Ledger.java

public double makeTxFees(Transaction tx) {
        if (!this.isValidTx(tx)) {
            return 0;
        }

        double inputValuesSum = tx.getInputs()
                .parallelStream()
                .map((x) -> new UTXO(x.prevTxHash, x.outputIndex))
                .filter((x) -> this.utxoPool.contains(x))
                .mapToDouble((x) -> this.utxoPool.getTxOutput(x).value)
                .sum();

        double outputValuesSum = tx.getOutputs()
                .parallelStream()
                .mapToDouble((x) -> x.value)
                .sum();

        return inputValuesSum - outputValuesSum;
    }