When MS Excel, Libreoffice opens - they create an empty spread sheet with name "Sheet1". 

XSSFSheet (and potentially other implementations) calls it "Sheet0" that is inconvenient. 

    public XSSFSheet createSheet() {
        String sheetname = "Sheet" + (sheets.size());
        int idx = 0;
        while(getSheet(sheetname) != null) {
            sheetname = "Sheet" + idx;
            idx++;
        }
        return createSheet(sheetname);
    }


it is worth to change it to 

    public XSSFSheet createSheet() {
        String sheetname = "Sheet" + (sheets.size() + 1);
        int idx = 0;
        while(getSheet(sheetname) != null) {
            sheetname = "Sheet" + idx;
            idx++;
        }
        return createSheet(sheetname);
    }