Module 3 · Exceptions, files and the interview set Lesson 8 of 15
Reading a file and writing a report
Reading a CSV into objects and writing a summary out is the shape of the first task in most enterprise teams. It is also the smallest project that proves you can work in a real codebase.
The concept
The fresher file task is always the same: read a CSV, do something to each row, write a report. Files.readAllLines(Path.of("data.csv")) gives you the lines, splitting each on a comma gives you the fields, and Files.writeString or a BufferedWriter writes the output.
Write it as three methods — read, process, write — and the whole thing becomes testable and explainable in an interview. Handle a missing file with a readable message rather than a stack trace, and skip the blank last line rather than crashing on it, which is what real exports always contain.
Map<String, Integer> totals = new LinkedHashMap<>();
try (BufferedReader r = new BufferedReader(new FileReader("raw.csv"))) {
r.readLine(); // skip the header
String line;
while ((line = r.readLine()) != null) {
String[] cells = line.split(",");
if (cells.length < 3 || cells[1].isBlank()) continue;
totals.merge(cells[1].trim(),
Integer.parseInt(cells[2].trim()), Integer::sum);
}
}
try (PrintWriter w = new PrintWriter("report.csv")) {
totals.forEach((city, n) -> w.println(city + "," + n));
}Common mistakes
The mistakes are hard-coding an absolute path from your own laptop, and using split(",") on text where a field contains a comma inside quotes. Use line.split(",", -1) so trailing empty fields are kept, and print the first three rows before trusting any of the parsing.
Worked example
Reading line by line with split(",") and building a Student per line gives you a list you can total, sort and print — the same program an MIS team would ask a fresher to write.
Your practice task
Add a malformed line to your CSV on purpose and make the program report the line number and keep going instead of crashing.
Do this before moving on. Reading a solution teaches the shape of it; typing it teaches you.
Key takeaway
The fresher file task is always the same: read a CSV, do something to each row, write a report.
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