使用Java 8流API将列表的列表展平为列表
我有以下代码,使用Java 8流API可能会更简单:
I have the following code which could be much simpler using Java 8 stream API:
List<List<String>> listOfListValues;
public List<String> getAsFlattenedList() {
List<String> listOfValues= new ArrayList<>();
for (List<String> value: listOfListValues) {
listOfValues.add(String.valueOf(value));
}
return listOfValues;
}
我在SO上搜索了一个解决方案,发现了这一点:
I searched for a solution on SO and found this:
listOfListValues.stream()
.flatMap(List::stream)
.collect(Collectors.toList());
但这与我想要的功能不同.
But this doesn't do the same what I want.
在这里您只需要简单" map
:
You require only a "simple" map
here:
List<List<String>> listOfListValues;
public List<String> getAsFlattenedList() {
return listOfListValues.stream()
.map(String::valueOf)
.collect(toList());
}
flatMap
而是用于将一个Stream
转换为另一个Stream
,这在您需要比当前可用的条目更多或更少的条目时有意义(或者仅是新映射的Stream
对其进行向下过滤/映射/等操作)是有道理的,例如:
flatMap
is rather used to transform one Stream
to another, which makes sense if you need more entries or less then currently available (or just a newly mapped Stream
to filter/map/etc it further down), e.g.:
-
计算所有列表的所有唯一字符串:
count all the unique strings of all lists:
listOfListValues.stream()
.flatMap(List::stream) // we want to count all, also those from the "inner" list...
.distinct()
.count()
在一定大小后截断条目:
truncate entries after a certain size:
listOfListValues.stream()
.flatMap(list -> {
if (list.size() > 3) // in this case we use only some of the entries
return Stream.concat(list.subList(0, 2).stream(), Stream.of("..."));
else
return list.stream();
})
.collect(Collectors.toList());
为多个感兴趣的键展平地图的值:
flattening values of a map for several interested keys:
Map<Key, List<Value>> map = new HashMap<>();
Stream<Value> valueStream = interestedKeys.stream()
.map(map::get)
.flatMap(List::stream);