parse a fixed-width file and convert to tab-separated (fields trimmed)

Superfile.java

import java.io.*;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.Calendar;
import java.util.zip.GZIPInputStream;

public final class Superfile {
    public static String SuperfileToTSV(String inputFileName, String outputFileName) {
        try {
            FileInputStream instream = new FileInputStream(inputFileName);
            InputStreamReader rdr;
            GZIPInputStream gz = null;
            if (inputFileName.endsWith(".gz")) {
                gz = new GZIPInputStream(instream, 4096);
                rdr = new InputStreamReader(gz, "ibm437"); //aliases: ibm437 437 ibm-437 cspc8codepage437 cp437 windows-437
            } else {
                rdr = new InputStreamReader(instream, "ibm437"); //aliases: ibm437 437 ibm-437 cspc8codepage437 cp437 windows-437
            }
            BufferedReader in = new BufferedReader(rdr);

            FileOutputStream outFile = new FileOutputStream(outputFileName);
            OutputStreamWriter outStream = new OutputStreamWriter(outFile, "UTF-8");
            short[] fldEnds = {2, 10, 16, 21, 28, 29, 59, 89, 119,
                    149, 162, 167, 171, 201, 211, 220, 229, 233, 237,
                    245, 253, 261, 269, 277, 282, 285, 288, 293, 305,
                    314, 325, 328, 345, 350};
            StringBuilder sb = new StringBuilder(400000);
            String ln = in.readLine();
            int rowsInBuffer = 0;
            while (ln != null && ln.length() > 300) {
                int pstart = 0;
                for (int pend : fldEnds) {
                    sb.append(ln.substring(pstart, pend).trim());
                    sb.append('\t');
                    pstart = pend;
                }
                sb.append("\n");
                if (++rowsInBuffer > 999) {
                    outStream.write(sb.toString());
                    sb.setLength(0);
                    rowsInBuffer = 0;
                }
                ln = in.readLine();
            }
            if (rowsInBuffer > 0) {
                outStream.write(sb.toString());
            }
            if (gz != null) {
                gz.close();
            }
            instream.close();
            outStream.close();
            outFile.close();
        } catch (Exception x) {
            // IOException, FileNotFoundException, UnsupportedEncodingException
            return "Error: Exception=" + x.getMessage();
        }

        // next steps (done in CFML):
        // copy stg_superfile from '[outputFileName]' with (format 'text');
        // then select * from import_superfile()
        // then call superfile_post_import()
        return "Success: Conversion completed";
    }
}