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[![GraalVM](https://github.com/daggerok/fold-java/actions/workflows/ci.yaml/badge.svg)](https://github.com/daggerok/fold-java/actions/workflows/ci.yaml)\nKotlin to the rescue\n\n[fold the universe: reduction with java 8 streams API](https://dzone.com/articles/folding-the-universe-part-iii-java-8-list-and-stre)\n\n\u003c!--I don't know why, but people so confused about that simple operation...--\u003e\n\n_DomainEvents.kt_\n\n```kotlin\n@file:JvmName(\"DomainEvents\")\n\ninterface DomainEvent {\n  val type: Class\u003cout DomainEvent\u003e\n    get() = this.javaClass\n}\n\nclass CounterCreatedEvent : DomainEvent\nclass CounterIncrementedEvent : DomainEvent\nclass CounterDecrementedEvent : DomainEvent\n\nfun Aggregate.apply(event: DomainEvent): Aggregate = when (event) {\n  is CounterCreatedEvent -\u003e this.on(event)\n  is CounterIncrementedEvent -\u003e this.on(event)\n  is CounterDecrementedEvent -\u003e this.on(event)\n  else -\u003e throw IllegalStateException(\"unexpected unsupported domain event occur: $event\")\n}\n```\n\n_Some.java_\n\n```java\nOptional\u003cAggregate\u003e foldLeft(Aggregate initialState, DomainEvent... events) {\n  if (events.length \u003c= 0) return Optional.empty();\n  Aggregate accumulator = initialState;\n  for (DomainEvent event : events)\n    accumulator = DomainEvents.apply(accumulator, event);\n  return Optional.of(accumulator);\n}\n```\n\n## Run tests\n\n```bash\n./gradlew clean test\n```\n\n## Links\n\n- [lazy-java](https://github.com/daggerok/lazy-java)\n- [java-examples](https://github.com/daggerok/java-examples)\n- [learn-jvm](https://github.com/daggerok/learn-jvm)\n- [fold-java](https://github.com/daggerok/fold-java)\n","project_url":"https://awesome.ecosyste.ms/api/v1/projects/github.com%2Fdaggerok%2Ffold-java","html_url":"https://awesome.ecosyste.ms/projects/github.com%2Fdaggerok%2Ffold-java","lists_url":"https://awesome.ecosyste.ms/api/v1/projects/github.com%2Fdaggerok%2Ffold-java/lists"}