Re: Bytes coming through as -1

Nigel Wade <>
Thu, 03 Aug 2006 10:03:48 +0100
AndrewTK wrote:

I am trying to read binary data from a network input stream but am
having a hard time at it. Consider the following:

      public static void main(String[] args) throws Exception {
              ServerSocket serversock = new ServerSocket(1234);

              while(true) {
                      try {
                              Socket s = serversock.accept();
                              test(s.getInputStream(), s.getOutputStream() );
                      } catch(Exception e) {

      public static void test(InputStream in, OutputStream out) throws
IOException {
              byte[] trash = new byte[10];
              int rcount;
              while( (rcount = != -1) {
                      for(byte b : trash) {
                              if(b ==-1) {System.out.print(".");}

So far straightforward. It reads from the input stream and discards the
bytes. It also prints a full stop each time it encounters a byte
/inside the buffer/ that comes through as -1.

In theory, this should never happen: if a byte is -1, it comes out of
the read() method as such, which is normally a flag for read(byte[],
int, int) to stop reading.

However, the above code does print out a full set of dots when I run
the program - I have a form which connects to this server program and
sends a ZIP file to it. Try:

<form action="" method="POST"
<input type="file" name="zfile" />
<input type="submit value="send" />

When I try to read the ZIP data, expecting -1 at the end of the stream,
reading gets -1 bang in the middle of the data. I am aware that EOS
would occur long after the file data ended, but that's not the point -
I can't even reach the end of the file data.

Any suggestions?


I think you are confusing what is read from the stream and the return value of
read(). The read() method returns -1 when it encounters the end of the stream,
this has nothing whatever to do with the bytes which are read. The return value
of read() is either the number of bytes read or -1, not any of the bytes which
was actually read (all of which could be -1).

Nigel Wade, System Administrator, Space Plasma Physics Group,
            University of Leicester, Leicester, LE1 7RH, UK
E-mail :
Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555

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