数字写入字节/ Java

Question

I need to get a numeric from a table in database and put it a ChannelBuffer in a field with 4 bytes of width.

The number can be 1 to 9999 eg. 237 and in ChannelBuffer I need to fill the 4 bytes.

ChannelBuffer bufcmd = ChannelBuffers.directBuffer(4);
bufcmd.writeByte(0x00); // 1º digit of 4 bytes
bufcmd.writeByte(0x00); // 2º digit of 4 bytes
bufcmd.writeByte(0x00); // 3º digit of 4 bytes
bufcmd.writeByte(0x01); // 4º digit of 4 bytes

How can I fill this directly with 237 value?

The entire code is:

String strcmd = "dyd#"; //from database

ChannelBuffer bufcmd = ChannelBuffers.directBuffer(100);

bufcmd.writeByte(0x78);
bufcmd.writeByte(0x78);
bufcmd.writeByte((5 + strcmd.length()));
bufcmd.writeByte(0x80);
bufcmd.writeByte((4 + strcmd.length()));



// TODO: convert 1 to 9999 to HEX 
// (id of command from database eg: 3)
// but filling 4 bytes
bufcmd.writeByte(0x00);
bufcmd.writeByte(0x00);
bufcmd.writeByte(0x00);
bufcmd.writeByte(0x03);


bufcmd.writeBytes(ChannelBuffers.copiedBuffer(strcmd, CharsetUtil.US_ASCII));
bufcmd.writeShort(buf.readUnsignedShort());
bufcmd.writeShort(Crc.crc16Ccitt(bufcmd.toByteBuffer(2, 4)));
bufcmd.writeByte(0x0D);
bufcmd.writeByte(0x0A);
channel.write(bufcmd);

Answer 1

This converts to big endian, binary:

   int theNumber = 237;
   byte[] buf = new byte[4];
   for( int i = 0; i < 4; i++ ){
   buf[3 - i] = (byte)(theNumber & 0xFF);
       theNumber = theNumber >> 8;
   }

Note that java.io.DataOutputStream and similar classes have method writeInt(int v) and similar methods for other types, which simplifies tasks like the one you appear to have considerabls.

The code below converts to decimal ASCII, most significant digit first, leading zeros:

   int theNumber = 237;
   byte[] buf = new byte[4];
   for( int i = 0; i < 4; i++ ){
   buf[3 - i] = (byte)(theNumber % 10 + 0x30);
       theNumber = theNumber / 10;
   }

Answer 2

Without knowing which encoding etc you want in your bufcmd, I'm going to take a wild guess that "whatever String is using is okay"

int theNumber = 237; // The value
String theString = Integer.toString(theNumber); //The string representation of the value
for(int i=0; i<4;i++)
{
      bufcmd.writeByte( theString.charAt(i) );
}

Added: A full implementation which takes into account string lengths and adds zero padding and also translates the character into an integer, if you do not need this, remove the Character.getNumericValue() call that wraps around the character..

int theNumber = 237; // The value
String theString = Integer.toString(theNumber); //The string representation of the value
int len = theString.length(); //Bytes in the string
int requiredLen = 4; // 3 = 0003, 42 = 0042
int padding = requiredLen-len; //How many 0 we need to send first

while( padding-- > 0 )
{
    bufcmd.writeByte( 0 );
}

for(int i=0; i<len;i++)
{
    bufcmd.writeByte( Character.getNumericValue(theString.charAt(i)) );
}

You can also calculate the value of a position instead of translating it to a string using Math.pow.

Here is a test to prove that the method works. http://www.compileonline.com/compile_java_online.php

public class HelloWorld{

     public static void main(String []args){
        System.out.println("Hello World");
        int theNumber = 237; // The value
        String theString = Integer.toString(theNumber); //The string representation of the value
        int len = theString.length(); //Bytes in the string
        int requiredLen = 4; // 3 = 0003, 42 = 0042
        int padding = requiredLen-len; //How many 0 we need to send first

        while( padding-- > 0 )
        {
            System.out.println( 0 );
        }

        for(int i=0; i<len;i++)
        {
            System.out.println( Character.getNumericValue(theString.charAt(i)) );
        }        

     }
}