Java Basics

hallo Everybody,

public class update
int lengthcode = 10;
byte[] lengtharray = {0,(byte)lengthcode};
byte command = 20;
byte[] command = {0,command};


String client = "hallo";
byte[] stringLength = {0,(byte)client.length};

byte[] stringBytes = client.getBytes("UTF-8");
byte[] temp1 = new byte[ stringLength.length + stringBytes.length ] ;
System.arraycopy( strarrayLength, 0, temp1, 0, strarrayLength.length ) ;
System.arraycopy( stringBytes, 0, temp1, strarrayLength.length, stringBytes.length ) ;

/////////////// now i need to add the length of the stringlength with the lengtharray and update the lengtharay without disturbing the other elements
all the other elements i should have


how can i do that

With Regards,
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zzynxSoftware engineerCommented:

>>byte command = 20;
>>byte[] command = {0,command};

won't compile (twice the same variable name used)
Is your intention to write these to stream only?
... if so, you'll find it a lot easier to forget about the arrays - just write with DataOutputStream
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zzynxSoftware engineerCommented:
Sure this is right?
>> System.arraycopy(command,0,lengtharray,lengtharray.length,command.length);

public static void arraycopy(Object src,  // the source array.
                                         int srcPos,  // starting position in the source array.
                                         Object dest, // the destination array.
                                         int destPos, // starting position in the destination data.            // <<<<< lengtharray.length???
                                         int length)  // the number of array elements to be copied.
ashok3sepAuthor Commented:
I had to send a certain set of fields to the server in the properway

so i am using a byte[] array to write to the stream
initially i am sending a length of the data i need to send

but later on when i send strings to the server the length of the string has to be addted to the initial length

so how can i do that.

that was my question.
hope u understand now almost clearly

With regards
zzynxSoftware engineerCommented:
What about the remarks I made?
I think the are still valid.
ashok3sepAuthor Commented:
about your remarks you are right.

i have mis typed it

it should be
byte[] command_code

and NOT command

With Regards,
zzynxSoftware engineerCommented:
And what about the 4th parameter of
>> System.arraycopy(command,0,lengtharray,lengtharray.length,command.length);

Could you please comment what you're trying to do with this line?

From what you say, you should probably do something more like

String s = "qwerty";
byte[] bytes = s.getBytes("UTF8");
byte length = (byte)bytes.length;
dis.write(length); // (Using DataOutputStream)

There's no need to be copying bytes into arrays first, unless you want to hang onto them for later use, in which case, you should probably be using a dedicated class to hold these. You would then give that class something like a writeToStream method that does the above
ashok3sepAuthor Commented:
i am appending the commandcode[] values  to the lengtharray[] values

so for example it will look like this if you print it out

// so now i give a string and append to the same array

String str = "hallo";

the question is i have already created a length of 10 in the array.

now i need to add this str.length to that value 10 so that it automatically becomes 16

i dont want to do it manually.

when i speicify the string i has to be updated.
how can i acheive it

With Regards,

>>i am appending the commandcode[] values  to the lengtharray[] values

>>now i need to add this str.length to that value 10 so that it automatically becomes 16

and if you have to compute the total length, why do you do that *before* you have assembled all the data?
ashok3sepAuthor Commented:
I would like to thank you for your pateince to answer my queries so politely.

This is the way that i should model my program to follow the protocol rule.

only INIT command doesnt have DATA field
all other commands have DATA field

1st field ------------- length , ------- 2 bytes
2nd field -.--------- commandcode, --------2 bytes
3 field ----------- if there is some data to be added (data can be a STRING or WORD ) --------- 2* n bytes

if my data is string then the string length is added to the 1st field length.

4th field ------------ Checksum ------------- 2 bytes

all these fields i write to a byte array and then to the socket.

with regards,

a. How is the 1st length field set out? A word with high byte followed by low?
b. How do you compute the checksum?
ashok3sepAuthor Commented:
Hallo CEHJ,

It is the same way as you say

a high byte and then a low byte

for example if length is 8
then {0,8} is stored in a bytearray[]
checksum is calculated in the way given below

public byte[] calculateCheckSum(byte[] identify)
      int sum = 0;
      message = identify;
      for(int i = 0; i< message.length; i+=2)
            sum += (message[i] * 256);
            sum += (message[i+1]);
byte[] checksumdata = {(byte)(sum / 256) ,(byte)(sum % 256 ) };

byte[] temp3 = new byte[ message.length + checksumdata.length] ;

System.arraycopy(message, 0, temp3, 0, message.length ) ;
System.arraycopy(checksumdata, 0, temp3, message.length, checksumdata.length ) ;
message = temp3 ;
return message;


Hope you can understand


           byte[] init = new byte[4];

            // Assume command '20' (these should be named constants, not literals)
            init[2] = 0;
            init[3] = 20;

            // Get some test data
            final int TEST_LENGTH = 300;
            StringBuffer sb = new StringBuffer(TEST_LENGTH);
            String letters = "abcdefghijklmnopqrstuvwxyz";
            for(int i = 0;i < TEST_LENGTH;i++) {
                  sb.append(letters.charAt((int)(Math.random() * letters.length())));
            String data = sb.toString();

            // Get data and length
            byte[] dataBytes = data.getBytes("UTF8");
            int length = dataBytes.length;
            if (length > 0xFFFF) {
                  throw new RuntimeException("True data length cannot be represented properly");

            // Set length
            init[0] = (byte)(length >> 8);
            init[1] = (byte)(length & 0xFF);

            // Calc checksum (you probably need to check and test the checksum routine)
            byte[] checksumData = calculateCheckSum(init);

            // Write to stream

ashok3sepAuthor Commented:
Hallo CEHJ,

what you say is correct,
but i want in the way that everything has to be appended to INIT array and then finally writing this array to the stream.
thats what my protocol rule says,

if i send in otherways it says your command cannot be identified .
please check your structure.

With regasrds,

Can you show the bit where you write to the stream?
ashok3sepAuthor Commented:
//// INIT Command------------

//// Here is how i write to a stream.
first i gather all the datas and put it in a byte array and then finally i write to the stream.

////Here begins the code

int lengthByte = 6;
byte[] init = createMessage.createLengthField(lengthByte);
byte command = 2;
init = createMessage.createCommandField(init,command);
init = createMessage.createCheckSumField(init);

With Regards,
ashok3sepAuthor Commented:
I would like to have my points refunded to me again as i did the piece of code myself.

while ( bis.available() < 0)
byte[] identifyAcknowledgement = new byte[8];
for(int i = 0; i < identifyAcknowledgement.length ; i++)


PAQed with points refunded (25)

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