Java Json to CSV converter using Jackson

JsonToCsv.java

package whatever.converters;

import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
import java.util.ArrayList;
import java.util.Collection;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;

public class JsonToCsv {

  public static void main(String[] args) throws IOException {
    String homeDir = System.getProperty("user.home");
    Path inFile = Paths.get(homeDir).resolve("Downloads").resolve("inputfile.json");
    Path outFile = inFile.resolveSibling(inFile.getFileName() + ".csv");
    flattenFile(inFile, outFile, ";", StandardCharsets.UTF_8);
  }

  public static void flattenFile(Path inFile, Path outFile, String delimiter, Charset charset) throws IOException {
    try(InputStream is = Files.newInputStream(inFile);
        OutputStream os = Files.newOutputStream(outFile, StandardOpenOption.CREATE, StandardOpenOption.WRITE, StandardOpenOption.TRUNCATE_EXISTING)) {
      flattenStream(is, os, delimiter, charset);
    }
  }

  public static void flattenStream(InputStream is, OutputStream os, String delimiter, Charset charset) throws IOException {
    List<Map<String, Object>> lines = readJson(is);
    Set<String> headers = new LinkedHashSet<>();
    lines.stream().flatMap(map -> map.keySet().stream()).forEach(headers::add);
    writeCsv(os, headers, lines, charset, delimiter);
  }

  private static List<Map<String, Object>> readJson(InputStream is) throws IOException {
    ObjectMapper objectMapper = new ObjectMapper();
    List<Map<String, Object>> lines = new ArrayList<>();

    Object obj = objectMapper.readValue(is, Object.class);
    if(obj instanceof List) {
      for (Object elt : (List<?>) obj) {
        lines.add(flatten(elt));
      }
    } else {
      lines.add(flatten(obj));
    }
    return lines;
  }

  public static Map<String, Object> flatten(Object in) {
    Map<String, Object> line = new LinkedHashMap<>();
    flattenObj(in, line, "");
    return line;
  }

  private static void flattenObj(Object in, Map<String, Object> out, String prefix) {
    if(in instanceof Map) {
      flattenMap((Map<?, ?>) in, out, prefix);
    } else if(in instanceof List) {
      flattenList((List<?>) in, out, prefix);
    } else {
      out.put(prefix, in);
    }
  }

  private static void flattenMap(Map<?, ?> in, Map<String, Object> out, String prefix) {
    in.forEach((k, v) -> {
      String header = appendSuffix(prefix, k);
      flattenObj(v, out, header);
    });
  }

  private static void flattenList(List<?> in, Map<String, Object> out, String prefix) {
    int i = 0;
    for (Object e : in) {
      String header = appendSuffix(prefix, i++);
      flattenObj(e, out, header);
    }
  }

  private static String appendSuffix(String header, Object suffix) {
    if(header != null) {
      return header + '.' + suffix;
    } else {
      return String.valueOf(suffix);
    }
  }

  private static void writeCsv(OutputStream os, Collection<String> headers, Collection<Map<String, Object>> lines, Charset charset, String delimiter) throws IOException {
    BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(os, charset));
    bw.write(String.join(delimiter, headers));
    bw.newLine();

    for (Map<String, Object> kvs : lines) {
      List<String> line = new ArrayList<>(headers.size());
      for (String header : headers) {
        line.add(toCsvString(kvs.get(header), delimiter));
      }
      bw.write(String.join(delimiter, line));
      bw.newLine();
    }
    bw.flush();
  }

  private static String toCsvString(Object v, String delimiter) {
    return Optional.ofNullable(v).map(o -> {
      String s = o.toString();
      if(s.contains(delimiter)) {
        s = '"' + s + '"';
      }
      return s;
    }).orElse("");
  }
}