Select Existing Sheet - apache-poi

Question

How do I select an existing sheet that I know the name of the sheet? I want to append code to a sheet but in different columns. In the code below, I am referring to the wb.creatsheet("Summary") line.
def sql_a = """
SELECT MOPACTIVITY.MOPID FROM MOPUSER.MOPACTIVITY WHERE UPPER(MOPACTIVITY.mopid) = 'JONATHAN MORNINGSTAR'
ORDER BY MOPACTIVITY.MOPID
"""
def date = new Date()
def dts = date.format("yyyy-MM-dd-HH-mm-ss")
File file = new File('X:/CCG/' + dts + ' CCG REPORT.xls')
file.write("")
Workbook wb = new HSSFWorkbook()
try {
Sheet sheet = wb.createSheet("Summary")
sheet.setColumnWidth(0,8.43 * 256)
int rowNum = 0
Row row = sheet.createRow(rowNum)
Cell cellMOPID = row.createCell(0)
cellMOPID.setCellStyle(headerStyle)
cellMOPID.setCellValue("")
db.eachRow(sql_a) {
rowNum++
row = sheet.createRow(rowNum)
cellMOPID = row.createCell(0)
cellMOPID.setCellValue(it.MOPID)
file.append(it.MOPID + "\n")
}
} catch (Exception e) { System.out.println(e.toString()) }

Answer 1

If you look at the JavaDocs for the Workbook interface, you'll see that as well as createSheet(String) there's also a getSheet(String) method. Call that with your existing sheet name, and you'll get the existing sheet to work with
Your code will want to be something like:
Workbook wb = WorkbookFactory.create(new File("input.xls"));
Sheet sheet = wb.getSheet("Summary");
// Read and add to the existing sheet here!