Converting long[64] to byte[512] in Java?

Question

I\'m porting a process over to Java. There\'s already working versions in C# and C++.

I have a section in C# that I do Marshal.Copy(...) to convert 64 ulongs to 512

Answer 1

ByteBuffer works well for this: just put in 64 long values and get a byte[] out using the array() method. The ByteOrder class can handle endian issues effectively. For example, incorporating the approach suggested in a comment by wierob:

private static byte[] xform(long[] la, ByteOrder order) {
    ByteBuffer bb = ByteBuffer.allocate(la.length * 8);
    bb.order(order);
    bb.asLongBuffer().put(la);
    return bb.array();
}

Addendum: The resulting byte[] components are signed, 8-bit values, but Java arrays require nonnegative integer index values. Casting a byte to an int will result in sign extension, but masking the higher order bits will give the unsigned value of byte b:

int i = (int) b & 0xFF;

This answer elaborates on the applicable operator precedence rules. This related answer demonstrates a similar approach for double values.