Convert Short to 3 bytes array

Question

I need to transfer numbers over UDP. The protocol specifies the numbers to be 3 bytes.

So for example I need ID: 258 converted to a byte array:

byte[] a = new byte[3]

possible duplicate of [Convert integer into byte array (Java)](http://stackoverflow.com/questions/1936857/convert-integer-into-byte-array-java) — Zangdak, Jul 30 '14 at 13:07
All the answers I found convert it into 2 byte, or 4 byte arrays. I specifically need 3 bytes. — Alexander Ciesielski, Jul 30 '14 at 13:12
If one of the responses answers your question, you can mark it as accepted. — Zangdak, Jul 30 '14 at 13:57

Zangdak · Accepted Answer · 2014-07-31T13:09:02.913

3

I think it should work:

Little Endian:

byte[] convertTo3ByteArray(short s){

   byte[] ret = new byte[3];
   ret[2] = (byte)(s & 0xff);
   ret[1] = (byte)((s >> 8) & 0xff);
   ret[0] = (byte)(0x00);

   return ret;

}

short convertToShort(byte[] arr){

    if(arr.length<2){
        throw new IllegalArgumentException("The length of the byte array is less than 2!");
    }

    return (short) ((arr[arr.length-1] & 0xff) + ((arr[arr.length-2] & 0xff ) << 8));       
}

Big Endian:

byte[] convertTo3ByteArray(short s){

   byte[] ret = new byte[3];
   ret[0] = (byte)(s & 0xff);
   ret[1] = (byte)((s >> 8) & 0xff);
   ret[2] = (byte)(0x00);

   return ret;

}

short convertToShort(byte[] arr){

    if(arr.length<2){
        throw new IllegalArgumentException("The length of the byte array is less than 2!");
    }

    return (short) ((arr[0] & 0xff) + ((arr[1] & 0xff ) << 8));

}

edited Jul 31 '14 at 13:09

answered Jul 30 '14 at 13:14

Zangdak

242
4
22

It won't unless you replace Short with short ;) But nice approach +1 – flotothemoon Jul 30 '14 at 13:31
2

Don't you want the first byte (most significant) to be 0? If you convert a 16-bit integer to a 24-bit integer by adding 0's to the end, you make the number bigger. – Cruncher Jul 30 '14 at 13:38
It seems your solution return a BIG_ENDIAN byte[], I was asking for little. But thanks anyway – Alexander Ciesielski Jul 30 '14 at 14:28
I added a version for both big endian and little endian. I may mixed them up, tell me if I'm wrong. – Zangdak Jul 30 '14 at 14:51
Funny thing, I convert 258 to array and then back, it gives me 258. But when I convert 675, it gives me back -93. Why is that? – Alexander Ciesielski Jul 31 '14 at 11:52
Works perfectly for me. Do you need the methods to convert your 3 bytes array to short? – Zangdak Jul 31 '14 at 12:59
I added the two corresponding methods to convert a byte array to short. – Zangdak Jul 31 '14 at 13:09

morpheus05 · Answer 2 · 2014-07-31T13:19:39.283

You could use a DataInputStream:

ByteArrayOutputStream bout = new ByteArrayOutputStream();
DataOutput out = new DataOutputStream();

out.write(0); //to have 3 bytes
out.writeShort(123);


byte[] bytes = bout.toByteArray();

If you have to send other data to a later time (e.g. string or something) you then can simple append this new data:

ByteArrayOutputStream bout = new ByteArrayOutputStream();
DataOutput out = new DataOutputStream();

out.write(0); //to have 3 bytes
out.writeShort(123);


out.writeUTF("Hallo UDP");

byte[] bytes = bout.toByteArray();

Convert Short to 3 bytes array

2 Answers2