How to add table heading drop-down with Apache Poi

Question

I'm generating Excel tables with Apache POI, but my generated tables lack the drop-down menu on each header that appear when I "format as table" in Excel itself.

I'd like to generate this:

But instead I get this:

I'm following this blog post, and my code looks like this:

        XSSFTable table = sheet.createTable();
        table.setDisplayName("Data");
        CTTable ctTable = table.getCTTable();
        ctTable.setDisplayName("Data");
        ctTable.setId(1L);
        ctTable.setName("DATA");
        CTTableStyleInfo table_style = ctTable.addNewTableStyleInfo();
        table_style.setName("TableStyleMedium9");
        table_style.setShowColumnStripes(false);
        table_style.setShowRowStripes(true);

Each column is then created like this:

            CTTableColumn column = ctColumns.addNewTableColumn();
            column.setName(headers.get(i));
            column.setId(i + 1);

What am I missing?

Answer 1

Thanks to Alan Hay for the clue - the solution is to add an auto-filter, but this needs to be added as a CTAutoFilter for each individual column of the CTTable. The working solution looks like this:

    CTTableColumns ctColumns = ctTable.addNewTableColumns();
    CTAutoFilter autofilter = ctTable.addNewAutoFilter();
    ctColumns.setCount(table_headers.size());

    for(int i = 0; i < table_headers.size(); i++) {
        CTTableColumn column = ctColumns.addNewTableColumn();
        column.setName(table_headers.get(i));
        column.setId(i + 1);
        CTFilterColumn filter = autofilter.addNewFilterColumn();
        filter.setColId(i + 1);
        filter.setShowButton(true);
    }

When auto-sizing columns, it's also necessary to add extra width for the drop down menu:

    for(int i = 0; i < table_headers.size(); i++) {
        sheet.autoSizeColumn(i);
        // Include width of drop down button
        sheet.setColumnWidth(i, sheet.getColumnWidth(i) + 1000);
    }