java 8 splitting bytes into chunks

 public List<byte[]> getFileChunks(byte[] mainFile) {

    int sizeMB = 1 * 1024 * 1024;
    List<byte[]> chunks = new ArrayList<>();
    for (int i = 0; i < mainFile.length; ) {
        byte[] chunk = new byte[Math.min(sizeMB, mainFile.length - i)];
        for (int j = 0; j < chunk.length; j++, i++) {
            chunk[j] = mainFile[i];
        }
        chunks.add(chunk);
    }
    return chunks;
}

or if you want a functional solution, then try this:
public List<byte[]> getFileChunks(byte[] mainFile) {
    final int sizeMB = 1 * 1024 * 1024;
    return IntStream.iterate(0, i -> i + sizeMB)
                    .limit((mainFile.length + sizeMB - 1) / sizeMB)
                    .mapToObj(i -> Arrays.copyOfRange(mainFile, i, Math.min(i + sizeMB, mainFile.length)))
                    .collect(Collectors.toList());
}



or

public static byte[][] divideArray(byte[] source, int chunksize) {
     
            byte[][] ret = new byte[(int)Math.ceil(source.length / (double)chunksize)][chunksize];
     
            int start = 0;
     
            for(int i = 0; i < ret.length; i++) {
                ret[i] = Arrays.copyOfRange(source,start, start + chunksize);
                start += chunksize ;
            }
     
            return ret;
    }