Project 1B: Applying and Testing Data Structures version 1.0


In most of your prior programming assignments, you’ve relied on some external source of truth to tell you that your code was correct.

In this project, you’ll use your deque from project 1A to solve a real world problem. Along the way, you’ll also write tests of your application to convince yourself that everything works correctly.

Unlike Project 1A, this project will be more highly scaffolded to reduce the workload in anticipation of the midterm.

Warning: The autograder output for this project is going to be terse and unhelpful. This is because we want you to use your tests to build confidence in your code. You will not be able to rely on the autograder for correctness.

Getting the Needed Files

Skeleton Files

As before, pull the skeleton using the command git pull skeleton master.

In the skeleton, we have provided the following files:

In addition you will create the following files:

You’ll also need to cd into your library-sp18 folder. Once you’re there, use git pull origin master. If everything works as it should, you should see file called words.txt appear in the library-sp18/data folder.

In summary, these should be the shell commands to use:

$ git pull skeleton master
$ cd library-sp18
$ git pull origin master

If that didn’t work, try git submodule update --recursive --remote from your personal sp18-??? folder.

Note: We’ve set up your git repository so that it will reject words.txt by default. For those of you who know that this means: Please don’t use the -f command to force this large file into your repo. It will slow down the autograder.

Additional Optional Manual Download

For this assignment, you’ll also need a correct deque implementation. You are welcome to use your LinkedListDeque or your ArrayDeque from Part1A. If you’re not sure if your solution to project 1a is correct, or you just didn’t finish, you may download a correct implementation of LinkedListDeque by left clicking on this link, and manually copying and pasting the displayed code.

Task 1: Deque Interface

This first task is going to be a little tedious, but it won’t take long.

Create an interface in a new file named that contains all of the methods that appear in both ArrayDeque and LinkedListDeque. See the project 1a spec for a concise list. In IntelliJ, you’ll use “New->Java Class”. IntelliJ will assume you want a class, so make sure to replace the class keyword with interface.

After creating this interface and adding all the methods, modify your LinkedListDeque and/or ArrayDeque so that they implement the Deque interface by adding implements Deque<Item> to the line declaring the existence of the class. If you used something other than Item for your generic type parameter, use that instead. Add @Override tags to each method that overrides a Deque method.

Note: If you’re using the provided solution for LinkedListDeque, which relies on some inheritance black magic, your class definition should look like
public class LinkedListDeque<Item> extends LinkedList<Item> implements Deque<Item>.

Task 2: wordToDeque

Create a new file called, and add a method with the signature shown below:

Don’t write any code yet for this method. For now, just have it return null so that can compile.

Given a String, wordToDeque should return a Deque where the characters appear in the same order as in the String. For example, if the word is “persiflage”, then the returned Deque should have ‘p’ at the front, followed by ‘e’, and so forth. Don’t implement wordToDeque yet!

Uncomment the code in TestPalindrome and run the test contained in the file (e.g. by right-clicking on it and picking Run TestPalindrome). You should fail the provided test. Your goal is to now pass this test by correctly implementing wordToDeque. Once you’ve passed the test, move on to the next part of this assignment. Make sure that you don’t delete the weird line static Palindrome palindrome = new Palindrome();. It’s not useful for this task of the assignment, but it’ll be necessary later.

Tip: Search the web to see how to get the i-th character in a String.

Tip: Inserting chars into a Deque<Character> is just like inserting ints into a LinkedListDeque<Integer>.

Note: The careful reader of testWordToDeque might wonder why we didn’t just create a correct Deque and then call assertEquals. The reason is that our Deque class does not provide an equals method and thus it won’t work the way you expect. We’ll be talking about this in class soon.

Tip: If you’re failing your own test and can’t figure why, remember you have a debugger. Use it! Don’t just stare at your code looking for the bug. That’s too slow and tedious.

Task 3: isPalindrome

Task 3A: isPalindrome Testing

Now that you’re passing the test and getting to enjoy the nice green glow of the success bar, let’s break things again.

Modify your so that it now has a second method with the signature below.

For now, have it return a dummy value. A dummy value is just some arbitrary thing you select, i.e. either true or false. Before we write the method itself, we’re going to write a test. Let’s start by discussing what isPalindrome is supposed to do.

The isPalindrome method should return true if the given word is a palindrome, and false otherwise. A palindrome is defined as a word that is the same whether it is read forwards or backwards. For example “a”, “racecar”, and “noon” are all palindromes. “horse”, “rancor”, and “aaaaab” are not palindromes. Any word of length 1 or 0 is a palindrome.

‘A’ and ‘a’ should not be considered equal. You don’t need to do anything special for capital letters to work properly. In fact, if you forget that capital letters exist, your code will work fine.

Add at least one test to TestPalindrome that tests the isPalindrome method. You’ll probably find the assertTrue and assertFalse methods to be useful. You’re also welcome to use any other methods in the JUnit documentation. Ideally, you should write several tests, and not just one, but it’s up to you. It’s ok to have multiple asserts in one test, though don’t go too wild. Make sure to annotate your tests with @Test, otherwise JUnit won’t run your tests.

When you run TestPalindrome again, your code should fail, hopefully multiple tests.

Tip: As an example, assertFalse(palindrome.isPalindrome("cat")); tests to ensure that “cat” is not considered a palindrome.

Tip: If you’re looking for more interesting things to test, read this section carefully for any interesting corner cases, and ensure that your tests check those corner cases.

Task 3B: isPalindrome

Now that you have a failing test, implement the isPalindrome method. Use your wordToDeque method to give yourself an easier time. While you can technically not use a Deque at all, we strongly encourage you to do so. It’s a good exercise in understanding how your choice of data structures (in this case, Deque) will have a profound affect on how you write your code.

Once you’ve passed your own tests, you’re ready to move on. Keep in mind that our autograder is going to be very quiet, so you’ll want to make sure your tests are thorough so that you feel good about your code. At the very least, you should have at least one test that checks that some word is a palindrome, and one that checks that some word is not a palindrome, as well as two interesting corner cases.

Tip: Consider recursion. There’s a really beautiful solution that uses recursion. You’ll need to create a private helper method for this to work.

Tip: Don’t use the get method of Deque. That will just make things unnecessarily complicated.

Just for fun: Uncomment the code in the provided class and you’ll get a list of all palindromes of length 4 or more in English (assuming you also downloaded the provided words file).

Tip: If you’re failing your own test and can’t figure why, remember you have a debugger. Use it! Don’t just stare at your code looking for the bug. That’s too slow and tedious.

Task 4: Generalized Palindrome and OffByOne

For this task, you will do six things. Our suggested order is given below:

However, you’re welcome to do them in any order you choose. For this task, rather than carefully enumerating everything you’re supposed to do, as in the tasks above, we will simply give a general description of your goal.

In this task, your ultimate goal is to add a third public method to your Palindrome class with the following signature:

The method will return true if the word is a palindrome according to the character comparison test provided by the CharacterComparator passed in as argument cc. A character comparator is defined as shown below:

/** This interface defines a method for determining equality of characters. */
public interface CharacterComparator {

    /** Returns true if characters are equal by the rules of the implementing class. */
    public boolean equalChars(char x, char y);

For this task, you’ll also create a class called, which should implement CharacterComparator such that equalChars returns true for characters that are different by exactly one. For example the following calls to obo should return true. Note that characters are delineated in Java by single quotes, in contrast to Strings, which use double quotes.

OffByOne obo = new OffByOne();
obo.equalChars('a', 'b');
obo.equalChars('r', 'q');

However, the three calls below should return false:

obo.equalChars('a', 'e');
obo.equalChars('z', 'a');
obo.equalChars('a', 'a');

Spec clarification, 2/7/2018: Characters in Java include non-alphabetical characters. For example ‘%’ and ‘&’ are off by one. This might seem strange (especially since they’re not even next to each other on the keyboard), but char values in Java are really just integers. For example ‘%’ is actually just another way of writing 37, and ‘&’ is another way of writing 38. That is, the code below is valid and the first print statement will print 38. The second two print statements are exactly equivalent, and they both simply print 1.

int x = '&';
System.out.println(x);         // prints 38
System.out.println(38 - 37);   // prints 1
System.out.println('&' - '%'); // prints 1

Thus the method call below should return true:

obo.equalChars('&', '%');

Similarly, ‘a’ and ‘B’ are NOT off by one, since ‘a’ - ‘B’ is 31. If you’re curious about the specific values for many familiar characters, see the table at the bottom of this wikipedia article. [end of spec clarification from 2/7/2018]

To allow for odd length palindromes, we do not check the middle character for equality with itself. So “flake” is an off-by-1 palindrome, even though ‘a’ is not one character off from itself.

As with our earlier isPalindrome method, any zero or 1 character word is considered a palindrome.

Tip: Make sure to include @Override when implementing equalChars. While it has no effect on the function of your program, it’s a good habit for the reasons detailed in lecture.

Tip: To calculate the difference between two chars, simply compute their difference in java. For example int diff = 'd' - 'a'; would return diff as -3.

Tip: Even though a good solution for Palindrome and OffByOne should not explicitly worry about non-alphabetical characters or uppercase letters, your tests may be run on code which is not a good solution, and thus may have errors that only apply to non-alphabetical characters.

Just for fun: Try printing out all off-by-one palindromes of length 4 or more in English (assuming you also downloaded the provided dictionary) by modifying For example “flake” is an off-by-1 palindrome since “f” and “e” are one letter apart, and “k” and “l” are one letter apart.

Tip: If you’re failing your own test and can’t figure why, remember you have a debugger. Use it! Don’t just stare at your code looking for the bug. That’s too slow and tedious.

Task 5: OffByN

In this last part, you will implement a class OffByN, which should implement the CharacterComparator interface, as well as a single argument constructor which takes an integer. In other words, the callable methods and constructors will be:

The OffByN constructor should return an object whose equalChars method returns true for characters that are off by N. For example the call to equal chars below should return true, since “a” and “f” are off by 5 letters, but the second call would return false since “f” and “h” are off by 4 letters.

OffByN offBy5 = new OffByN(5);
offBy5.equalChars('a', 'f');  // true
offBy5.equalChars('f', 'a');  // true
offBy5.equalChars('f', 'h');  // false

For this task, if you choose to write tests, you’ll need to make your own test file. Make sure to include the appropriate import statements, which you can copy and paste from our two provided test files. Unlike the other test files, you’re welcome to use new in your tests. Due to technical limitations we will not test your file if you create one, but you still might find it useful to create such tests.

Just-for-fun: Try modifying so that it outputs a list of offByN palindromes for the N of your choosing.

Just-for-more-fun: For what N are there the most palindromes in English? What is the longest offByN palindrome for any N?

Frequently Asked Questions

I can’t get the provided LinkedListDeque solution to compile.

Make sure your class definition is
public class LinkedListDeque<Item> extends LinkedList<Item> implements Deque<Item>.

My implementation of LinkedListDeque or ArrayDeque won’t compile.

Make sure your class definition ends with implements Deque<Item>, or if your code doesn’t use Item, replace this with whatever parameter type variable you used.

My code passes all my tests, but fails some autograder tests.

Try to think about corner cases that you may not be covering. Then write your own tests that test these corner cases, and ensure your code passes them.

The Autograder doesn’t like my tests but I’m almost certain that

Make sure that you’re not using words.txt anywhere in your tests. The autograder does not have access to this file.

Only my tests are failing. What tests am I missing?

Probably corner cases.

I’m failing the TestPalindrome or TestOffByOne tests but I feel like my tests are good.

Make sure your TestPalindrome does not say new Palindrome anywhere. Similarly, make sure your TestOffByOne does not say new OffByOne anywhere.

What’s with the weird static Palindrome palindrome = new Palindrome(); stuff?

In order to create JUnit tests for your JUnit tests, we had to resort to some clever hacks, and the easiest way involved this weird stuff. Sorry. Luckily in TestOffByN you don’t have to worry about it.