Java 8 stream Map<String, List<String>> sum of values for each key

Question

I am not so familiar with Java 8 (still learning) and looking to see if I could find something equivalent of the below code using streams.

The below code mainly tries to get corresponding double value for each value in String and then sums it up. I could not find much help anywhere on this format. I am not sure if using streams would clean up the code or would make it messier.

// safe assumptions - String/List (Key/Value) cannot be null or empty
// inputMap --> Map<String, List<String>>

Map<String, Double> finalResult = new HashMap<>();
for (Map.Entry<String, List<String>> entry : inputMap.entrySet()) {
    Double score = 0.0;
    for (String current: entry.getValue()) {
        score += computeScore(current);
    }
    finalResult.put(entry.getKey(), score);
}

private Double computeScore(String a) { .. }

Answer 1

You could also use the forEach method along with the stream API to yield the result you're seeking.

Map<String, Double> resultSet = new HashMap<>();
inputMap.forEach((k, v) -> resultSet.put(k, v.stream()
            .mapToDouble(s -> computeScore(s)).sum()));

s -> computeScore(s) could be changed to use a method reference i.e. T::computeScore where T is the name of the class containing computeScore.