Question

I have a list of Integer

List<Integer> listInt;

I want to generate a MD5 hash for the listInt how should I approach the problem, I know that I have to use

MessageDigest.getInstance("MD5").digest(byte[])

How do I convert the listInt to bytes.

Answer 1

You need to convert each integer value to 4 bytes, forming an array of bytes that is 4*listInt.size() bytes long.

One way to do this is to make use of NIO’s ByteBuffer class:

ByteBuffer buf = ByteBuffer.allocate(4*listInt.size());
buf.order(ByteOrder.BIG_ENDIAN);
for (int i = 0; i < listInt.size(); ++i)
    buf.putInt(listInt.get(i));
byte[] barr = buf.array();

Note that there are two common ways of converting an array of integers to an array of bytes. The difference, however, is simply the choice of Endianness. Some people choose to calculate the Big Endian byte representation of integers; others choose to calculate the little Endian representation of integers. It doesn’t matter which method you pick as long as you calculate the byte representation of each integer consistently.