I have a structure such as Map<String,List<Map<String,Object>>
. I want to apply a function to the map as follows. The method takes a key and uses a
Map<String,Object>
of the list. Each key has several Map<String,Object>
in the list. How can I apply the process method to the map's key for each value of Map<String,Object>
? I was able to use to forEach loops(see below) but I have a feeling this is not the best way to solve the problem in a functional way.
TypeProcessor p=new TypeProcessor.instance();
//Apply this function to the key and each map from the list
// The collect the Result returned in a list.
Result process(String key, Map<String,Object> dataPoints);
List<Result> list = new ArrayList<>();
map.forEach(key,value) -> {
value.forEach(innerVal -> {
Result r=p.process(key,innerVal);
list.add(r):
});
});
Method 2: List ListofKeys = new ArrayList(map. keySet()); We can convert Map keys to a List of Values by passing a collection of map values generated by map. values() method to ArrayList constructor parameter.
Java 8 stream map() map() method in java 8 stream is used to convert an object of one type to an object of another type or to transform elements of a collection. map() returns a stream which can be converted to an individual object or a collection, such as a list.
It seems from your code that you want to apply process
for the entire Map
, so you could do it like this:
List<Result> l = map.entrySet()
.stream()
.flatMap(e -> e.getValue().stream().map(value -> process(e.getKey(), value)))
.collect(Collectors.toList());
Well, assuming map
contains key
, you don't need any foreach
. Just obtain the value from the outer map
, stream it, map to your new object and collect to a List
:
List<Result> list =
map.get(key)
.stream()
.map(v -> p.process(key,v))
.collect(Collectors.toList());
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