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CS2110 Assignment A3. Linked Lists
Linked Lists
Preamble
This assignment begins our discussions of data structures. In this assignment, you will implement a data structure called a doubly linked list. Read the whole handout before starting. Near the end, we give important instructions
on testing.
At the end of this handout, we tell you what and how to submit. We will ask you for the time spent in doing A3,
so please keep track of the time you spend on it. We will report the minimum, average, and maximum.
Learning objectives
• Practice learning something by reading about it.
• Learn about and master the complexities of doubly linked lists.
• Learn a little about inner classes.
• Learn a little about generics.
• Learn and practice a sound methodology in writing and debugging a small but intricate program.
Collaboration policy and academic integrity
You may do this assignment with one other person. Both members of the group should get on the CMS and do
what is required to form a group well before the assignment due date. Both must do something to form the group:
one proposes, the other accepts.
People in a group must work together. It is against the rules for one person to do some programming on this assignment without the other person sitting nearby and helping. Take turns “driving” —using the keyboard and mouse.
With the exception of your CMS-registered group partner, you may not look at anyone else’s code, in any form,
or show your code to anyone else (except the course staff), in any form. You may not show or give your code to another student in the class. This all applies also to code from similar assignments in previous semesters of 2110.
Getting help
If you don’t know where to start, if you don’t understand testing, if you are lost, etc., SEE SOMEONE IMMEDIATELY —an instructor, a TA, a consultant. If you find yourself spending more than an hour or two on one issue, not
making any progress, STOP and get help. Some pondering and thinking is helpful, but too much of it just wastes
your time. A little in-person help can do wonders. See the course webpage for contact information.
Learning about linked lists
Part of this assignment is for you to learn about an interesting data structure by reading about it —without an
instructor explaining everything it to you. Your first task it to read the two-page entry “Linked lists” in the Java Hypertext. You will learn about (1) Singly linked lists, (2) doubly linked lists, and circular linked lists —all with and
without headers. You won’t see much about applications. Take our word for it: You will see many applications of
linked lists in the rest of this course!
This assignment
This assignment gives you a skeleton for class DLList<E> (where E is any class-type). The class also contains
a definition of Node (it is an inner class; see below) and asks you to complete several methods. The methods to
write are indicated in the skeleton. You must also develop a JUnit test class, DLListTest, that thoroughly tests the
methods you write. We give important directions on writing and testing/debugging below.
CS2110 Spring 2018 Assignment A3. Linked Lists Due on the CMS by: See the CMS 2
Generics
The definition of the doubly linked list class has DLList<E> in its header. Here, E is a “type parameter”. To
declare a variable v that can contain (a pointer to) a linked list whose values are of type Integer, use:
DLList<Integer> v; // (replace Integer by any class-type you wish)
Similarly, to create an object whose list-values will be of type String use the new-expression:
new DLList<String>()
We will introduce you to generic types more thoroughly later in the course. For now, read the pdf files in the first
two lines of the JavaHyperText entry for “generic” and look at slides of 15 Feb. lecture.
Inner classes
Class Node is declared as a public component of class DLList. It is called an inner class. Its fields and some of
its methods are private, so you cannot reference them outside class DLList, e.g. in a JUnit testing class. But the
methods in DLList can and should refer to the fields of Node, even though they are not public, because Node is a
component of DLList. Thus, inner classes provide a useful way to allow one class but not others to reference the
components of the inner class. We will discuss inner classes in depth in a later recitation. For now, read the pdf file
linked to in the first line of the JavaHyperText entry for “inner class” and look at slides for 15 Feb. lecture.
The constructor in class Node has access modifier package . So classes in the same package can use the constructor. For example, in the JUnit testing class, to obtain the first node of doubly linked list b of Integers and
store it in variable node, use:
DLList<Integer>.Node node= b.first();
Describing the time a method takes
This is your first look at estimating the time an algorithm takes.
Consider storing the number of ‘e’s in a String s:
int n= 0;
for (int k= 0; k < s.length(); k= k+1) {
if (s.charAt(k) == ‘e’) n= n+1;
}
The repetend consists of an if-statement. At each iteration, it either (1) evaluates the boolean expression, finds it
true, and executes the assignment or (2) evaluates the boolean expression and finds it false. We say that execution of
the if-statement takes constant time, because it does at most two things (evaluate an expression, execute an assignment) and the time it takes for either of those things is always the same.
The time to execute the for-loop obviously depends on how long String s is. If s contains n characters, the
repetend is executed n times. In detail, what is executed or evaluated during execution of the for-loop?
• n= 0; k= 0; // once
• k < s.length() // n+1 times
• k= k+1; // n times
• s.charAt(k) == ‘e’ // n times
• n= n+1; // at most n times
So we see that execution of this loop executes or evaluates at most 4n +3 things, each of which takes constant
time. We say that it takes time proportional to n, the length of String s, or it takes time linear in the length of the
string.
CS2110 Spring 2018 Assignment A3. Linked Lists Due on the CMS by: See the CMS 3
These terms constant time and linear time will be used in discussing this assignment.
What to do for this assignment
1. Start a project a3 (or another name) in Eclipse. Select directory src in the project and use menu item File ->
New -> Package to create a package named LinkedList. Put file DLList.java into the package —you can
do this by dragging the file on top of LinkedList. Insert into package LinkedList a new JUnit test class
(menu item File -> New -> JUnit Test Case) named DLListTest.java. Write the 6 methods indicated in
class DLList.java, testing each thoroughly, before moving on to the next one, in the JUnit test class. Inner
class Node is complete; do not change it.
Test each method thoroughly. It’s best to write a separate testing procedure for each one. We tell you later about how
to do the testing. Note that if a method is supposed to throw an exception in certain cases, then you must test that it
is thrown properly. Look at JavaHyperText entry “JUnit testing” for information on how to do this.
2. On the first line of file DLList.java, replace nnnn by your netids and hh and mm by the hours and minutes
you spent on this assignment. If you are doing the project alone, replace only the first nnnn. Please do all this
carefully. If the minutes is 0, replace mm by 0. We wrote a program to extract these times, and when you don’t
actually replace hh and mm but instead write in free form, that causes us trouble. Also, please take a few minutes
to tell us what you thought of this assignment.
3. Submit the assignment (both classes) on the CMS before the end of the day on the due date.
Grading: The correctness of the 6 methods you write is worth 62. The testing of each is worth 4-5 points: we will
look carefully at class DLListTest. If you don’t test a method properly, points might be deducted in two places:
(1) the method might not be correct and (2) it was not tested properly.
Further guidelines and instructions
Note that some methods that you will write have an extra comment in the body, giving more instructions and hints
on how to write it. Follow these instructions carefully. Also, writing some methods in terms of calls on previously
written methods may save you time.
Writing a method that changes the list: Five of the methods you write change the list in some way. These methods
are short, but you have to be extremely careful to write them correctly. It is best to draw the linked list before the
change; draw what it looks like after the change; note which variables have to be changed; and then write the code.
Not doing this is sure to cause you trouble.
Be careful with a method like prepend(v) because a single picture does not tell the whole story. Here, two cases must be considered: the list is empty and it is not empty. So two sets of before-and-after diagrams should be
drawn. This will probably mean a method that uses an if-statement.
Methodology on testing: Write and test one group of methods at a time! Writing all and then testing will waste your
time, for if you have not fully understood what is required, you will make the same mistakes many times. Good programmers write and test incrementally, gaining more and more confidence as each method is completed and tested.
Determining what test cases to use: Please read the pdf file found in the JavaHyperText at entry “testing”, especially the last part of that pdf file. It is important. Also, refer to the notes on the recitation on testing.
What to test and how to test it: Determining how to test a method that changes the linked list can be time consuming and error prone. For example: after inserting 6 before 8 in list [2, 7, 8, 5], you must be sure that the list is now [2,
7, 6, 8, 5]. What fields of what objects need testing? What prev and next fields? How can you be sure you didn’t
change something that shouldn’t be changed?
CS2110 Spring 2018 Assignment A3. Linked Lists Due on the CMS by: See the CMS 4
To remove the need to think about this issue and to test all fields automatically, you must must must do the following. In class DLList, FIRST write function gnirtSot as best you can. In writing it, do not use field size. Instead, use only fields last in class DLList and the prev and val fields of nodes. You can look at how we wrote
toString —that can help you. Do not put in JUnit a testing procedure for gnirtSot, because it will be tested
when testing procedure prepend, just as getters were tested in testing a constructor in A1.
For example, after completing gnirtSot, you can test that it works properly on the empty list using this
method:
@Test
public void testConstructor() {
DLList<Integer> b= new DLList<Integer>();
assertEquals(“[]”, b.toString());
assertEquals(“[]”, b.gnirtSot());
assertEquals(0, b.size());
}
Now write procedure prepend. Testing prepend will fully test gnirtSot. You are testing those two method
together. Each call on prepend will be followed by 3 assertEquals calls, similar to those in testConstructor:
@Test
public void testAppend() {
DLList<String> ll= new DLList<String>();
ll.prepend(“Sampson”);
assertEquals(“[Sampson]”, ll.toString());
assertEquals(“[Sampson]”, ll.gnirtSot());
assertEquals(1, ll.size());
}
The call ll.toString() tests field first, all fields next, and all fields val. The call ll.gnirtSot() tests
field last, all fields prev, and all fields val. (Remember, gnirtSot is toString in reverse.) The call on ll.-
size() tests field size. Thus, all fields are tested.
You must test all methods that change the linked list with three such assertEquals calls. That way, you don’t
have to think about what fields to test; you test them the all.
Would you have thought of using toString and gnirtSot like this? It is useful to spend time thinking not
only about writing code but also about how to simplify testing.

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