Friday, May 13, 2011

A Java CSV File Reader

One of the most common types of data file is a CSV (Comma Separated Value) file. They can be exported by many popular applications, notable spreadsheet programs like Excel and Numbers. They are easy to read into your Java programs once you know how.

Reading the file is as simple as reading a text file. The file has to be opened, a BufferedReader object is created to read the data in a line at a time.

Once a line of data has been read, we make sure that it's not null, or empty. If it is, we've hit the end of the file and there's no more data to read. If it isn't, we then use the split() method that's a member of Java's String object. This will split a string into an array of Strings using a delimiter that we give it.

The delimiter for a CSV file is a comma, of course. Once we've split() the string, we have all the element in an Array from which our Java programs can use the data. For this example, I just use a for loop to print out the data, but I could just as well sort on the values of one of the cells, or whatever I need to do with it in my program.

The Steps

  1. Open the file with a BufferedReader object to read it a line at a time.

  2. Check to see if we've got actual data to make sure we haven't finished the file.

  3. Split the line we read into an Array of String using String.split()

  4. Use our data.


The Program
// CSVRead.java
//Reads a Comma Separated Value file and prints its contents.

import java.io.*;
import java.util.Arrays;

public class CSVRead{

 public static void main(String[] arg) throws Exception {

  BufferedReader CSVFile = 
        new BufferedReader(new FileReader("Example.csv"));

  String dataRow = CSVFile.readLine(); // Read first line.
  // The while checks to see if the data is null. If 
  // it is, we've hit the end of the file. If not, 
  // process the data.

  while (dataRow != null){
   String[] dataArray = dataRow.split(",");
   for (String item:dataArray) { 
      System.out.print(item + "\t"); 
   }
   System.out.println(); // Print the data line.
   dataRow = CSVFile.readLine(); // Read next line of data.
  }
  // Close the file once all data has been read.
  CSVFile.close();

  // End the printout with a blank line.
  System.out.println();

 } //main()
} // CSVRead


Downloads

This program, and an example CSV file to use it with (a section of a spreadsheed I use to keep track of my integrated circuits) are available at my code archive.

Writing to CSV Files with Java

Writing to a CSV file is as simple as writing a text file. In this case, we write a comma between each field, and a newline at the end of each record.

Give it a try, starting with TextSave.java, modify it appropriately, then see what your favorite spreadsheet program thinks of the results.
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