Java 8 Stream API收集器

Java 8 Stream API收集器 - addAll不适用于参数Object，对于Object类型，addAll未定义(Java 8 Stream API Collector - addAll not applicable for argument Object, addAll is undefined for type Object)

我的代码没有编译，我不太清楚为什么。这是代码：

ArrayList<ClassificationData> classifications = productData .stream() .filter(p -> CollectionUtils.isNotEmpty(p.getClassifications())) .flatMap(p -> p.getClassifications().stream()) .collect(groupingBy(ClassificationData::getName, mapping(ClassificationData::getFeatures, Collector.of(LinkedHashSet<FeatureData>::new, (a,b) -> b.addAll(a), (a,b) -> { b.addAll(a); return b; }) ))) .entrySet() .stream() .map(e -> { ClassificationData c = new ClassificationData(); c.setName(e.getKey()); c.setFeatures(e.getValue()); return c; }) .collect(Collectors.toCollection(ArrayList::new));

而错误：

(a,b) -> b.addAll(a), The method addAll(Collection<? extends FeatureData>) in the type Collection<FeatureData> is not applicable for the arguments (Object) b.addAll(a); The method addAll(Object) is undefined for the type Object c.setFeatures(e.getValue()); The method setFeatures(Collection<FeatureData>) in the type ClassificationData is not applicable for the arguments (Object)

我也尝试过Set :: add和Set :: addAll，结果非常相似。

编辑：

我最终得到了这段代码。请告诉我是否有更干净的方法来做到这一点，还是可以的？

ArrayList<ClassificationData> classifications = productData .stream() .filter(p -> CollectionUtils.isNotEmpty(p.getClassifications())) .flatMap(p -> p.getClassifications().stream()) .collect(groupingBy(ClassificationData::getName, mapping(ClassificationData::getFeatures, toCollection(LinkedHashSet::new) ))) .entrySet() .stream() .map(e -> { ClassificationData c = new ClassificationData(); c.setCode(e.getKey()); c.setName(e.getKey()); c.setFeatures(e.getValue() .stream() .filter(CollectionUtils::isNotEmpty) .flatMap(p -> p.stream()) .filter(distinctByKey(FeatureData::getName)) .collect(toCollection(ArrayList::new))); return c; }) .collect(toCollection(ArrayList::new));

My code is not compiling and I'm not quite sure why. Here is the code:

ArrayList<ClassificationData> classifications = productData .stream() .filter(p -> CollectionUtils.isNotEmpty(p.getClassifications())) .flatMap(p -> p.getClassifications().stream()) .collect(groupingBy(ClassificationData::getName, mapping(ClassificationData::getFeatures, Collector.of(LinkedHashSet<FeatureData>::new, (a,b) -> b.addAll(a), (a,b) -> { b.addAll(a); return b; }) ))) .entrySet() .stream() .map(e -> { ClassificationData c = new ClassificationData(); c.setName(e.getKey()); c.setFeatures(e.getValue()); return c; }) .collect(Collectors.toCollection(ArrayList::new));

And the errors:

(a,b) -> b.addAll(a), The method addAll(Collection<? extends FeatureData>) in the type Collection<FeatureData> is not applicable for the arguments (Object) b.addAll(a); The method addAll(Object) is undefined for the type Object c.setFeatures(e.getValue()); The method setFeatures(Collection<FeatureData>) in the type ClassificationData is not applicable for the arguments (Object)

I've also tried Set::add and Set::addAll with pretty much the same result.

EDIT:

I've ended up with this code. Please tell me if there is a cleaner way to do this, or is it ok?

ArrayList<ClassificationData> classifications = productData .stream() .filter(p -> CollectionUtils.isNotEmpty(p.getClassifications())) .flatMap(p -> p.getClassifications().stream()) .collect(groupingBy(ClassificationData::getName, mapping(ClassificationData::getFeatures, toCollection(LinkedHashSet::new) ))) .entrySet() .stream() .map(e -> { ClassificationData c = new ClassificationData(); c.setCode(e.getKey()); c.setName(e.getKey()); c.setFeatures(e.getValue() .stream() .filter(CollectionUtils::isNotEmpty) .flatMap(p -> p.stream()) .filter(distinctByKey(FeatureData::getName)) .collect(toCollection(ArrayList::new))); return c; }) .collect(toCollection(ArrayList::new));