How to convert a Java Long to byte[] for Cassandra?

Question

Lazy programmer alert. :)

Cassandra stores column values as bytes (Java example). Specifying a LongType comparator compares those bytes as a long. I want the value

Answer 1

You can crack the bytes apart by using shifts and mask, or a bit easier is ByteBuffer.wrap to wrap an 8 long byte array and using the putLong method. You must set the ByteOrder first using the ByteBuffer.order method.

Answer 2

I would write the long to a ByteArrayOutputStream wrapped in a DataOutputStream and then retrieve the raw bytes, although this will always give you your data in big endian byte order (most significant byte first):

public static byte[] getBytes(Long val)
    throws IOException
{
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    DataOutputStream dos = new DataOutputStream(baos);
    dos.writeLong(val);
    return baos.toByteArray();
}

If you want to be able to specify the endianness, you can use the ByteBuffer class:

public static byte[] getBytes(Long val)
{
    ByteBuffer buf = ByteBuffer.allocate(8);
    buf.order(ByteOrder.BIG_ENDIAN);
    buf.putLong(val);
    return buf.array();
}

Answer 3

Here is cut and paste from java 6 DataOutputStream.writeLong

public final void writeLong(long v) throws IOException {
    writeBuffer[0] = (byte)(v >>> 56);
    writeBuffer[1] = (byte)(v >>> 48);
    writeBuffer[2] = (byte)(v >>> 40);
    writeBuffer[3] = (byte)(v >>> 32);
    writeBuffer[4] = (byte)(v >>> 24);
    writeBuffer[5] = (byte)(v >>> 16);
    writeBuffer[6] = (byte)(v >>>  8);
    writeBuffer[7] = (byte)(v >>>  0);
    out.write(writeBuffer, 0, 8);
incCount(8);
}

Here are modifications for your case

public final byte[] longToBytes(long v) {
    byte[] writeBuffer = new byte[ 8 ];

    writeBuffer[0] = (byte)(v >>> 56);
    writeBuffer[1] = (byte)(v >>> 48);
    writeBuffer[2] = (byte)(v >>> 40);
    writeBuffer[3] = (byte)(v >>> 32);
    writeBuffer[4] = (byte)(v >>> 24);
    writeBuffer[5] = (byte)(v >>> 16);
    writeBuffer[6] = (byte)(v >>>  8);
    writeBuffer[7] = (byte)(v >>>  0);

    return writeBuffer;
}

Answer 4

You can use Cassandra's utility class: ByteBufferUtil.bytes(long n)