Programming Assignment 4 – Interfaces

This programming assignment will help you practice writing interfaces and classes. You can get the starter repository here:

https://github.com/ucsd-cse11-s20/pa4-starter

You will upload several Java files for this assignment, detailed below.

You will also complete a survey on your experiences in the class the past two weeks.

Different assignments in this course have different collaboration policies. On this assignment, you cannot share code publicly on Piazza or with any other students in the course. If you need to share code, ask in a private Piazza post or in 1-on-1 hours. Still do ask any public code questions about lecture code, quizzes that are past, or other conceptual questions, just no code from this PA.

FAQ

  1. In class ReplyTweet, should be field replyTo be a TextTweet object instead of a Tweet?
    • replyTo field should be of type Tweet, because you can reply to any Tweet. See the sidenote in the writeup, “A Twitter-specific pattern is writing a “thread” by replying to one’s own Tweets repeatedly.” Having replyTo as a Tweet object helps us achieve the “thread” function.
  2. If replyTo is a Tweet, how can I use the fields, like author, inside it?
    • Let’s think about the examples in Lecture 7 — how do UnionRegion and IntersectRegion work if they don’t know which specific type the two Region fields have? How did they avoid needing to see use the center field of CircRegion, or the upperLeft field of RectRegion?
  3. Should class ReplyTweet implement Tweet?
    • Yes, and the writeup has been updated to explicitly say that.
  4. 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.
  5. 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.
  6. 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.
  7. For unrollThread methods in Tweets.java, 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.”
  8. 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! I think we didn’t make a big deal of this in lecture and it’s mentioned quickly in the reading, but they do have to start with that word, like boolean testAdd(Tester t) { .... }
  9. 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!
  10. 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.

Tweets

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

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

  • public boolean isStartOfThreadBy(String author);
  • public int totalLikes();
  • public String unrollThread();

Then, write two classes:

  • TextTweet, implements Tweet and has three fields:
    • contents, a String
    • likes, an int
    • author, a String

    This class should implement the methods as follows:

    • isStartOfThreadBy should return true when the given author is the same as the author of this TextTweet, false otherwise.
    • totalLikes should return the number of likes on this TextTweet object

    • unrollThread should return a string in the following format:

      <author>
<n> likes
<content>

      where <author> is replaced by the author’s name, <n> is replaced by the number of likes on the TextTweet, and <content> is replaced by the contents of the Tweet. The string should end in a new line ("\n" character).

  • ReplyTweet, which should implement Tweet and has four fields:
    • contents, a String
    • likes, an int
    • author, a String
    • replyTo, a Tweet

    This class should implement the methods as follows:

    • isStartOfThreadBy should return true when the given author is the same as the author of this ReplyTweet and the replyTo tweet is also the start of a thread by the same author.
    • totalLikes should return the total number of likes on this ReplyTweet object plus the total likes of its replyTo Tweet. For example, a thread of tweets that is 4 replies long should sum the likes on all 4 of those tweets.

    • unrollThread should return a string in the following format:

      <replyTo content>
<author>
<n> likes
<content>

      where the bottom three parts are the same format as in TextTweet, and the first part is the unrolled version of the replyTo Tweet. Like for TextTweet, this should end in a new line character.

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.

Numbers

This code will go in the file ExamplesNumber.java, 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:

  • A field int n and a constructor that takes a single int
  • An implementation of all the methods above.
    • numerator should return the value of n
    • denominator should return 1
    • add should return a new Number that represents adding this whole number to the one provided as an argument. Note that the argument could be either a Fraction or a WholeNumber
    • multiply should return a new Number that represents multiplying this whole number to the one provided as an argument. Note that the argument could be either a Fraction or a WholeNumber
    • toString should return the value of n as a String, so if n is 500, it should return "500"
    • toDouble should return the value of n as a double

Fraction should have:

  • A field int n representing the numerator
  • A field int d representing the denominator
  • An implementation of all the methods above:
    • numerator should return the value of n
    • denominator should return the value of d
    • add should return a new Number that represents adding this fraction to the one provided as an argument. Note that the argument could be either a Fraction or a WholeNumber
    • multiply should return a new Number that represents multiplying this fraction by the one provided as an argument. Note that the argument could be either a Fraction or a WholeNumber
    • toString should return a String in the format "n/d" where n and d are the corresponding fields. So if n and d were 1 and 2, this should be "1/2"
    • toDouble should return the value of n/d as a double. So if n is 1 and d is 2, this should return 0.5

A reminder about arithmetic and fractions:

\[n = \frac{n}{1}\] \[\frac{n}{d_1} + \frac{m}{d_2} = \frac{d_1m + d_2n}{d_1d_2}\] \[\frac{n}{d_1} \cdot \frac{m}{d_2} = \frac{nm}{d_1d_2}\]

Exploration

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()

Submission

Complete the survey!

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

  • On the Gradescope upload screen, you can keep clicking “Browse Files” to select more than one file for your upload; you can select them one at a time or use your operating system’s multi-select (Shift-Click usually works) to select them all and drag them onto the upload area, or other options that you find that work.
  • You can also make a zip archive of all of your files and upload them all at once if you prefer.

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?