ApachePOI Null Pointer Exception - excel

Question

I am running into a strange null pointer exception when attempting to create a sheet using ApachePOI. I require a more modular code as this is part of a larger system. I have been unable to find any similar examples online, all examples are stored within a main method.
I am also wondering when it is necessary to include a workbook.write(outputStream) does this need to be in every method that a new sheet/workbook/row/cell is created or only when I am writing data to the sheets (like in the write method)?
public class ExcelWriter {
private XSSFWorkbook workbook;
private String outputFile;
private ArrayList<XSSFSheet> sheets;
private static int sheetCount;
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//Constructor --> initializes a workbook
public ExcelWriter(String outputFile) throws IOException {
XSSFWorkbook workbook = new XSSFWorkbook();
this.outputFile=outputFile;
sheets=new ArrayList<XSSFSheet>();
sheetCount=0;
FileOutputStream os = new FileOutputStream(outputFile);
workbook.write(os);
os.close();
}
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//Method to create a new sheet in the workbook, add to sheet list
public void newSheet(String sheetName) throws IOException {
sheets.add(workbook.createSheet(sheetName)); //ERROR HERE****
sheetCount++;
FileOutputStream os = new FileOutputStream(outputFile);
workbook.write(os);
os.close();
}
//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//Method to write a cell into a worksheet given sheet row and column
public void write(int sheet,int row, int col, String data) {
XSSFSheet tempSheet=sheets.get(sheet);
System.out.println("yest");
// Specific row number
XSSFRow tempRow = tempSheet.createRow(row);
// Specific cell number
XSSFCell cell = tempRow.createCell(col);
// putting value at specific position
cell.setCellValue(data);
// writing the content to Workbook
OutputStream os;
try {
os = new FileOutputStream(outputFile);
workbook.write(os);
} catch (IOException e) {
e.printStackTrace();
}
}