How to convert a byte array to its numeric value (Java)?

Question

I have an 8 byte array and I want to convert it to its corresponding numeric value.

e.g.

byte[] by = new byte[8];  // the byte array is stored in 'by'

// CONVERSION OPERATION
// return the numeric value

I want a method that will perform the above conversion operation.

Solution 1

Assuming the first byte is the least significant byte:

long value = 0;
for (int i = 0; i < by.length; i++)
{
   value += ((long) by[i] & 0xffL) << (8 * i);
}

Is the first byte the most significant, then it is a little bit different:

long value = 0;
for (int i = 0; i < by.length; i++)
{
   value = (value << 8) + (by[i] & 0xff);
}

Replace long with BigInteger, if you have more than 8 bytes.

Thanks to Aaron Digulla for the correction of my errors.

Solution 2

One could use the Buffers that are provided as part of the java.nio package to perform the conversion.

Here, the source byte[] array has a of length 8, which is the size that corresponds with a long value.

First, the byte[] array is wrapped in a ByteBuffer, and then the ByteBuffer.getLong method is called to obtain the long value:

ByteBuffer bb = ByteBuffer.wrap(new byte[] {0, 0, 0, 0, 0, 0, 0, 4});
long l = bb.getLong();

System.out.println(l);

Result

4

I'd like to thank dfa for pointing out the ByteBuffer.getLong method in the comments.

---

Although it may not be applicable in this situation, the beauty of the Buffers come with looking at an array with multiple values.

For example, if we had a 8 byte array, and we wanted to view it as two int values, we could wrap the byte[] array in an ByteBuffer, which is viewed as a IntBuffer and obtain the values by IntBuffer.get:

ByteBuffer bb = ByteBuffer.wrap(new byte[] {0, 0, 0, 1, 0, 0, 0, 4});
IntBuffer ib = bb.asIntBuffer();
int i0 = ib.get(0);
int i1 = ib.get(1);

System.out.println(i0);
System.out.println(i1);

Result:

1
4

Solution 3

Simply, you could use or refer to guava lib provided by google, which offers utiliy methods for conversion between long and byte array. My client code:

    long content = 212000607777l;
    byte[] numberByte = Longs.toByteArray(content);
    logger.info(Longs.fromByteArray(numberByte));

Solution 4

If this is an 8-bytes numeric value, you can try:

BigInteger n = new BigInteger(byteArray);

If this is an UTF-8 character buffer, then you can try:

BigInteger n = new BigInteger(new String(byteArray, "UTF-8"));

Solution 5

You can also use BigInteger for variable length bytes. You can convert it to Long, Integer or Short, whichever suits your needs.

new BigInteger(bytes).intValue();

or to denote polarity:

new BigInteger(1, bytes).intValue();

View more solutions

Comments

[Laurence Gonsalves]

I would've voted for this answer if it ended right after the first code snippet, or if it included some code to convert the string into a "numeric value". As-is, the second half of your answer seems like a non sequitur.

[Vincent Robert]

Not what I meant in the first place, I changed my answer

[Aaron Digulla]

-1 bytes are signed values! And replace pow() with shift (<<)! "value = (value << 8) + (by[i] & 0xff)"

[dfa]

what about ByteBuffer.wrap(new byte[] {0, 0, 0, 1, 0, 0, 0, 4}).getLong()? this method should read next 8 byte and convert them to a long

[coobird]

@dfa: Thanks for pointing that out, it sure seems to work -- I'll edit the answer. :)

[suraj]

Does the shift operator(<<) has right to left precedence? How does above code works? Its working all fine for me. Just wanna know the working. Thanx in advance

[suraj]

@Mnementh : Does the shift operator(<<) has right to left precedence? How does above code works? Its working all fine for me. Just wanna know the working. Thanx in advance

[Luke]

In case anyone else has the same issue I did, in the first example, by[i] must be cast to a long, otherwise it only works for values less than 2^32. That is, value += ((long)by[i] & 0xffL) << (8 * i);

[Luke]

Just a note that I needed to use 0xFFL, otherwise the cast from int 0xFF to long had a whole lot of incorrect 1 bits set when I printed Long.toHexString(l).

[will.fiset]

google.github.io/guava/releases/21.0/api/docs/com/google/com‌​mon/…

[Gray]

You need to use 0xFFL otherwise you get sign extension.