# 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):

```java
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.