How to convert int[] to byte[]

As Brian says, you need to work out how what sort of conversion you need.

Do you want to save it as a "normal" image file (jpg, png etc)?

If so, you should probably use the Java Image I/O API.

If you want to save it in a "raw" format, the order in which to write the bytes must be specified, and then use an IntBuffer and NIO.

As an example of using a ByteBuffer/IntBuffer combination:

import java.nio.*;
import java.net.*;

class Test
{   
    public static void main(String [] args)
        throws Exception // Just for simplicity!
    {
        int[] data = { 100, 200, 300, 400 };

        ByteBuffer byteBuffer = ByteBuffer.allocate(data.length * 4);        
        IntBuffer intBuffer = byteBuffer.asIntBuffer();
        intBuffer.put(data);

        byte[] array = byteBuffer.array();

        for (int i=0; i < array.length; i++)
        {
            System.out.println(i + ": " + array[i]);
        }
    }
}

Maybe use this method

byte[] integersToBytes(int[] values)
{
   ByteArrayOutputStream baos = new ByteArrayOutputStream();
   DataOutputStream dos = new DataOutputStream(baos);
   for(int i=0; i < values.length; ++i)
   {
        dos.writeInt(values[i]);
   }

   return baos.toByteArray();
}  

You need to decide how you convert 1 integer to a set of bytes first.

Most probably (?) 1 integer to 4 bytes, and use the shift (>> or <<) operators to get each byte out (watch that byte ordering!). Copy to a byte array 4 times the length of the integer array.

if your intent is to save to file you maybe want to save directly in a file using FileOutputStream.write:

    OutputStream os = new FileOutputStream("aa");
    int[] rgb = { 0xff, 0xff, 0xff };
    for (int c : rgb) {
        os.write(c);
    }
    os.close();

since it:

Writes the specified byte to this output stream. The general contract for write is that one byte is written to the output stream. The byte to be written is the eight low-order bits of the argument b. The 24 high-order bits of b are ignored.

  /** int[] --> byte[] */
  private byte[] toByte(int[] data) throws IOException {
    
    byte[] bytes = new byte[data.length];
    for (int i = 0; i < bytes.length; i++) {
      bytes[i] = (byte) data[i];
    }
    return bytes;
  }

really simple ？？？