isValid.java

    public boolean isValidTx(Transaction tx) {
        boolean areInputsInUTXOPool = !tx.getInputs().stream()
            .filter(i -> !utxoPool.contains(new UTXO(i.prevTxHash, i.outputIndex)))
            .findFirst()
            .isPresent();
        boolean areOutputValuesNonNegative = !tx.getOutputs().stream()
            .filter(o -> o.value < 0)
            .findFirst()
            .isPresent();
        double inputsValue = tx.getInputs().stream()
            .map(i -> utxoPool.getTxOutput(new UTXO(i.prevTxHash, i.outputIndex)))
            .filter(Objects::nonNull)
            .map(o -> o.value)
            .reduce(0.0, (x, y) -> x + y);
        double outputsValue = tx.getOutputs().stream()
            .map(o -> o.value)
            .reduce(0.0, (x, y) -> x + y);
        boolean areUTXOClaimedOnlyOnce = !tx.getInputs().stream()
            .map(i -> new UTXO(i.prevTxHash, i.outputIndex))
            .collect(Collectors.groupingBy(UTXO::hashCode))
            .entrySet()
            .stream()
            .filter(x -> x.getValue().size() > 1)
            .findFirst()
            .isPresent();
        boolean areAllInputSignaturesValid = !tx.getInputs().stream()
            .filter(i -> {
                    Optional<Transaction.Output> output = Optional.ofNullable(utxoPool.getTxOutput(new UTXO(i.prevTxHash, i.outputIndex)));
                    if (output.isPresent())
                        return !Crypto.verifySignature(output.get().address, tx.getRawDataToSign(i.outputIndex), i.signature);
                    else
                        return false;
                })
            .findFirst()
            .isPresent();
        if (!areInputsInUTXOPool || !areOutputValuesNonNegative || outputsValue > inputsValue || !areUTXOClaimedOnlyOnce || !areAllInputSignaturesValid)
            return false;
        return true;
    }