如果您只想按collectionId对元素进行分组，则可以尝试类似

If you just want to group elements by collectionId you could try something like

List<AnswerCollection> collections = answerRows.stream()
    .collect(Collectors.groupingBy(x -> x.collectionId))
    .entrySet().stream()
    .map(e -> { AnswerCollection c = new AnswerCollection(); c.addAll(e.getValue()); return c; })
    .collect(Collectors.toList());

以上代码将每个collectionId产生一个AnswerCollection.

Above code will produce one AnswerCollection per collectionId.

对于Java 6和Apache Commons Collections，以下代码使用Java 8流产生与上述代码相同的结果:

With Java 6 and Apache Commons Collections, the following code produce the same results as the above code using Java 8 streams:

ListValuedMap<Long, AnswerRow> groups = new ArrayListValuedHashMap<Long, AnswerRow>();
for (AnswerRow row : answerRows)
    groups.put(row.collectionId, row);
List<AnswerCollection> collections = new ArrayList<AnswerCollection>(groups.size());
for (Long collectionId : groups.keySet()) {
    AnswerCollection c = new AnswerCollection();
    c.addAll(groups.get(collectionId));
    collections.add(c);
}