Lab 19: Sortable Roster

After this lab, you will be able to:

• Implement the Comparable interface with a compareTo method
• Sort objects using Arrays.sort with natural ordering
• Add a multi-field sort with tiebreakers
• Read objects from a file, sort them, and write the sorted result back

What You’re Building

You will extend your Student class (or a fresh copy) to implement Comparable<Student>, enabling Arrays.sort to order students by GPA. When two students have the same GPA, the sort breaks ties alphabetically by name. The program reads a roster from a file, creates an array of Student objects, sorts them, and writes the sorted roster to an output file. This is a full-week lab because the interface pattern is new and the file round-trip adds integration complexity.

Timeline: This lab is assigned Monday and due Friday. Start with compareTo and test it in isolation before adding file I/O.

Concepts and Misconceptions

Checkpoints

Checkpoint 1: Implement Comparable and compareTo by GPA

What to do: Add implements Comparable<Student> to your class declaration. Write compareTo(Student other) that orders students by GPA in descending order (highest GPA first). Use Double.compare to avoid precision issues with subtraction.

What the test checks: compareTo returns a negative value when this student’s GPA is higher, a positive value when lower, and 0 when equal.

Debugging tip: If the sort order is backwards (ascending instead of descending), you have the arguments to Double.compare reversed. For descending GPA: Double.compare(other.gpa, this.gpa). If students with the same GPA produce a nonzero result, check that you are comparing only GPA in this step, not name.

Checkpoint 2: Name Tiebreaker and Arrays.sort

What to do: When two students have the same GPA, break the tie by comparing names alphabetically (ascending). Use this.name.compareTo(other.name) for the tiebreaker. In your main method, create an array of students and call Arrays.sort.

What the test checks: An array of students with mixed and duplicate GPAs is sorted by descending GPA, then ascending name.

Debugging tip: If the tiebreaker is not working, make sure you only fall through to the name comparison when the GPA comparison returns 0. A common pattern: int result = Double.compare(other.gpa, this.gpa); if (result != 0) return result; return this.name.compareTo(other.name);. If Arrays.sort throws a NullPointerException, check that every slot in your array has been assigned a Student object.

Checkpoint 3: Load from File, Sort, and Write Output

What to do: Read a roster file where each line contains a name, ID, and GPA separated by commas. Parse each line into a Student object and store them in an array. Call Arrays.sort on the array. Write the sorted roster to an output file using PrintStream, one student per line (use toString).

What the test checks: The output file contains all students in the correct sorted order, with each line matching the expected toString format.

Debugging tip: If parsing fails, print each line before splitting it. Check that you handle leading/trailing whitespace with trim(). When parsing GPA, use Double.parseDouble, not Integer.parseInt. If the output file is empty, verify that you close the PrintStream after writing.

How to Debug

1. Test compareTo in isolation. Create three students in main — one with a higher GPA, one lower, one equal — and print the results of compareTo for each pair. Verify the signs are correct before using Arrays.sort.

2. Print the array before and after sorting. Use Arrays.toString or a loop to print every element. This makes it obvious if the sort is working or if null elements are present.

3. Separate concerns. Get the sort working with hardcoded data first. Only then add file reading. This way, if something breaks, you know which part is the cause.

Scoring