code: @Override
      public BigInteger getPrivKey() {
          final BigInteger key = findOrDerivePrivateKey();
          checkState(key != null, "Private key bytes not available");
          return key;
      }
docs: Returns the private key of this deterministic key. Even if this object isn't storing the private key,
  it can be re-derived by walking up to the parents if necessary and this is what will happen.
  @throws java.lang.IllegalStateException if the parents are encrypted or a watching chain.