Cómo leer y escribir un archivo Excel

try {
    POIFSFileSystem fs = new POIFSFileSystem(new FileInputStream(file));
    HSSFWorkbook wb = new HSSFWorkbook(fs);
    HSSFSheet sheet = wb.getSheetAt(0);
    HSSFRow row;
    HSSFCell cell;

    int rows; // No of rows
    rows = sheet.getPhysicalNumberOfRows();

    int cols = 0; // No of columns
    int tmp = 0;

    // This trick ensures that we get the data properly even if it doesn't start from first few rows
    for(int i = 0; i < 10 || i < rows; i++) {
        row = sheet.getRow(i);
        if(row != null) {
            tmp = sheet.getRow(i).getPhysicalNumberOfCells();
            if(tmp > cols) cols = tmp;
        }
    }

    for(int r = 0; r < rows; r++) {
        row = sheet.getRow(r);
        if(row != null) {
            for(int c = 0; c < cols; c++) {
                cell = row.getCell((short)c);
                if(cell != null) {
                    // Your code here
                }
            }
        }
    }
} catch(Exception ioe) {
    ioe.printStackTrace();
}

Workbook wb = new HSSFWorkbook();
//Workbook wb = new XSSFWorkbook();
CreationHelper createHelper = wb.getCreationHelper();
Sheet sheet = wb.createSheet("new sheet");

// Create a row and put some cells in it. Rows are 0 based.
Row row = sheet.createRow((short)0);
// Create a cell and put a value in it.
Cell cell = row.createCell(0);
cell.setCellValue(1);

// Or do it on one line.
row.createCell(1).setCellValue(1.2);
row.createCell(2).setCellValue(
createHelper.createRichTextString("This is a string"));
row.createCell(3).setCellValue(true);

// Write the output to a file
FileOutputStream fileOut = new FileOutputStream("workbook.xls");
wb.write(fileOut);
fileOut.close();