@ray0625 就是一个简单的 reduce ，关键字搜 java8 stream reduce

stream 风格
```
public static List<Element> retainElementList(List<List<Element>> elementLists) {
checkNotNull(elementLists, "elementLists should not be null!");
Optional<List<Element>> result = elementLists.parallelStream().filter(elementList -> elementList != null && ((List) elementList).size() != 0).reduce((a, b) -> {
a.retainAll(b);
return a;
});
return result.orElse(new ArrayList<>());
}
```

觉得转型成功的人都很厉害…