Having an instance of a Map<A, Set<B>>
where B
is a POJO with property Integer price
(among others), how do I elegantly transform this map into a Map<A, Integer>
where Integer
denotes the sum of prices
of a given set, for each key A
?
You can use stream like this:
Map<A, Integer> result = map.entrySet().stream()
.collect(Collectors.toMap(
Map.Entry::getKey,
b -> b.getValue().stream()
.mapToInt(B::getPrice)
.sum()
));
That should help you out. Using the Streaming-API from Java 8: (In this example the class "PricePojo" is your B and "String" is your A)
Map<String, Integer> newMap = map.entrySet().stream()//
.collect(Collectors.toMap(Entry::getKey, entry -> entry.getValue().stream().mapToInt(PricePojo::getPrice).sum()));
You could also go the way of implementing a Map.Entry yourself like in the following example:
Map<String, Integer> newMap = map.entrySet().stream()//
.map(entry -> new Map.Entry<String, Integer>() {
private int sum = entry.getValue().stream().mapToInt(PricePojo::getPrice).sum();
@Override
public String getKey() {
return entry.getKey();
}
@Override
public Integer getValue() {
return sum;
}
@Override
public Integer setValue(Integer value) {
return sum = value.intValue();
}
}).collect(Collectors.toMap(Entry::getKey, Entry::getValue));
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