Homework 1A (CS 111/112/17/19): Classes and Methods

Due: Tuesday, February 1st, 2022 at 11:59pm ET

Collaboration Policy: You may collaborate as much as you want on this assignment (this is more flexible than the normal course policy). The goal is to get everyone up to speed on Java. You are required to turn this in, but it will be weighted lightly in final grades. That said, we strongly encourage you to actively try writing these on your own while collaborating, as assignments after we come back together will assume you can do these sorts of problems on your own.

Need help? Find us, as well as questions and answers from other students, on Ed. Here's our office hours schedule.

Handin Instructions are at the end of the document.

Learning Objectives

• Practice writing classes, methods, and tests in Java
• Learn how to raise errors in Java

Stencil Code and Assignment Setup

Using this GitHub Classroom link, accept the assignment and clone on your machine. See our GitHub Guide for more detailed instructions.

Next, check out our IntelliJ Setup Guide and make sure your IntelliJ has been downloaded and configured properly for this course. If you're having trouble, the guide includes a Common Bugs/FAQ section which might help.

In order to use JUnit (the framework you will be using to write tests in this class), you will need to add both the hamcrest-core-1.3.jar and junit-4.13.2.jar files as dependencies to your IntelliJ project. If you're not sure how to do this, check out the Adding Jars/Dependencies section of the IntelliJ Setup Guide.

Please make sure to read our gradescope submission guide before handing in your assignment!

Style Expectations

Note: Click on blue headers throughout for a fun surprise :)

While you may have seen references to the course style guide, for this first assignment we'll only be expecting the following style requirements:

• Basic Javadocs for all methods (you can look in the class notes for examples of these)
• Method and variable names are in camelCase
• Class names are in UpperCamelCase

For this assignment (1A), you should not use access modifiers for fields (private fields could break our autograder) If you don't know what they are yet, don't worry about it.

When creating fields make sure to name them exactly what is given in the handout. If you do not do this, the autograder will fail.

The handout guides you through developing the code in stages. You will turn in one set of files with your cumulative work on all tasks. You do not need to maintain versions of your code from each task separately.

Relevant Documentation

Java is a large language with lots of optional code libraries and different ways of doing things. Honestly, we believe that going into the Java documentation for the kinds of questions arising in this homework will likely leave you more confused and less confident. The lecture materials should have everything you need. If not, ask us on Ed. We'll get to using the documentation on the next assignment.

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Problem Setup

It's the early 2000s! Brown wants to make a C@B-like website and you've been tasked to represent all the information about courses, faculty, and students. Each faculty member teaches one course. Students must take two courses each. Faculty are permitted to give grades to students taking the course that they are teaching.

Courses

Let's start by creating a class representing a Course.

Task: Create a class named Course.

• The class should have three fields:
  • department, representing the department the course is being taught on (a String like "CSCI"),
  • courseNumber, representing the course number of the course (an int like 200),
  • credits, representing the number of credits the course gives (a double that should be one of 0.5, 1, or 1.5).
• The constructor of the class should take values for all three fields as inputs and set the field variables to these values.

At this point, make sure you are able to run your code and create an object from the Course class. You can do this by writing a JUnit test in the Homework1ATest.java file (more details about testing can be found in the Testing your work and FAQ sections of this handout). This will confirm that you have everything set up properly.

Task: Add a second constructor that takes only the department and course number as inputs, setting the credits to 1 (by default).
Since 1-credit courses are the default, we can reduce the chance of errors by allowing someone to omit the credits and having a constructor set the default value. In Java, a class can have multiple constructors, as long as their signatures are not the same. A constructor's signature is determined by the number and type of its parameters.

Task: Add an error-checking mechanism to the three-argument constructor.
The three-argument constructor allows someone to create an object with an invalid number of credits. Let's add some error checking to the constructor. Edit the body of the constructor to check whether the entered value is one of 0.5, 1, or 1.5. If it is not, use the following statement to raise an error message. Errors in Java are called exceptions and throwing one will stop your program.

throw new IllegalArgumentException("Invalid credits");

Note: Java has different kinds of exceptions for different kinds of errors. For now, we will use an IllegalArgumentException when an input is not from an expected set of values.

It helps for each object to have a string-based representation that we can use to print out the object in a readable form. In Java, every class has a default method named toString that shows how to display that class as a string (by returning a String—note the capital letter).

Task: Write a method called toString in the Course class that takes no input and returns a String, combining the department and the course number.
The string representation of a class should combine the department and the course number. For example, a course in department "CSCI" with number 200 should yield the string "CSCI200".

// Sample output
"CSCI200"

You can concatenate, or combine, strings with the + operator. You can also concatenate a String and a int with the + operator to create a new String in Java.

Here's a method stub for a generic toString method:

@Override
public String toString()

You do not have to write Javadocs for this method.

Why do we have to use an @Override tag?

This is because a version of the toString method is automatically defined on all objects (you can try this by trying to print a Course object without defining a toString method). To fix this you should add the @Override annotation above the method definition to tell IntelliJ that you know you are changing this already defined method.

(To give a more technical explanation, this is because the default toString method is actually found in the Object class, which all classes in Java inherit. Therefore, in order to tell Java to use your toString instead, we use the @Override tag).

Faculty and Students

Now let's create some classes to represent Faculty and Student.

Task: Create a class named Faculty.

• The class should have three fields:
  • name, representing the name of the faculty member (a String like "Tall Kathi"),
  • department, representing the department that the faculty member teaches in (a String like "CSCI"),
  • teaching, representing the course that they are currently teaching. For this assignment, we will assume that every faculty member is teaching exactly one course.
• The constructor of the class should take values for all three fields as inputs and set the field variables to these values.

Task: Define a method isTeaching for faculty that takes a Course and returns a boolean indicating whether the faculty member is teaching that course.

Note: Use == to check for sameness for this and all other comparison questions for this assignment.

Task: Create a class named Student.

• The class should have three fields:
  • name, representing the name of the student (a String like "Tall Aaron")
  • course1 and course2, representing the two courses that the student is taking (both of type Course). Assume that every student takes exactly two courses, and doesn't add or drop courses.
• The constructor of the class should take values for all three fields as inputs and set the field variables to these values.
• The constructor of Student should throw an IllegalArgumentException with the error message "Invalid courses" if the two courses are the same.

Task: Define a method isTaking in Student that takes a Course and returns a boolean indicating whether this course is one of the two courses that the student is taking.

Task: Define a method totalCredits in Student that takes no inputs and returns the sum of the credits for the student's two courses.

Task: Add a method called canGrade to the Faculty class.
Faculty can only grade students in their own courses. Add a method to the Faculty class called canGrade that takes a Student as input and returns a boolean indicating whether the faculty member is allowed to give a grade to that student.

Study Question: We could have represented courses within faculty and students as strings, such as "CSCI0200". What's the advantage of using objects instead? Is there an advantage to using the strings? (No need to turn this in, but think about it!)

Testing your work

We will be writing tests in JUnit, a testing framework for Java. The stencil code gives you a file Homework1ATest.java that includes the basic structure of a test. You'll test the methods and constructors that you wrote by adding to this file. See the FAQ section below for more in depth instructions on this!

Task: Write JUnit tests for the totalCredits method in the Student class.

Task: Write JUnit tests for the canGrade method in the Faculty class.

Task: Write JUnit tests to check that the Course constructor throws an exception if given an invalid number of credits.

For this assignment, you don't need to test any other methods. We're just trying to get you familiar with Java, JUnit, and how the pieces fit together.

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How to Hand In

In order to hand in your solutions to these problems, they must be stored in appropriately-named files with the appropriate package header in an appropriately-named directory.

Your solution code files should be in the sol package. This means that all your solution code should have a line at the top saying package sol; and they should be in the hw01-classes/sol/ directory.

After completing this assignment, your hw01a-classes/sol/ directory should contain the following files:

• AutograderCompatibility.java containing public class AutograderCompatibility
• Course.java containing public class Course
• Faculty.java containing public class Faculty
• Homework1ATest.java containing public class Homework1ATest
• Student.java containing public class Student
• TestRunner.java containing public class TestRunner

To hand in your homework, submit the following files to the Homework 1A: Implementation assignment on Gradescope (make sure to exclude the AutograderCompatibility.java and TestRunner.java files from your submission):

• Course.java containing public class Course
• Faculty.java containing public class Faculty
• Homework1ATest.java containing public class Homework1ATest
• Student.java containing public class Student

Once you have handed in your homework, you should receive an email, more or less immediately, confirming your turn-in.

Note on Autograder Compatibility: There should be a class in the stencil code named AutograderCompatibility. Using this class is required to ensure that your submission is working correctly with the autograder. You will be penalized if your code does not work with the autograder.

If Gradescope gives you message “The autograder failed to execute correctly. Please ensure that your submission is valid. Contact your course staff for help in debugging this issue. Make sure to include a link to this page so that they can help you most effectively," uncomment the main method of AutograderCompatibility and check that it compiles and runs. If the Gradescope autograder still doesn't work, come to hours or post on Ed for help.

FAQ

How do I use decimals in Java? What is a double?
A double is a data type that represents a decimal number. You can initialize a double similarly to an int, like so:

double myDouble = 0.4;

How do I test Exceptions?

The format for checking Exceptions is as follows:

@Test(expected = <exception-type>.class)
public void <test-name>() {
    // code that results in exception
}

In practice, it looks something like this:

@Test(expected = IllegalArgumentException.class)
public void courseIllegalArgumentException() {
    Course cs200 = new Course("CSCI", 200, 2.0);
}

Where creating a Course with 2.0 credits throws an IllegalArgumentException.

How do I test methods?

The format for testing a method is as follows:

@Test
public void <test-name>() {
    // code that tests the specific method 
}

In practice, it looks something like this:

@Test
public void testOne() {
    String name = this.professor.name;
    assertEquals(name, "Milda");
    Course cs200 = new Course("CSCI", 200, 1.0);
    assertEquals(cs200.credits, 1, 0.01);
}

Here we're first checking if name equals Milda. In the second example, we check to see if credits returned is within 0.01 of 1.0. We need to include 0.01 as an argument in this case because double comparisons sometimes have slight imprecisions.

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Please let us know if you find any mistakes, inconsistencies, or confusing language in this or any other CS200 document by filling out the anonymous feedback form!