C++:
void OTWriter::setChecksum()
{
    uint16_t value = 0;
    unsigned char* bytePos = reinterpret_cast<unsigned char*>(&otData);
    for (int i = 16; i < sizeof(otData) - 2; i++)
        value += bytePos[i];

    otData.checkSum = SYSTEM_USE_LITTLE_ENDIAN ? __builtin_bswap16(value) : value;
}

JAVA:
// TODO
// This calculation is wrong
int Checksum (byte[] data) {
    int checkSum = 0;
    for(byte b : data) {
        checkSum += b & 0xffff;
    }
    return checkSum;
}