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Hi everyone,

I'm working on an app that saves data on a text file, as we know, formatting text file is awful and not very user friendly. I'd like to save the information I've collected on my program on a nicely formatted excel file. Given the file it's already created, just need to open it and add records to it. How can I approach to doing this?

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Good call — and a nice follow-up by pointing at a library that manipulates Excel directly. Modern, practical options are:

  • Java: Apache POI (HSSF for .xls, XSSF for .xlsx, SXSSF for large streaming writes).
  • Python: openpyxl (for .xlsx; pandas can drive openpyxl or xlsxwriter).
    Prefer .xlsx for formatting and modern features; .xls is older and more limited.

A typical Java workflow (open existing workbook, append a row, save safely):

try (FileInputStream in = new FileInputStream("data.xlsx");
     Workbook wb = WorkbookFactory.create(in)) {
    Sheet s = wb.getSheetAt(0);
    int last = s.getLastRowNum();
    Row r = s.createRow(last + 1);
    r.createCell(0).setCellValue("value");
    try (FileOutputStream out = new FileOutputStream("tmp.xlsx")) {
        wb.write(out);
    }
}
Files.move(Paths.get("tmp.xlsx"), Paths.get("data.xlsx"),
           StandardCopyOption.REPLACE_EXISTING);

A minimal Python using openpyxl to append and save:

from openpyxl import load_workbook
wb = load_workbook("data.xlsx")
ws = wb.active
ws.append(["value1", 42, 3.14])
wb.save("data.xlsx")

Practical tips not in the replies: always write to a temporary file then atomically replace the original to avoid corruption and to handle Windows locking. If files are large, use streaming writers (POI SXSSF or openpyxl write_only) to keep memory low. Keep a preformatted template workbook (headers, column widths, styles) and copy it as the base — applying cell styles programmatically is possible but slower. Watch types (numbers vs strings) and dates (use datetime objects so libraries convert properly). Finally, handle concurrency: if multiple processes/users may add rows, use file locks or a centralized store (database) to avoid race conditions.

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Great stuff! Thanks

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