Programming Assignment 4

Due Date: Tuesday, February 7, 10:00PM Pacific Time

Learning Goals


Different assignments in this course have different collaboration policies. On this assignment, you can collaborate with anyone in the course, including sharing code. In your submission, give credit to all students and course staff who helped you with this assignment by noting their name and how you used their ideas or work. Note that using someone’s work without giving credit to them is a violation of academic integrity.

Starter Code

You can get the starter code at

Submission Checklist


A Twitter-specific pattern is writing a “thread” by replying to one's own Tweets repeatedly.

In a file, write an interface called Tweet with three methods:

Then, write two classes:

You can, but don’t have to, use an abstract class to avoid duplicated work as you see fit. Add constructors as appropriate to initialize the fields on objects of these classes (whether or not you use an abstract class).

For each method, write at least two tests for it in a class called Tweets using the Tester library. A “test” is a use of checkExpect that checks the results of the method call against an expected value.

Since there are 6 total method implementations, you should have at least 12 tests. Tests are graded manually, your implementation is graded automatically.


This code will go in the file, any tests in a class called ExamplesNumber that you add to that file.

We saw in our reading that representing fractional numbers like 0.6 with doubles can be fraught. Some languages and libraries do support exact fractions, and we can implement classes that act like them in Java. We won’t be able to use the built-in + and * operators, because these are only defined for numbers and strings, but we can define methods for the operations we care about. We can represent numbers with an interface:

interface Number {
  int numerator();
  int denominator();
  Number add(Number other);
  Number multiply(Number other);
  String toString();
  double toDouble();

(We could specify more methods, but for the purposes of this assignment, these six will be sufficient.)

Your task is to create two classes that implement the interface above. One should be called WholeNumber and represent whole integers (including negative numbers). The other should be called Fraction and represent mixed numbers.

WholeNumber should have:

Fraction should have:

A reminder about arithmetic and fractions:



At the end of the ExamplesNumber class in a place marked clearly with a comment that says // Exploration, write code to perform four calculations:

  1. The result of 0.1 + 0.2 + 0.3 using built-in double arithmetic in Java
  2. The result of 0.1 + (0.2 + 0.3) using built-in double arithmetic in Java
  3. The result of (1) using your exact fractions, showing the result via toString()
  4. The result of (2) using your exact fractions, showing the result via toString()


Then you will submit all of your files to the pa4 assignment on Gradescope:

We will automatically grade the correctness of the methods and classes you write. Tests and exploration sections will be graded manually. In addition, we may give you feedback on any part of the code, including automatically graded parts, that we want you to respond to after grading.

Extra Challenges (not for credit)

Challenge (not required for credit): Many fractions, like \(2/4\) or \(27/6\), are not in their simplest form. Make it so that the constructor for Fraction always creates a fraction object with numerator and denominator in their most reduced form.

Challenge (not required for credit): Create a ReplyTweet that is a reply to itself. Do you think this is possible on Twitter?


  1. Why is the autograder producing this error “…” for me?
    • As a general reminder, it would help us a lot if your provide your submission link when it’s a Gradescope-related question (just copy the URL from the URL bar when looking at your submission and other students won’t be able to see your code from a Gradescope link).
    • In addition, you should have your own tests, and you should write your own tests and try things out to make sure you understand what your code does before submitting it. You can share your tests with us privately on Piazza and we can discuss what’s happening in them as a way to debug as well.
  2. My code does not run on Gradescope because it’s still WIP. How can I check that the part I finished is correct?
    • For the methods you haven’t finished yet, you can put implementations for them that intentionally return the wrong answer, like an empty string or false, to make it so all the tests will run.
  3. VSCode is red underlining the import tester statement. Is this an error?
    • Before trying to assess the error, try compiling your code and running it. Sometimes VSCode will erroneously underline imports that actually do work. If the code does not compile, make sure that the file that is trying to import the tester is in the same folder as tester.jar.
  4. For unrollThread methods in, I cannot pass the tests on Gradescope, but the expected result and my program result look exactly the same.
    • This is probably because of a missing or extra newline character or space character that is hard to spot with our eyes. The autograder has been updated to give a hint:”(Hint) If your result looks the same as the reference output but still gets an error, there may be extra space characters or newline characters in your result. You may try to print the length of your output String locally to see if it matches your expectation.”
  5. I wrote test methods with the Tester, but ./run is telling me that no tests ran.
    • Tester methods have to start with “test” at the beginning! like boolean testAdd(Tester t) { .... }
  6. Am I required to write tests in the ExamplesNumber class?
    • We won’t grade tests written there, but we encourage you to write them to gain confidence in your code!
  7. Should the unrollThread method of ReplyTweet start with just the contents of the replyTo tweet, or the entire unrolled tweet?
    • It should contain the entire unrolled tweet, not just the contents field.
  8. In class ReplyTweet, should be field replyTo be a TextTweet object instead of a Tweet?
    • replyTo field should be of type Tweet. Since TextTweet and ReplyTweet both implement the Tweet interface, TextTweet and ReplyTweet both have a shared type called Tweet. (Like for example in the class we saw CircleRegion and SquareRegion were both of type Region.). Having replyTo as a Tweet object helps us achieve the “thread” function, where you can reply to your’s own Tweets repeatedly.