import com.google.code.externalsorting.ExternalSort;

//... inputfile: input file name
//... outputfile: output file name
// next command sorts the lines from inputfile to outputfile
ExternalSort.mergeSortedFiles(ExternalSort.sortInBatch(new File(inputfile)), new File(outputfile));
// you can also provide a custom string comparator, see API

import com.google.code.externalsorting.CsvExternalSort;
import com.google.code.externalsorting.CsvSortOptions;

// provide a comparator
Comparator<CSVRecord> comparator = (op1, op2) -> op1.get(0).compareTo(op2.get(0));
//... inputfile: input file name
//... outputfile: output file name
//...provide sort options
CsvSortOptions sortOptions = new CsvSortOptions
				.Builder(comparator, CsvExternalSort.DEFAULTMAXTEMPFILES, CsvExternalSort.estimateAvailableMemory())
				.charset(Charset.defaultCharset())
				.distinct(false)
				.numHeader(1)
				.skipHeader(false)
				.format(CSVFormat.DEFAULT)
				.build();
// container to store the header lines
ArrayList<CSVRecord> header = new ArrayList<CSVRecord>();

// next two lines sort the lines from inputfile to outputfile
List<File> sortInBatch = CsvExternalSort.sortInBatch(file, null, sortOptions, header);
// at this point you can access header if you'd like.
CsvExternalSort.mergeSortedFiles(sortInBatch, outputfile, sortOptions, true, header);