learning-hour-maker/ideas/slice-with-lambda/exercise/README.md

Exercise folder — Slice with Lambda (learning hour)

Contents:

• java/ — pre-factored exercise repo (prefactored-longrange; runs with mvn test)
• solution/ — reference end state after step 3 (extracted method taking the lambda)
• rehearse-with-agent.md — how Willem rehearses this with qwen-code + java-refactoring-mcp

Reference solution

See solution/. End state of the production code (simplified):

public Map<IceCream, Integer> longRangeForecast() {
    return longRangeForecast(daysForward -> {
        scorer.updateSelection();
        return scorer.lookupWeather(daysForward);
    });
}

public Map<IceCream, Integer> longRangeForecast(
        LongFunction<Boolean> dayOffsetToWeather) {
    var expectedWeather = collectExpectedWeather(dayOffsetToWeather);
    // ... reset loop + scoring loop, unchanged ...
}

private static List<Boolean> collectExpectedWeather(
        LongFunction<Boolean> dayOffsetToWeather) {
    var expectedWeather = new ArrayList<Boolean>();
    for (Instant interestingDate : DATES) {
        long daysForward = ChronoUnit.DAYS.between(ANCHOR_DATE, interestingDate);
        expectedWeather.add(dayOffsetToWeather.apply(daysForward));
    }
    return expectedWeather;
}

Notes on the end state:

• Key move: scorer is used only inside the lambda; the extracted method collectExpectedWeather has no dependency on Scorer at all — all its inputs are parameters.
• The public no-arg overload keeps callers unchanged; it injects the "real" weather lambda. The test calls the overload with a pure daysForward -> true instead.
• The test is pure and glanceable: alwaysSunny weather gives {Strawberry:12, Chocolate:12, Vanilla:17, } (approved file included).
• scorer.updateSelection() turned out to be deletable (it only mutates static Scorer.flavour state that this method never reads). Leaving it in the lambda is fine for step 3; deleting it is a debrief aha.