From Evernote:

Long2BigIntegerTest

import java.math.BigInteger;

public class Long2BigIntegerTest {

        /**

        * @param args

        */

        public static void main(String[] args) {

              System. out.println("----------------1------------------" );

               long ii = -1L;

              System. out.println("source:[" + ii + "]" );

               byte buf[] = longToBytes(ii);

              BigInteger bi = new BigInteger(1,buf);

              System. out.println("unsign BigInteger : ["   + bi + "]") ;

              bi = new BigInteger(-1,buf);

              System. out.println("sign BigInteger : ["   + bi + "]") ;

               long rr = readLong(buf, 0);

              System. out.println(" readLong : ["   + rr + "]") ;

              System. out.println("-----------------------------------" );

              

              System. out.println("----------------100-----------------" );

              ii = -100L;

              System. out.println("source:[" + ii + "]" );

              buf = longToBytes(ii);

              bi = new BigInteger(1,buf);

              System. out.println("unsign BigInteger : ["   + bi + "]") ;

              bi = new BigInteger(-1,buf);

              System. out.println("sign BigInteger : ["   + bi + "]") ;

              rr = readLong(buf, 0);

              System. out.println(" readLong : ["   + rr + "]") ;

              System. out.println("-----------------------------------" );

              

              System. out.println("-----------------+1-----------------" );

              ii = 1L;

              System. out.println("source:[" + ii + "]" );

              buf = longToBytes(ii);

              bi = new BigInteger(1,buf);

              System. out.println("unsign BigInteger : ["   + bi + "]") ;

              bi = new BigInteger(-1,buf);

              System. out.println("sign BigInteger : ["   + bi + "]") ;

              rr = readLong(buf, 0);

              System. out.println(" readLong : ["   + rr + "]") ;

              System. out.println("-----------------------------------" );

              

              System. out.println("--------------Long.MAX_VALUE--------------" );

              ii = Long. MAX_VALUE;

              System. out.println("source:[" + ii + "]" );

              buf = longToBytes(ii);

              bi = new BigInteger(1,buf);

              System. out.println("unsign BigInteger : ["   + bi + "]") ;

              bi = new BigInteger(-1,buf);

              System. out.println("sign BigInteger : ["   + bi + "]") ;

              rr = readLong(buf, 0);

              System. out.println(" readLong : ["   + rr + "]") ;

              System. out.println("-----------------------------------" );

              

              System. out.println("--------------Long.MIN_VALUE--------------" );

              ii = Long. MIN_VALUE;

              System. out.println("source:[" + ii + "]" );

              buf = longToBytes(ii);

              bi = new BigInteger(1,buf);

              System. out.println("unsign BigInteger : ["   + bi + "]") ;

              bi = new BigInteger(-1,buf);

              System. out.println("sign BigInteger : ["   + bi + "]") ;

              rr = readLong(buf, 0);

              System. out.println(" readLong : ["   + rr + "]") ;

              System. out.println("-----------------------------------" );

              

              System. out.println("--------------Long.MIN_VALUE-100--------------" );

              ii = Long. MIN_VALUE;

              System. out.println("source:[" + ii + "]" );

              buf = longToBytes(ii);

              bi = new BigInteger(1,buf);

              System. out.println("unsign BigInteger : ["   + bi + "]") ;

              bi = new BigInteger(-1,buf);

              System. out.println("sign BigInteger : ["   + bi + "]") ;

              rr = readLong(buf, 0);

              System. out.println(" readLong : ["   + rr + "]") ;

              System. out.println("-----------------------------------" );

       }

       

        public static long readLong(byte[] b, int index) {

        long r = 0;

        for (int i = index; i < b.length; i++) {

            r <<= 8;

            r |= (b[i] & 0x00000000000000ff);

        }

        return r;

    }

    

    public static byte [] longToBytes(long n) {

        byte[] buf = new byte[8];

        for (int i = buf.length - 1; i >= 0; i--) {

            buf[i] = ( byte) (n & 0x00000000000000ff);

            n >>= 8;

        }

        return buf;

    }

}