Note that in addition to the answer of mcfinnigan, you must know that when you use the code:

FileOutputStream fos = new FileOutputStream(f); // create a file output stream around f
ftpClient.retrieveFile("/" + ftpFile.getName(), fos);

Then an empty file will be created on your filesystem on the first line. Then if the 2nd line throws an exception, because no remote file exist for path "/" + ftpFile.getName(), the empty file will still be on your filesystem.

So I've done a little LazyInitOutputStream with Guava to handle that:

public class LazyInitOutputStream extends OutputStream {

  private final Supplier<OutputStream> lazyInitOutputStreamSupplier;

  public LazyInitOutputStream(Supplier<OutputStream> outputStreamSupplier) {
    this.lazyInitOutputStreamSupplier = Suppliers.memoize(outputStreamSupplier);
  }

  @Override
  public void write(int b) throws IOException {
    lazyInitOutputStreamSupplier.get().write(b);
  }

  @Override
  public void write(byte b[]) throws IOException {
    lazyInitOutputStreamSupplier.get().write(b);
  }

  @Override
  public void write(byte b[], int off, int len) throws IOException {
    lazyInitOutputStreamSupplier.get().write(b,off,len);
  }


  public static LazyInitOutputStream lazyFileOutputStream(final File file) {
    return lazyFileOutputStream(file,false);
  }

  public static LazyInitOutputStream lazyFileOutputStream(final File file,final boolean append) {
    return new LazyInitOutputStream(new Supplier<OutputStream>() {
      @Override
      public OutputStream get() {
        try {
          return new FileOutputStream(file,append);
        } catch (FileNotFoundException e) {
          throw Throwables.propagate(e);
        }
      }
    });
  }

I've encoutered this problem while using Spring integration remote.file packages, with the FTP/SFTP file download features. I use it like that to solve this empty file problem:

try ( OutputStream downloadedFileStream = LazyInitOutputStream.lazyFileOutputStream(destinationfilePath.toFile()) ) {
  remoteFileSession.read(source, downloadedFileStream);
}

If you want to open reports after exporting/saving it, then this will definately work

       JasperReportBuilder report = DynamicReports.report();// a new report
       report.setDataSource(dataSource);

        String path = "d:\\PriceReport"+Math.random()+".pdf";
        File generatedfile= new File(path);
        FileOutputStream fileOutStream = new FileOutputStream(generatedfile);
        report.toPdf(fileOutStream);
        fileOutStream.close();
        fileOutStream.flush();
         if(generatedfile.exists()){
             if(Desktop.isDesktopSupported()){
                Desktop.getDesktop().open(generatedfile);
             }else{
                System.out.println("Not Supported by your desktop");
             }
         }else{
             System.out.println("File does not Exists");
         }

Use the constructor for appending material to the file:

FileOutputStream(File file, boolean append)
Creates a file output stream to write to the file represented by the specified File object.

So to append to a file say "abc.txt" use

FileOutputStream fos=new FileOutputStream(new File("abc.txt"),true);