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CSE 12 PA4: Iterators

Due date: Thursday, October 31 @ 11:59 PM PDT

There is an FAQ post on Piazza. Please read that post first if you have any questions.

Learning goals

  • Implement a data structure similar to Java’s Iterator with generic types
  • Write JUnit tests to verify proper implementation

Testing and Implementation of ListIterator [100 points]

You will implement an iterator and write JUnit tests to ensure that your implementation functions correctly.

Read the entire write-up before you start coding.

Download the starter code and put it in a directory in your working environment.

You will find the following files:

+-- PA4
|    +-- MyListIteratorCustomTester.java    Edit this file
|    +-- MyListIteratorPublicTester.java

Copy over MyLinkedList.java from PA3 to this directory, because you will also edit this file.

Part 1: JUnit Testing (20 points)

We provide two test files:

  • MyListIteratorPublicTester.java
    • This contains all the public test cases we will use to grade your MyListIterator implementation (visible on Gradescope).
  • MyListIteratorCustomTester.java
    • Contains some of the headers and description to the hidden test cases we will use to grade your MyListIterator implementation (hidden until the PA is graded). ⚠️There are hidden tests on Gradescope not described in the write-up. Make sure to write additional tests to verify your implementation's correctness! ⚠️

Your task: Implement the missing unit tests in MyListIteratorCustomTester.java based on the description in the Tests table below.

  • Your tests will be graded by checking if they pass on a good implementation and fail on a bad implementation. If a test fails on a good implementation, then the test is written incorrectly. If a test passes on a bad implementation, it may be written incorrectly or may be not be rigorous enough (try adding more asserts).
  • Some of your tests will be run on several bad implementations. You will receive 2 pts for every bad implementation your test fails (if your test also passes on the good implementation).
  • DO NOT CHANGE THE TEST HEADERS!

Tests Table: List of test cases to write and their description

Test Cases Description Point Value
testNextEnd Call next when iterator is at the end of a list. 3
testPreviousStart Call previous when iterator is at the beginning of a list. 3
testAddInvalid add an invalid element to the list 2
testCantSet set an element when canRemoveOrSet is false. 2
testCantRemove remove an element when canRemoveOrSet is false. 2
testHasNextEnd Call hasNext when the iterator is at the end of a list. 2
testHasPreviousStart Call hasPrevious when the iterator is at the beginning of a list. 2
testPreviousIndexStart Call previousIndex when the iterator is at the beginning of a list. 2
testNextIndexEnd Call nextIndex when the iterator is at the end of a list. 2

How to compile and run the testers:

Running the tester on UNIX based systems (including a mac):

  • Compile: javac -cp ../libs/junit-4.13.2.jar:../libs/hamcrest-2.2.jar:. MyListIteratorPublicTester.java
  • Execute: java -cp ../libs/junit-4.13.2.jar:../libs/hamcrest-2.2.jar:. org.junit.runner.JUnitCore MyListIteratorPublicTester

Running the tester on Windows systems:

  • Compile: javac -cp ".;..\libs\junit-4.13.2.jar;..\libs\hamcrest-2.2.jar" MyListIteratorPublicTester.java
  • Execute: java -cp ".;..\libs\junit-4.13.2.jar;..\libs\hamcrest-2.2.jar" org.junit.runner.JUnitCore MyListIteratorPublicTester

You should run the above commands in the starter directory. To run the custom tester, replace references to MyListIteratorPublicTester with MyListIteratorCustomTester in the above commands.

⚠️Your code must first compile in order to receive credit for the different test cases. You will receive a zero if your code doesn’t compile.

Part 2: Implementation of ListIterator (75 points)

Overview:

It is important to thoroughly test out your MyLinkedList implementation before working on this, as a buggy MyLinkedList implementation will give you a hard time testing your ListIterator. The resubmission assignment for PA3 is currently open and all test cases are visible.

You should also import java.util.NoSuchElementException, java.util.ListIterator, and java.util.Iterator apart from the imports you used in PA3.

Inside MyLinkedList.java, create a class named MyListIterator that implements the ListIterator interface as an inner class (contained inside the definition of the MyLinkedList class). This is the approach you should use to create your MyListIterator.

public class MyLinkedList<E> extends AbstractList<E> {
    // class variables

    // Node inner class

    // MyLinkedList methods

    protected class MyListIterator implements ListIterator<E> {

        // class variables here

        // MyListIterator methods
        public boolean hasNext() {
            // your code here
        }
        // more methods, etc.
    }
}

The MyListIterator is able to traverse the list by moving a space at a time in either direction. The MyListIterator can also add, set, and remove elements in the list. However, unlike the add, set, and remove implemented in PA3 for MyLinkedList that can directly use a specific index, the MyListIterator must first move positions (using next and previous) to modify the desired index in add, set, and remove.

Public methods to add to MyLinkedList.java

Once you are sure your iterator is working, you should add the following methods in MyLinkedList to override the AbstractList implementations. Each of these should just create a new MyListIterator and return it. Note: Make sure you are adding these methods to MyLinkedList.java and not MyListIterator.java

public ListIterator<E> listIterator() {
    return new MyListIterator();
}

public Iterator<E> iterator() {
    return new MyListIterator();
}

MyListIterator Implementation:

  • Your methods should properly throw IllegalStateException, NoSuchElementException, and NullPointerException exceptions. This means you DO NOT HAVE TO IMPLEMENT throwing ALL exceptions as the official Javadoc would indicate. Please refer to the table below. Note that our test cases do not check for the exception message, just that the correct exception was thrown.
  • You should use the getters and setters defined in the Node class to access and modify Node objects. Do not modify or access the Node’s member variables directly.
  • Do not use MyLinkedList methods to write iterator methods. If you use a MyLinkedList method in an iterator method, you will get a 0 for that method. We will use our MyLinkedList class when running test cases for MyListIterator, and it will throw an exception if a MyLinkedList method is used. However, feel free to use the Node methods.
  • Do not define any helper methods in the MyLinkedList class. If you have helper methods, they should go in the inner class MyListIterator.
  • DO NOT import and use Java’s built-in LinkedList. If we see that you import java.util.LinkedList; you will not receive any credit! ⚠️
  • Do not make any instance variables private, they should have the default access modifier.
  • You may add private static final variables to be used as constants, but do not add any other instance variables and do not add any static variables.

Instance Variable Descriptions

Instance Variable Description
Node left,right Two node references to determine the iterator location.
It is useful to have a number of state fields to simplify the writing of the methods below. Since the cursor of the ListIterator exists logically between 2 nodes, it is useful to just keep references to the next Node and the previous Node.
int idx Int value of the index of the next node.
boolean forward Determine the current moving direction of the iterator. forward is true after calling next(), false after calling previous(). forward can be either true or false after initialization (because it is only used for remove and set), however in our examples it is initially true.
boolean canRemoveOrSet Helper boolean variable to keep track of whether the current state of the iterator allows remove or set operation. It should be false initially.
True after calling next or previous. False after calling add or remove.

Method Descriptions

Method Name Description Exceptions to Throw
public MyListIterator() Constructor that is used to initialize the iterator. None
public boolean hasNext() Return true if there is an element node when going in the forward (head to tail) direction from the current iterator position. Sentinel (dummy) nodes do not count as element nodes. None
public E next() Return the next element in the list when going forward, and move the iterator forward by one node. Throw a NoSuchElementException if there is no such element.
public boolean hasPrevious() Return true if there is an element node when going in the backward (tail to head) direction from the current iterator position. Sentinel (dummy) nodes do not count as element nodes. None
public E previous() Return the next element in the list when going backward, and move the iterator backward by one node. Throw a NoSuchElementException if there is no such element.
public int nextIndex() Return the index of the element that would be returned by a call to next().
Return the list size if at the end of the list. 		None
public int previousIndex() Return the index of the element that would be returned by a call to previous().
Return -1 if at the start of the list. 		None
public void add(E element) Insert the given item into the list immediately before the element that would be returned by next().
If we call previous() immediately following add, the newly added item would be returned.
The value of the current index of the list iterator is increased by one. 		Throw a NullPointerException if element is null.
public void set(E element) For the node returned by the most recent next/previous call, replace its value with the new value element. Throw a NullPointerException if element is null.
Throw an IllegalStateException if neither next nor previous were called, or if add or remove have been called since the most recent next/previous call.
public void remove() Remove the last element node returned by the most recent next/previous call. Throw an IllegalStateException if neither next nor previous were called, or if add or remove have been called since the most recent next/previous call.

Examples for the MyListIterator Methods can be found in the Appendix.

Part 3: Coding Style (5 points)

Coding style is an important part of ensuring readability and maintainability of your code. We will grade your code style in all submitted code files according to the style guidelines. Namely, there are a few things you must have in each file/class/method:

  • File header
  • Class header
  • Method header(s)
  • Inline comments
  • Proper indentation
  • Descriptive variable names
  • No magic numbers (Exception: Magic numbers can be used for testing.)
  • Reasonably short methods (if you have implemented each method according to the specification in this write-up, you’re fine). This is not enforced as strictly.
  • Lines shorter than 80 characters
  • Javadoc conventions (@param, @return tags, /** comments */, etc.)

A full style guide can be found here and a sample styled file can be found here. If you need any clarifications, feel free to ask on Piazza.

Submission Instructions

Turning in your code

  • Submit all of the following files to [Gradescope].
    • MyLinkedList.java
    • MyListIteratorCustomTester.java

Important: Even if your code does not pass all the tests, you will still be able to submit your homework to receive partial points for the tests that you passed. Make sure your code compiles in order to receive partial credit.

How your assignment will be evaluated [100 points]

  • Correctness [95 points] You will earn points based on the autograder tests that your code passes. If the autograder tests are not able to run (e.g., your code does not compile or it does not match the specifications in this writeup), you may not earn credit.
    • Tester [20 points]
      • The autograder will test your implementation of the Junit tests. Your unit tests are expected to fail or pass based on the Part 1 Test Table.
    • MyListIterator [75 points]
      • The autograder will test your implementation of MyListIterator on the public test cases given in MyListIteratorPublicTester.java and hidden test cases not described in this PA writeup.
  • Coding Style [5 points]
    • MyLinkedList.java will be graded on style. MyListIteratorCustomTester.java will only be graded on file header.

Appendix

Example for MyListIterator Methods

It might be helpful to consider that the iterator has (size+1) positions in the list: just after the sentinel head node (i.e. just before the first node containing data), between the 0 and 1 index, ..., just before the sentinel tail node (i.e., just after the last node containing data).

As seen in the figure below, the iterator has 4 positions when the size of the MyLinkedList is 3. Green numbers indicate the indices for MyLinkedList, while blue numbers indicate the indices for MyListIterator.

Suppose we already have a MyLinkedList with 3 nodes:
head <-> c <-> s <-> e <-> tail

MyListIterator should look like this after initialization:

After calling the constructor, idx should be 0, forward can be true or false (here we set it to true), canRemoveOrSet should be false, left should be pointing to the sentinel head node, and right should be pointing to the sentinel head node’s next node.

After calling next():

“c” should be returned. Since we just called next(), canRemoveOrSet is set to true and forward is set to true.

After calling add("a"):

MyLinkedList size as well as MyListIterator idx increments, canRemoveOrSet changes to false.
If next() is called now, “s” should be returned. If remove() is called now, an IllegalStateException should be thrown since canRemoveOrSet is false.

After calling previous():

“a” is returned by the iterator. forward is set to false since the iterator is going in the reverse direction now. idx decrements, canRemoveOrSet is again true.

After calling remove():

MyLinkedList size decrements. “a” is removed from the linked list. canRemoveOrSet is set to false.