Question

I’m writing a UDP Client to transfer a file to a UDP server. First I try to measure the length of the file, devided by the buffer length to be sent in UDP packet, to get the number of packets required to be sent. I send this number to the server first to acknowledge it. But on the server side, the transform from byte array of the receiving packet into the initial number just failed. Can anyone help me out of this? Here is my code on the client side:

    DatagramSocket socket=new DatagramSocket(); 
    File f = new File(filename);
long fileSize = f.length();
    byte[] buffer = new byte[16384]; 
    long packetNumber = (fileSize/(buffer.length))+1;
    DatagramPacket sendPacket=new DatagramPacket(buffer,buffer.length,addr,srvPort);
    String str=Long.toString(packetNumber);
    buffer = str.getBytes();
    socket.send(sendPacket);

And here is the code on the server side:

    DatagramSocket socket=new DatagramSocket(port);     
byte[] buffer=new byte[16384];
    DatagramPacket receivePacket=new DatagramPacket(buffer, buffer.length);
    while(true)
        {
        socket.receive(receivePacket);
        if (receivePacket.getData().toString().trim()!=null)            
        {

        String str=receivePacket.getData().toString();
        System.out.println(str);
        long pcount=Long.parseLong(str);
                    System.out.println(pcount+" packets to be received.");
                    break;
                    }
                    }

But on the server side the variable pcount can always not be resolved, and when I try to print out the str, it writes out some “[B@60991f” or sth, weird.

Answer 1

This code doesn’t make any sense.

Most networks won’t let you send a datagram over 534 bytes reliably.

At present you are sending 16384 bytes of zero value, because you aren’t putting anything into the buffer: instead you are creating a new buffer after creating the DatagramPacket. So you aren’t sending anything yet.

And you aren’t receiving anything yet either. The result of String.trim() cannot be null. You must reset the byte array in a DatagramPacket before every receive(), because it shrinks to the size of the actual received packet, so unless you reset it it keeps getting smaller and smaller. The result of toString() on a byte array does not include its contents, so parsing it is futile.

You need to study several basic Java programming concepts: too many to answer here.