May be you need something like:
      public byte[] pack(long value){
        long tmp = value;
        int size = 0;
        do{
            size ++;
            tmp <<= Byte.SIZE - 1;
        }while (tmp != 0);
        byte[] bytes = new byte[size];
        for ( int i = 0; i < bytes.length ; i++){
            tmp = value;
            for ( int j = 0 ;j < i ; j++){
                tmp >>= Byte.SIZE - 1;
            }
            bytes[i] = (byte)(tmp & Byte.MAX_VALUE);
        }
        return bytes;
    }
I belive that an unpack method is pretty clear, then you have packing;

Just use a DataOutputStream.

The unsigned arithmetic can be performed using longs. When you are ready to
output the data you mask the low order 4 bytes into an int. Then write the int
directly to the network using DataOutputStream.writeInt(). Java I/O is all
network byte order (big endian) so there's no need to worry about byte
swapping.

The following code will write the unsigned value 0xfedcba01 to a file:

package tests;

import java.io.DataOutputStream;
import java.io.FileOutputStream;

public class IntOutput {
   public static void main(String[] args) {
       // create a 32bit, "unsigned" integer.
       long l = 0xfedcba01L;
       
       // mask off the 32bit low order bits into an int.
       int i = (int) l & 0xffffffff;
       System.out.println(l);
       System.out.println(i);
       
       try {
           // open a DataOutputStream, in this case to a file, but it could
           // just as easily be  network connection.
           FileOutputStream fos = new FileOutputStream("tmp.tmp");
           DataOutputStream dos = new DataOutputStream(fos);
           
           dos.writeInt(i);
           
           dos.close();
           fos.close();
       }
       catch(Exception e) {
           System.err.println(e);
       }
   }
}

When you run it the output is:
4275878401
-19088895

The contents of the output file, when viewed using od are:

$ od -t x4 tmp.tmp
0000000 fedcba01      HEX
$ od -D tmp.tmp
0000000 4275878401    Unsigned int
$ od -t d4 tmp.tmp
0000000 -0019088895   Signed int