14 Oct 2013

Java: Incrementally read/stream a CSV file

I’ve been doing some work which involves reading in CSV files, for which I’ve been using OpenCSV, and my initial approach was to read through the file line by line, parse the contents and save it into a list of maps.

This works when the contents of the file fit into memory but is problematic for larger files where I needed to stream the file and process each line individually rather than all of them after the file was loaded.

I initially wrote a variation on totallylazy’s https://code.google.com/p/totallylazy/source/browse/src/com/googlecode/totallylazy/Strings.java?spec=svna4a6ac3d443db02821d434a5d6560cb77ec4ef4a&r=a4a6ac3d443db02821d434a5d6560cb77ec4ef4a#55 to do this and while I was able to stream the file I made a mistake somewhere which meant the number of maps on the heap was always increasing.

After spending a few hours trying to fix this Michael suggested that it’d be easier to use an iterator instead and I ended up with the following code:

public class ParseCSVFile {
    public static void main(String[] args) throws IOException
    {
        final CSVReader csvReader = new CSVReader( new BufferedReader( new FileReader( "/path/to/file.csv" ) ), '\t' );
        final String[] fields = csvReader.readNext();

        Iterator<Map<String, Object>>() lazilyLoadedFile = return new Iterator<Map<String, Object>>()
        {
            String[] data = csvReader.readNext();

            @Override
            public boolean hasNext()
            {
                return data != null;
            }

            @Override
            public Map<String, Object> next()
            {
                final Map<String, Object> properties = new HashMap<String, Object>();
                for ( int i = 0; i < data.length; i++ )
                {
                    properties.put(fields[i], data[i]);
                }

                try
                {
                    data = csvReader.readNext();
                }
                catch ( IOException e )
                {
                    data = null;
                }

                return properties;
            }

            @Override
            public void remove()
            {
                throw new UnsupportedOperationException();
            }
        };
    }
}

Although this code works it’s not the most readable function I’ve ever written so any suggestions on how to do this in a cleaner way are welcome.

About the author

I'm currently working on short form content at ClickHouse. I publish short 5 minute videos showing how to solve data problems on YouTube @LearnDataWithMark. I previously worked on graph analytics at Neo4j, where I also co-authored the O'Reilly Graph Algorithms Book with Amy Hodler.