import java.io.*;

public class main {
  static String prelude =
    "public class main {\n" +
    "  public static void main(String[] args) {\n";
  static String postlude =
    "  }\n" +
    "}\n";

  public static void main(String[] args) throws IOException {
    String code = loadTextFile("input/main.java", null);
    if (code == null)
      throw new RuntimeException("Nothing to do (no input/main.java)");
    saveTextFile("output/main.java", prelude + code + postlude);
    if (false) System.out.println("output/main.java saved!");
  }

  /** writes safely (to temp file, then rename) */
  public static void saveTextFile(String fileName, String contents) throws IOException {
    File file = new File(fileName);
    File parentFile = file.getParentFile();
    if (parentFile != null)
      parentFile.mkdirs();
    String tempFileName = fileName + "_temp";
    FileOutputStream fileOutputStream = new FileOutputStream(tempFileName);
    OutputStreamWriter outputStreamWriter = new OutputStreamWriter(fileOutputStream, "UTF-8");
    PrintWriter printWriter = new PrintWriter(outputStreamWriter);
    printWriter.print(contents);
    printWriter.close();
    if (file.exists() && !file.delete())
      throw new IOException("Can't delete " + fileName);

    if (!new File(tempFileName).renameTo(file))
      throw new IOException("Can't rename " + tempFileName + " to " + fileName);
  }

  public static String loadTextFile(String fileName, String defaultContents) throws IOException {
    if (!new File(fileName).exists())
      return defaultContents;

    FileInputStream fileInputStream = new FileInputStream(fileName);
    InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream, "UTF-8");
    return loadTextFile(inputStreamReader);
  }

  public static String loadTextFile(Reader reader) throws IOException {
    StringBuilder builder = new StringBuilder();
    try {
      BufferedReader bufferedReader = new BufferedReader(reader);
      String line;
      while ((line = bufferedReader.readLine()) != null)
        builder.append(line).append('\n');
    } finally {
      reader.close();
    }
    return builder.length() == 0 ? "" : builder.substring(0, builder.length()-1);
  }
}

Began life as a copy of #2000317