[Node[E]] indices
public IDLList ()
public boolean add (int index, E elem)
public boolean add (E elem)
public boolean append (E elem)
public E get (int index)
public E getHead ()
public E getLast ()
public int size()
public E remove ()
public E removeLast ()
public E removeAt (int index)
public boolean remove (E elem)
public String toString()
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