Convert List of Maps to a single Map with VAVR-CodePudding

I'm struggling to reduce a List of Maps to a single Map using Vavr

<List<Map<String, Object>>>

<Map<String, Object>>

I tried it with flatmap/reduce/merge but unfortunately with no happy end :-(

Can someone provided an example how to do it with io.vavr.collection.List/io.vavr.collection.Map ?

CodePudding user response：

I have no idea what vavr has to do with this.

list.stream()                             // A
  .flatMap(map -> map.entrySet().stream()) // B
  .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

[A] Now you have a stream of Map<String, Object>.

[B] Now you have a stream of map entries. Now you have a stream of key/value pairs.

Then just collect them into a map by providing a function to extract a key from a Map.Entry object, and a function that extracts a value.

Note that if you have a map of type Map<String, Object>, I give it 99% odds you're doing something wrong. Java is nominally, statically, and strongly typed, after all.

CodePudding user response：

You have several options.

For instance, given the following data set (sorry for its simplicity):

Map<String, Object> map1 = HashMap.of("key1", "value1", "key2", "value2");
Map<String, Object> map2 = HashMap.of("key3", "value3", "key4", "value4");
Map<String, Object> map3 = HashMap.of("key5", "value5", "key6", "value6");
List<Map<String, Object>> list = List.of(map1, map2, map3);

You can combine the different Maps with fold, for instance:

Map<String, Object> result = list.fold(HashMap.empty(), (m1, m2) -> m1.merge(m2));

You can use reduce as well:

Map<String, Object> result = list.reduce((m1, m2) -> m1.merge(m2));

Both of them use the Map merge method.