title: Count not null row in Excel file using Apache POI
body: <p>Does Apache POI provide any function for us to count the number of "not-null" rows in a sheet of an Excel file?<br>
  At the first time, I have an Excel sheet with 10 data rows, the function worksheet.getPhysicalNumberOfRows() returns the exact number (10). But after that, I delete 3 rows, then that function still gets 10 rows. Maybe the total number of rows was cached anywhere by POI. What does getPhysicalNumberOfRows() mean? As its API described: "Returns the number of physically defined rows (NOT the number of rows in the sheet)", but I do not understand what "physically defined" mean. Can you help me on this issue?<br>
  Thank you so much!</p>