title: Assign 'NULL'/'EMPTY' to blank cells in R instead of NA using read.xlsx
body: <p>I am reading my data using read.xlsx by the following lines of code:</p>
  
  <pre><code> Data &lt;- read.xlsx(file.path(directory,file), sheetName=data,
                      colClasses=Types, stringsAsFactors=FALSE, endRow = dataEnd)
  </code></pre>
  
  <p><code>Types</code> consists of the allowed data values for various columns of the data. So after I have read the data all the blank values and wrong data is assigned <code>NA</code>. Whereas I would like to specifically set <code>NULL</code> or <code>Empty</code> to the blank values instead of <code>NA</code>. So that only the wrong data will consist of <code>NA</code> values.</p>
  
  <p>One way was to read the data twice once without the attribute <code>colClasses</code> and once with the attribute but that definetly is not the optimal way. Is there any attribute that can be included to <code>read.xlsx</code> so that all the blank values will be assigned a particular string value upon reading?</p>
  
  <p>Edit : 
  by using na.strings we are setting blank values to NA. I wanted an explicit string to be added to the blank cell instead of NA. So that is what my actual requirement is. Is it possible to do that while reading the data? 
  For example :
  If there is a cell with empty value, I need to add "empty" string to the cell.</p>