I have a list of map of string string(List<Map<String,String>>) like bellow,
[{key = "car", value = "bmw"},{key = "car", value = "suzuki"},{key = "car", value = "hyundai"},{key = "bike", value = "honda"},{key = "bike", value = "tvs"}]
I want to convert to below list of map string list string(List<Map<String, List<String>>>) or something equal
[{key = "car", value = ["bmw","suzuki","hyundai"]},{key = "bike", value = ["honda","tvs"]}]
Thanks in advance.
Use groupingBy to separate by key and then mapping to get the value of your singleton map
Map<String, List<String>> res =
list.stream()
.map(map -> map.entrySet().iterator().next())
.collect(Collectors.groupingBy(
entry -> entry.getKey(),
Collectors.mapping(
entry -> entry.getValue(),
Collectors.toList()
)
)
);
You do the same as above, but flatMap to get all the entries in a single 1-D stream
Map<String, List<String>> res =
list.stream()
.flatMap(map -> map.entrySet().stream())
.collect(Collectors.groupingBy(
entry -> entry.getKey(),
Collectors.mapping(
entry -> entry.getValue(),
Collectors.toList()
)
)
);
You'll have to iterate through each map in the list, and then through the entries of each map. For each entry, check if the key's already there, and if so, add the value to the list associated with that key. If the key's not in the result map, then create a new list there.
Map<String, List<String>> map = new HashMap<>();
for (Map<String, String> m : list) {
for (Map.Entry<String, String> e : m.entrySet()) {
String key = e.getKey();
if (!map.containsKey(key)) {
map.put(key, new ArrayList<String>());
}
map.get(key).add(e.getValue());
}
}
For each map in your original list, you find the first entry, and then you do the same as above: add that entry's value to the list that the entry's key maps to, and before that, add a new List to the map if the key doesn't exist there already.
Map<String, List<String>> map = new HashMap<>();
for (Map<String, String> m : list) {
Map.Entry<String, String> e = m.entrySet().iterator().next();
String key = e.getKey();
if (!map.containsKey(key)) {
map.put(key, new ArrayList<String>());
}
map.get(key).add(e.getValue());
}
Based on your example, you have a list of maps like this:
List<Map<String,String>> myListOfMaps = List.of(
Map.of("car", "bmw"),
Map.of("car", "suzuki"),
Map.of("car", "hyundai"),
Map.of("bike", "honda"),
Map.of("bike", "tvs")
);
Then you can use the groupingBy function of streams to do something similar to what you see below.
Stream your list, flatten the stream by streaming over the entrysets, group the entrysets by their keys, map to a list of values
Map<String,List<String>> myResultMap = myListOfMaps.stream()
.flatMap(m -> m.entrySet().stream())
.collect(Collectors.groupingBy(
Map.Entry::getKey, Collectors.mapping(Map.Entry::getValue, Collectors.toList())));
use result/print ..
myResultMap.forEach((k,v)-> {
System.out.println(k + " : " + v);
});
// car : [bmw, suzuki, hyundai]
// bike : [honda, tvs]
