Kotlin to the rescue
fold the universe: reduction with java 8 streams API
DomainEvents.kt
@file:JvmName("DomainEvents") interface DomainEvent { val type: Class<out DomainEvent> get() = this.javaClass } class CounterCreatedEvent : DomainEvent class CounterIncrementedEvent : DomainEvent class CounterDecrementedEvent : DomainEvent fun Aggregate.apply(event: DomainEvent): Aggregate = when (event) { is CounterCreatedEvent -> this.on(event) is CounterIncrementedEvent -> this.on(event) is CounterDecrementedEvent -> this.on(event) else -> throw IllegalStateException("unexpected unsupported domain event occur: $event") }Some.java
Optional<Aggregate> foldLeft(Aggregate initialState, DomainEvent... events) { if (events.length <= 0) return Optional.empty(); Aggregate accumulator = initialState; for (DomainEvent event : events) accumulator = DomainEvents.apply(accumulator, event); return Optional.of(accumulator); }./gradlew clean test