How do I convert a byte array to a long in Java?

Question

I am reading 8 bytes of data in from a hardware device. I need to convert them into a numeric value. I think I want to convert them to a long as that should fit 8 bytes. I am not very familiar with Java and low level data type operations. I seem to have two problems (apart from the fact there is almost no documentation for the hardware in question), The bytes are expecting to be unsigned, so I can't do a straight integer conversion. I am not sure what endianness they are.

Any advice would be appreciated.

---

Ended up with this (taken from some source code I probably should have read a week ago):

public static final long toLong (byte[] byteArray, int offset, int len)
{
   long val = 0;
   len = Math.min(len, 8);
   for (int i = (len - 1); i >= 0; i--)
   {
      val <<= 8;
      val |= (byteArray [offset + i] & 0x00FF);
   }
   return val;
}

Answer 1

Shifting bytes according to the endianness of the data is fairly straightforward. There is a small trick with the long datatype, however, because a binary operation on integral types of int or smaller promotes the operands to int. For left shifts larger than 31 bits, this will result in zero, since all of the bits have been shifted out of the int range.

So, force promotion to long by including a long operand in the calculation. Below, I do this by masking each byte with the value 0xFFL, which is a long, forcing the result to be a long.

byte[] buf = new byte[8];
/* Fill buf somehow... */
long l = ((buf[0] & 0xFFL) << 56) |
         ((buf[1] & 0xFFL) << 48) |
         ((buf[2] & 0xFFL) << 40) |
         ((buf[3] & 0xFFL) << 32) |
         ((buf[4] & 0xFFL) << 24) |
         ((buf[5] & 0xFFL) << 16) |
         ((buf[6] & 0xFFL) <<  8) |
         ((buf[7] & 0xFFL) <<  0) ;

Answer 2

Byte#longValue() should do it

And if not (thanks for the source example) you can use java.nio.ByteBuffer such as in

 public static long toLong(byte[] b) {
    ByteBuffer bb = ByteBuffer.allocate(b.length);
    bb.put(b);
    return bb.getLong();
}

The initial order is BIG_ENDIAN you can reed more here