Python's `sum()` works only on numbers and assumes an empty list sums to 0. Java's `reduce()` is quite a bit more flexible.

I use Joda Money and frequently find myself doing:

    final Money bucks = stream.map(Thing::getPrice).reduce(Dollars.ZERO, Money::plus);

If you don't pass in a baseline, you get optional:

    final Optional<Money> bucks = stream.map(Thing::getPrice).reduce(Money::plus);

Also, StreamEx improves the ergonomics of Java streams. The way I would implement the original example:

    EntryStream.of(cart.getProducts())
        .mapKey(Product::getPrice)
        .mapValue(BigDecimal::new)
        .mapKeyValue(BigDecimal::multiply)
        .reduce(BigDecimal.ZERO, BigDecimal::add);

Unfortunately BigDecimal is not especially ergonomic (it could use a `multiply(long)` method, which would eliminate the annoying `mapValue()` above). But unlike the python version of this, it will preserve scale. And streams work as-is on Money types.