Java Spring-boot：如何返回列表而不是单个结果？

Question

I'm new to Java and using the Spring-Boot framework. I have implemented a small method that given a transactionId gives me the list of all objects having that transactionId. I just have problems managing the list in the ServiceImpl. as I can't get a list back instead of a single result.

Answer 1

If the mapper is working for a single instance of your entities, you can simply iterate over the entity collection and map one entity at a time. Or you use Java8 streams:

List<StoredMessageModTrackEntity> entityList = repo.findAllByTransactionId(transactionId);
return entityList.stream().map(mapper::toDtoMapper).collect(Collectors.toList());

Unlike your example the above snippet will return an empty list, if entityList is empty. Unless you explicitly need to return null (by some weird API contract or similar) you should not use null for empty collections. This will just result in bulky code as all consumers will always need to check for null . If you really (and I mean really) need null , you can either keep your if-statement or use Optional :

Optional.ofNullable(entityList).filter(Objects::nonNull)
    .map(list -> list.stream().map(mapper::toDtoMapper)
    .collect(Collectors.toList())).orElse(null);