Rows to CSV in Java
Java interview exercise on Streams & lambdas, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write toCsv(rows) that renders a list of rows (each a list of strings) as CSV text: fields joined by commas, rows joined by newlines, no trailing newline. A field containing a comma or a double quote is wrapped in double quotes, with inner double quotes doubled. An empty rows list gives "".
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.