用lambda计数嵌套的HashMap

Question

I am dealing with the following structure.

Map<String, HashMap<Integer, HashMap<Integer, Integer>>> ...

I would like to increment a counter with the values in the inner HashMap. I could easily do that with an iterative function or with nested loop. However, I am looking for an elegant solution using lambdas.

Answer 1

You can use .values().stream() in order to get a Stream of the values in a Map . Then you can repeatedly apply .flatMap in order to peel away the nested structures until you get a Stream<Integer> . Then you can create an IntStream using .mapToInt to get the .sum() of all the values.

int sum = outerMostMap.values().stream()   // HashMap<Integer, HashMap<Integer, Integer>>
    .flatMap(map -> map.values().stream()) // HashMap<Integer, Integer>
    .flatMap(map -> map.values().stream()) // Integer
    .mapToInt(Integer::intValue)           // int
    .sum();