Programming Assignment 3 – Classes and Conditionals Practice

This programming assignment will help you practice writing classes and conditionals. You’ll also get a chance to do some open-ended exploration of Java features.

You will upload several Java files for this assignment, detailed below. The starter code is available here:

https://github.com/ucsd-cse11-s20/pa3

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.

We’ve added a file called CREDITS.txt for you to write these credits into.

FAQ

1. How do we test our methods in PA3? Do we need to use the tester package provide? A: You can test them in the same way as we did in class and previous two PAs. You can use the tester package if you want, but we’re not grading for it. PA4 we will explicitly require it in some cases to make sure everyone gets practice with it. [Piazza @269]

2. For hasMention() method in class TextTweet, I think I correctly searched for “@username”, but why did some tests on my Gradescope submission fail?
   A: There are some interesting cases for this method. For example, to check if we have the username “dummy” in “@dummy1 @dummy2”, hasMention() should return false in this case because username dummy1 and dummy2 is not the same as dummy, while “hello @dummy world” and “CSE 11 is a cool class @dummy” should return true.

3. For firstMention() method in class TextTweet, do we need to include the “@” (at character) or “ “ (space character) in the resultant substring? A: The idea of this method is to return just the username of the first mention. This means the “@” and “ “ are to be omitted.

Drill 1 (auto)

Write three classes as described below. They do not need to have any methods or constructors. Save them in a file called Drill1.java.

• A class named A with two fields, int f1 and String f2
• A class named B with two fields, A field1 and String field2
• A class named C with three fields, B fieldB, A fieldA, int field3

Drill 2 (auto)

Create a file called Drill2.java containing the following class definitions (you can copy/paste them from here):

class C1 {
  C2 other;
  C1(C2 other) {
    this.other = other;
  }
}

class C2 {
  int x;
  C2(int x) {
    this.x = x;
  }
}

Then add a class definition called Drill2 with the following fields:

• A field named first of type C2 with its x field equal to 10
• A field named second of type C1. It’s value should be a reference to a C1 object with its other field set to any C2 object other than the one stored in first (you can create another C2 object for this).
• A field named third of type C1. It’s value should be a reference to a C1 object with its other field the same C2 object as the one stored in the first field.

Drill 3 (auto)

In a file called Drill3.java, write a class called TextTweet that has two fields: one field called contents of type String, and one field called likes of type int. Give it a default constructor of two arguments that initializes those fields. In it, write the following methods:

• hasMention which takes a String called username and checks if the string @ followed by that username apppears in the Tweet contents, returning true if it does and false otherwise.
• hasLike which takes no arguments and returns true if the tweet one or more likes, false otherwise.
• firstMention which takes no arguments and returns a String containing the substring between the first appearance of the @ character in the contents and the first space character after that. If there is no space after the @, or if there’s no @, the empty string "" should be returned.

Also in Drill3.java, write a class called ReplyTweet that has three fields: one called replyTo of type TextTweet, one called contents of type String, and one called likes of type int. Give it a default constructor that initializes these fields. In it, write the following methods:

• morePopularReply which takes no arguments and returns true if this ReplyTweet has more likes than the TextTweet it is replying to
• allLikes which takes no arguments and returns the sum of the likes on this ReplyTweet and on the TextTweet it is replying to
• hasMention which takes a String called username and checks if the string @ followed by that username apppears in this ReplyTweet’s contents or in the TextTweet that is being replied to.

We highly recommend adding a class to this file called Drill3 that has any tests or examples you used to help verify that your methods worked as you expected.

Drill 4 (auto)

In a file called Drill4.java, write a class Drill4 with the following methods. For each, write at least three interesting tests, and include the expected values.

• phaseOfWater which takes an int and returns "vapor" if the number is greater than or equal to 100, "liquid" if the number is less than 100 and greater than 0, and "solid" if the number is less than or equal to 0.
• maxDifference which takes three ints and returns the largest absolute difference between any two of them. For example, maxDifference applied to 1, -1, and 5 should return 6 because the difference between -1 and 5 is 6 which is greater than 2 or 4 (the differences between -1 and 1, and between 1 and 5).
  Remember that the Java documentation lists lots of helpful methods in the Math class!
• ringArea, which takes two doubles representing the radius of an inner circle and an outer circle, and returns the area of the ring between them as a double. Assume both inputs are positive and that the first number is smaller than the second. Recall that the area of a circle is πr². You can use Math.PI, a field conveniently defined for us by Java, as a constant for the value of π.

Open-Ended 1 (manual)

Consider the following statements about Java programs:

• Statement A: In Java, two different classes can define a field with the same name and type.
• Statement B: In Java, one class can define two fields with the same name as long as they have different types.
• Statement C: In Java, two different methods in the same class can have the same parameter name, and arguments passed to one of those methods don’t affect the parameter in the other.

For each, write a small Java program that demonstrates whether it is true or false. Put them in the files Open1A.java, Open1B.java, and Open1C.java.

To show that a statement is true, write a Java program that matches the statement, doesn’t produce an error when run, and produces some meaningful output when ./run. This means you may want to include an Examples class with some instances of the classes you create to demonstrate this.

To show that a statement is false, write a Java program that matches the statement and produces an error when run, demonstrating that Java programs cannot do what the property says.

Include both the program and the output of running the program without error as your submission; you can upload screenshots as images clearly named with Open1, Open2, etc in the title, or copy-paste the text output into a comment in your code.

Open-Ended 2 (manual)

Create a class named R that has a field of type String and a field of type R. Give it a default constructor that initializes both fields. Put the class in a file called ExamplesR.java. Add an ExamplesR class to this file, and answer the following questions in that file:

1. Construct an example R object. Were you able to? Explain your example if you were able to, and explain why you think it’s not possible if you weren’t.

2. On Twitter, it’s possible to reply to a reply to a Tweet (that’s not a typo, it’s a reply to a reply). This is true of many systems, like email, Facebook comments, Piazza followups, and so on. With the class structure in Drill3 with ReplyTweet and TextTweet (that is, without changing the fields as described above), could you construct an example of a reply to a reply to a Tweet? Why or why not?

Submission

You will submit all of your files to the pa3 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.

The parts marked (auto) will be graded automatically in Gradescope based on tests we wrote. The parts marked (manual) will be graded by the course staff after the due date. 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.