[英]Converting Map<K, V> to Map<V,List<K>>
I have map as below 我有如下地图
Map<String, String> values = new HashMap<String, String>();
values.put("aa", "20");
values.put("bb", "30");
values.put("cc", "20");
values.put("dd", "45");
values.put("ee", "35");
values.put("ff", "35");
values.put("gg", "20");
I want to create new map in the format Map<String,List<String>>
, sample output will be as 我想以
Map<String,List<String>>
的格式创建新的地图,样本输出将为
"20" -> ["aa","cc","gg"]
"30" -> ["bb"]
"35" -> ["ee","ff"]
"45" -> ["dd"]
I am able to do by iterating through entity 我可以通过迭代实体来做
Map<String, List<String>> output = new HashMap<String,List<String>>();
for(Map.Entry<String, String> entry : values.entrySet()) {
if(output.containsKey(entry.getValue())){
output.get(entry.getValue()).add(entry.getKey());
}else{
List<String> list = new ArrayList<String>();
list.add(entry.getKey());
output.put(entry.getValue(),list);
}
}
Can this be done better using streams? 使用流可以做得更好吗?
groupingBy
can be used to group the keys by the values. groupingBy
可用于按值对键进行groupingBy
。 If used without a mapping
Collector
, it will transform a Stream
of map entries ( Stream<Map.Entry<String,String>>
) to a Map<String,List<Map.Entry<String,String>>
, which is close to what you want, but not quite. 如果在没有使用
mapping
Collector
,将变换Stream
映射条目(的Stream<Map.Entry<String,String>>
)到Map<String,List<Map.Entry<String,String>>
,这是接近你想要什么,但不完全。
In order for the value of the output Map
to be a List
of the original keys, you have to chain a mapping
Collector
to the groupingBy
Collector
. 为了使输出
Map
的值成为原始键的List
,您必须将mapping
Collector
到groupingBy
Collector
。
Map<String,List<String>> output =
values.entrySet()
.stream()
.collect(Collectors.groupingBy(Map.Entry::getValue,
Collectors.mapping(Map.Entry::getKey,
Collectors.toList())));
System.out.println (output);
Output : 输出:
{45=[dd], 35=[ee, ff], 30=[bb], 20=[aa, cc, gg]}
Note that in Java 8, you can also do better without using streams using Map.forEach
and Map.computeIfAbsent
. 请注意,在Java 8中,如果不使用
Map.forEach
和Map.computeIfAbsent
使用流,也可以Map.computeIfAbsent
。 This way, it is more concise than the old version with Map.Entry<String, String>
, entry.getValue()
, entry.getKey()
etc. 这样,它比使用
Map.Entry<String, String>
, entry.getValue()
, entry.getKey()
等的旧版本更简洁。
So you don't have to compare the old Java-7 iteration to that Java-8 stream solution, but to this one. 因此,您不必将旧的Java-7迭代与Java-8流解决方案进行比较,而是将其与此进行比较。
values.forEach( (key,value)->
groupBy.computeIfAbsent(value, x->new ArrayList<>())
.add(key)
);
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