Lesson 07

Streams vs readable loops

Use streams for clear transformations and loops when branching, validation, or naming intermediate steps makes the review easier.

streams
loops
readability

Good Code

src/main/java/dev/review/orders/DiscountCalculator.java

public Money discountFor(List<OrderLine> lines) {
    Money discount = Money.zero();

    for (OrderLine line : lines) {
        if (!line.isDiscountable()) {
            continue;
        }

        Money lineDiscount = policy.discountFor(line);
        discount = discount.plus(lineDiscount);
    }

    return discount;
}

Bad Code

src/main/java/DiscountCalculator.java

public Money discountFor(List<OrderLine> lines) {
    AtomicReference<Money> discount = new AtomicReference<>(Money.zero());

    lines.stream()
        .filter(line -> line.isDiscountable())
        .peek(line -> audit.log(line.id()))
        .map(line -> policy.discountFor(line))
        .forEach(value -> discount.set(discount.get().plus(value)));

    return discount.get();
}

Review Notes

What to review

Good Code

The good version uses a loop because the operation has branching and a named accumulation step. Each review question has a place to attach.

Bad Code

The bad version hides mutation inside a stream pipeline and uses peek for side effects. The stream looks functional, but it is harder to reason about.

Takeaways

The best Java iteration shape is the one that makes intent and failure points easiest to review.