ByteArrayOutputStream：奇怪的行为

Question

I'm writing a simple client-server application and I wanted to be able to take the attributes of a Header class, turn them into a byte[], send them to the other host, and then convert them back into an easily parsed Header. I was using a ByteArrayOutputStream to do this, but the results were not what I expected. For example, just to test it in main() I had:

Header h = Header();
h.setSource(111);
h.setDest(222);
h.setSeq(333);
h.setAck(444);
byte[] header = Header.convertHeaderToByteArray();
Header newHeader = new Header(headerArray);

Where convertHeaderToByteArray() looked like:

public byte[] convertHeaderToByteArray() {
    byte[] headerArray;
    ByteArrayOutputStream byteStream = new ByteArrayOutputStream(44);
    byteStream.write(srcPort);
    byteStream.write(dstPort);
    byteStream.write(seqNum);
    byteStream.write(ackNum);
    byteStream.write(controlBits);
    headerArray = byteStream.toByteArray();
    return headerArray;
}

And the Header(headerArray) constructor:

public Header(byte[] headerArray) {
    ByteArrayInputStream header = new ByteArrayInputStream(headerArray);
    srcPort = header.read();
    dstPort = header.read();
    seqNum = header.read();
    ackNum = header.read();
}

This definitely did not behave as expected. When I looked at those values at the end, srcPort was correct (111), dstPort was correct (222), seqNum was not correct (77), and ackNum was not correct (188).

After hours of reading and tinkering I couldn't get it right, so I tried to use ByteBuffer instead. Viola, correct results.

What is going on here? I read the documentation for both and although I spotted some differences I'm not seeing what the source of my error is.

Answer 1

OutputStream.write(int) writes a single byte. See the Javadoc. If you want to write wider values, you will have to use the writeXXX() methods of DataOutputStream, and the corresponding readXXX() methods of DataInputStream to read them.