Java By Comparison

Avoid Side Effects

class Inventory {

    List<Supply> supplies = new ArrayList<>();

    long countDifferentKinds() {
        List<String> names = new ArrayList<>();

        Consumer<String> addToNames = name -> names.add(name);

        supplies.stream()
                .filter(Supply::isUncontaminated)
                .map(Supply::getName)
                .distinct()
                .forEach(addToNames);
        return names.size();
    }
}

In theory, there are no side effects with functional programming. Everything’s just a function that takes data as input and produces new data as output. The data you pass along is immutable.

But in imperative and object-oriented programming, we rely on side effects (we change data and state through procedures or methods) all the time. In Java, we can now mix all these styles. ...