How to add a new line of text to an existing file in Java? [duplicate]

you have to open the file in append mode, which can be achieved by using the FileWriter(String fileName, boolean append) constructor.

output = new BufferedWriter(new FileWriter(my_file_name, true));

should do the trick

The solution with FileWriter is working, however you have no possibility to specify output encoding then, in which case the default encoding for machine will be used, and this is usually not UTF-8!

So at best use FileOutputStream:

    Writer writer = new BufferedWriter(new OutputStreamWriter(
        new FileOutputStream(file, true), "UTF-8"));

Starting from Java 7:

Define a path and the String containing the line separator at the beginning:

Path p = Paths.get("C:\\Users\\first.last\\test.txt");
String s = System.lineSeparator() + "New Line!";

and then you can use one of the following approaches:

1. Using Files.write (small files):

   try {
       Files.write(p, s.getBytes(), StandardOpenOption.APPEND);
   } catch (IOException e) {
       System.err.println(e);
   }
   

2. Using Files.newBufferedWriter(text files):

   try (BufferedWriter writer = Files.newBufferedWriter(p, StandardOpenOption.APPEND)) {
       writer.write(s);
   } catch (IOException ioe) {
       System.err.format("IOException: %s%n", ioe);
   }
   

3. Using Files.newOutputStream (interoperable with java.io APIs):

   try (OutputStream out = new BufferedOutputStream(Files.newOutputStream(p, StandardOpenOption.APPEND))) {
       out.write(s.getBytes());
   } catch (IOException e) {
       System.err.println(e);
   }
   

4. Using Files.newByteChannel (random access files):

   try (SeekableByteChannel sbc = Files.newByteChannel(p, StandardOpenOption.APPEND)) {
       sbc.write(ByteBuffer.wrap(s.getBytes()));
   } catch (IOException e) {
       System.err.println(e);
   }
   

5. Using FileChannel.open (random access files):

   try (FileChannel sbc = FileChannel.open(p, StandardOpenOption.APPEND)) {
       sbc.write(ByteBuffer.wrap(s.getBytes()));
   } catch (IOException e) {
       System.err.println(e);
   }
   

Details about these methods can be found in the Oracle's tutorial.

Try: "\r\n"

Java 7 example:

// append = true
try(PrintWriter output = new PrintWriter(new FileWriter("log.txt",true))) 
{
    output.printf("%s\r\n", "NEWLINE");
} 
catch (Exception e) {}

In case you are looking for a cut and paste method that creates and writes to a file, here's one I wrote that just takes a String input. Remove 'true' from PrintWriter if you want to overwrite the file each time.

private static final String newLine = System.getProperty("line.separator");

private synchronized void writeToFile(String msg)  {
    String fileName = "c:\\TEMP\\runOutput.txt";
    PrintWriter printWriter = null;
    File file = new File(fileName);
    try {
        if (!file.exists()) file.createNewFile();
        printWriter = new PrintWriter(new FileOutputStream(fileName, true));
        printWriter.write(newLine + msg);
    } catch (IOException ioex) {
        ioex.printStackTrace();
    } finally {
        if (printWriter != null) {
            printWriter.flush();
            printWriter.close();
        }
    }
}