docs: public Coin getFee() {
          Coin fee = Coin.ZERO;
          if (inputs.isEmpty() || outputs.isEmpty()) // Incomplete transaction
              return null;
          for (TransactionInput input : inputs) {
              if (input.getValue() == null)
                  return null;
              fee = fee.add(input.getValue());
          }
          for (TransactionOutput output : outputs) {
              fee = fee.subtract(output.getValue());
          }
          return fee;
      }