生成整数列表的MD5哈希

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.

Answer 2

First create a method that converts an int to a byte array:

  public static final byte[] intToByteArray(int value) 
  { 
    return new byte[]  { (byte)(value >>> 24), (byte)(value >> 16 & 0xff), (byte)(value >> 8 & 0xff), (byte)(value & 0xff) };
  }

Then use a ByteArrayOutputStream.

ByteArrayOutputStream baos = new ByteArrayOutputStream();

for (int i = 0; i < listInt.size(); i++)
{
  baos.write(intToByteArray(listInt[i]), 0, 4);
}

You will then get your full byte array to perform the md5 hash on by:

byte[] fullByteArray = baos.toByteArray();

Answer 3

byte data[] = new byte[listInt.size()*4];
for (int i = 0; i < listInt.size(); i++) {
  data[i*4]   = new Integer(listInt.get(i) >> 24).byteValue();
  data[i*4+1] = new Integer(listInt.get(i) >> 16).byteValue();
  data[i*4+2] = new Integer(listInt.get(i) >> 8).byteValue();
  data[i*4+3] = listInt.get(i).byteValue();
}

Sample code running: http://ideone.com/YJZVj