code: private void addMonth(int i) {
          Calendar cal = Calendar.getInstance();
          cal.set(getDisplayYear(), getDisplayMonth() - 1, 1);
          int month = cal.get(Calendar.MONTH) + 1;
          if (i > 0 && month == 12) {
              addYear(1);
          }
  
          if (i < 0 && month == 1) {
              addYear(-1);
          }
          cal.add(Calendar.MONTH, i);
  
          monthTextField.setText(String.valueOf(cal.get(Calendar.MONTH) + 1));
  
      }
docs: Add month count to month field
  
  @param i month to add