You can do it with using a FileOutputStream and the writeTo method.

ByteArrayOutputStream byteArrayOutputStream = getByteStreamMethod();
try(OutputStream outputStream = new FileOutputStream("thefilename")) {
    byteArrayOutputStream.writeTo(outputStream);
}

Source: "Creating a file from ByteArrayOutputStream in Java." on Code Inventions

ByteArrayOutputStream writes bytes to a byte array in memory. Not to any other destination, such as a file or a network socket. After writing the data, you can get the byte array by calling toByteArray() on it.

BufferedOutputStream wraps another, underlying OutputStream and provides buffering for that underlying stream, to make I/O operations more efficient. The underlying stream can be any kind of OutputStream, for example one that writes to a file or a network socket.

Why you might want to use buffering: Writing a large block of data to the file system is more efficient than writing byte by byte. If your program needs to write many small pieces of data, it's more efficient to first gather these small pieces in a buffer and then write the entire buffer to disk at once. This is what BufferedOutputStream does automatically for you.

There seem to be many links and other such stuff, but no actual code using pipes. The advantage of using java.io.PipedInputStream and java.io.PipedOutputStream is that there is no additional consumption of memory. ByteArrayOutputStream.toByteArray() returns a copy of the original buffer, so that means that whatever you have in memory, you now have two copies of it. Then writing to an InputStream means you now have three copies of the data.

The code using lambdas (hat-tip to @John Manko from the comments):

PipedInputStream in = new PipedInputStream();
final PipedOutputStream out = new PipedOutputStream(in);
// in a background thread, write the given output stream to the
// PipedOutputStream for consumption
new Thread(() -> {originalOutputStream.writeTo(out);}).start();

One thing that @John Manko noted is that in certain cases, when you don't have control of the creation of the OutputStream, you may end up in a situation where the creator may clean up the OutputStream object prematurely. If you are getting the ClosedPipeException, then you should try inverting the constructors:

PipedInputStream in = new PipedInputStream(out);
new Thread(() -> {originalOutputStream.writeTo(out);}).start();

Note you can invert the constructors for the examples below too.

Thanks also to @AlexK for correcting me with starting a Thread instead of just kicking off a Runnable.

The code using try-with-resources:

// take the copy of the stream and re-write it to an InputStream
PipedInputStream in = new PipedInputStream();
    new Thread(new Runnable() {
        public void run () {
            // try-with-resources here
            // putting the try block outside the Thread will cause the
            // PipedOutputStream resource to close before the Runnable finishes
            try (final PipedOutputStream out = new PipedOutputStream(in)) {
                // write the original OutputStream to the PipedOutputStream
                // note that in order for the below method to work, you need
                // to ensure that the data has finished writing to the
                // ByteArrayOutputStream
                originalByteArrayOutputStream.writeTo(out);
            }
            catch (IOException e) {
                // logging and exception handling should go here
            }
        }
    }).start();

The original code I wrote:

// take the copy of the stream and re-write it to an InputStream
PipedInputStream in = new PipedInputStream();
final PipedOutputStream out = new PipedOutputStream(in);
new Thread(new Runnable() {
    public void run () {
        try {
            // write the original OutputStream to the PipedOutputStream
            // note that in order for the below method to work, you need
            // to ensure that the data has finished writing to the
            // ByteArrayOutputStream
            originalByteArrayOutputStream.writeTo(out);
        }
        catch (IOException e) {
            // logging and exception handling should go here
        }
        finally {
            // close the PipedOutputStream here because we're done writing data
            // once this thread has completed its run
            if (out != null) {
                // close the PipedOutputStream cleanly
                out.close();
            }
        }   
    }
}).start();

This code assumes that the originalByteArrayOutputStream is a ByteArrayOutputStream as it is usually the only usable output stream, unless you're writing to a file. The great thing about this is that since it's in a separate thread, it also is working in parallel, so whatever is consuming your input stream will be streaming out of your old output stream too. That is beneficial because the buffer can remain smaller and you'll have less latency and less memory usage.

If you don't have a ByteArrayOutputStream, then instead of using writeTo(), you will have to use one of the write() methods in the java.io.OutputStream class or one of the other methods available in a subclass.

To convert a file to byte array, ByteArrayOutputStream class is used. This class implements an output stream in which the data is written into a byte array. The buffer automatically grows as data is written to it. The data can be retrieved using toByteArray() and toString().

To convert byte array back to the original file, FileOutputStream class is used. A file output stream is an output stream for writing data to a File or to a FileDescriptor.

The following code has been fully tested.

 public static void main(String[] args) throws FileNotFoundException, IOException {
            File file = new File("java.pdf");

            FileInputStream fis = new FileInputStream(file);
            //System.out.println(file.exists() + "!!");
            //InputStream in = resource.openStream();
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            byte[] buf = new byte[1024];
            try {
                for (int readNum; (readNum = fis.read(buf)) != -1;) {
                    bos.write(buf, 0, readNum); //no doubt here is 0
                    //Writes len bytes from the specified byte array starting at offset off to this byte array output stream.
                    System.out.println("read " + readNum + " bytes,");
                }
            } catch (IOException ex) {
                Logger.getLogger(genJpeg.class.getName()).log(Level.SEVERE, null, ex);
            }
            byte[] bytes = bos.toByteArray();

            //below is the different part
            File someFile = new File("java2.pdf");
            FileOutputStream fos = new FileOutputStream(someFile);
            fos.write(bytes);
            fos.flush();
            fos.close();
        }

how to write a byte array to a file using a FileOutputStream. The FileOutputStream is an output stream for writing data to a File or to a FileDescriptor.

public static void main(String[] args) {

        String s = "input text to be written in output stream";

        File file = new File("outputfile.txt");

        FileOutputStream fos = null;

        try {

            fos = new FileOutputStream(file);

            // Writes bytes from the specified byte array to this file output stream 
            fos.write(s.getBytes());

        }
        catch (FileNotFoundException e) {
            System.out.println("File not found" + e);
        }
        catch (IOException ioe) {
            System.out.println("Exception while writing file " + ioe);
        }
        finally {
            // close the streams using close method
            try {
                if (fos != null) {
                    fos.close();
                }
            }
            catch (IOException ioe) {
                System.out.println("Error while closing stream: " + ioe);
            }

        }

    }

Heh, sounds like they copied and pasted code from different sources? :-P No, seriously, unless you need to inspect the decompressed data, you can just use a BufferedOutputStream for both compression and decompression.