How to use ByteArrayOutputStream and DataOutputStream simultaneously in Java?

Use the former case - wrap DataOutputStream around the ByteArrayOutputStream. Just make sure you save the reference to the ByteArrayOutputStream. When you are finished, close() or at least flush() the DataOutputStream and then use the toByteArray method of the ByteArrayOutputStream.

You could use a stream approach if you connect your outputstream to an inputstream through a PipedInputStream/PipetOutputStream. Then you will consume the data from the inputstream.

Anyway if what you need to do is simple and doesn't not require a stream approach I would use a java.nio.ByteBuffer on which you have

• put(byte[] src) for your byte[]
• putInt(int value)
• and byte[] array() to get the content

Could you make a variable to hold on to the ByteArrayOutputStream and pass it into the DataOutputStream.

ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream dos = new DataOutputStream(baos);
dos.writeInt(1);
byte[] result = dos.toByteArray();

Like this:

ByteArrayOutputStream baos = new ByteArrayOutputStream();
DataOutputStream w = new DataOutputStream(baos);

w.writeInt(100);
w.write(byteArray);

w.flush();

byte[] result = baos.toByteArray();

Actually your second version will not work at all. DataOutputStream requires an actual target stream in which to write the data. You can't do new DataOutputStream(). There isn't actually any constructor like that.