Java 8流映射 <String, List<String> &gt;每个键的值总和

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

Map<String, Double> finalResult = inputMap.entrySet()
        .stream()
        .collect(Collectors.toMap(
                Entry::getKey,
                e -> e.getValue()
                      .stream()
                      .mapToDouble(str -> computeScore(str))
                      .sum()));

Above code iterates over the map and creates a new map with same keys & before putting the values, it first iterates over each value - which is a list, computes score via calling computeScore() over each list element and then sums the scores collected to be put in the value.

Answer 2

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 ie T::computeScore where T is the name of the class containing computeScore .

Answer 3

How about this one:

Map<String, Double> finalResult = inputMap.entrySet()
    .stream()
    .map(entry -> new AbstractMap.SimpleEntry<String, Double>(   // maps each key to a new
                                                                 // Entry<String, Double>
        entry.getKey(),                                          // the same key
        entry.getValue().stream()                             
            .mapToDouble(string -> computeScore(string)).sum())) // List<String> mapped to 
                                                                 // List<Double> and summed
    .collect(Collectors.toMap(Entry::getKey, Entry::getValue));  // collected by the same 
                                                                 // key and a newly 
                                                                 // calulcated value

The version above could be merged to the single collect(..) method:

Map<String, Double> finalResult = inputMap.entrySet()
    .stream()
    .collect(Collectors.toMap(
         Entry::getKey,                        // keeps the same key
         entry -> entry.getValue()
                       .stream()               // List<String> -> Stream<String>
                                               // then Stream<String> -> Stream<Double>
                       .mapToDouble(string -> computeScore(string)) 
                       .sum()));               // and summed 

The key parts:

• collect(..) performs a reduction on the elements using a certain strategy with a Collector .
• Entry::getKey is a shortcut for entry -> entry.getKey . A function for mapping the key.
• entry -> entry.getValue().stream() returns the Stream<String>
• mapToDouble(..) returns the DoubleStream. This has an aggregating operation sum(..) which sums the elements - together creates a new value for the Map.

Answer 4

Regardless of whether you use the stream-based or the loop-based solution, it would be beneficial and add some clarity and structure to extract the inner loop into a method:

private double computeScore(Collection<String> strings) 
{
    return strings.stream().mapToDouble(this::computeScore).sum();
}

Of course, this could also be implemented using a loop, but ... that's exactly the point: This method can now be called, either in the outer loop, or on the values of a stream of map entries.

The outer loop or stream could also be pulled into a method. In the example below, I generalized this a bit: The type of the keys of the map does not matter. Neither does whether the values are List or Collection instances.

As an alternative to the currently accepted answer, the stream-based solution here does not fill a new map that is created manually. Instead, it uses a Collector .

(This is similar to other answers, but I think that the extracted computeScore method greatly simplifies the otherwise rather ugly lambdas that are necessary for the nested streams)

import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collectors;

public class ToStreamOrNotToStream
{
    public static void main(String[] args)
    {
        ToStreamOrNotToStream t = new ToStreamOrNotToStream();

        Map<String, List<String>> inputMap =
            new LinkedHashMap<String, List<String>>();
        inputMap.put("A", Arrays.asList("1.0", "2.0", "3.0"));
        inputMap.put("B", Arrays.asList("2.0", "3.0", "4.0"));
        inputMap.put("C", Arrays.asList("3.0", "4.0", "5.0"));

        System.out.println("Result A: " + t.computeA(inputMap));
        System.out.println("Result B: " + t.computeB(inputMap));
    }

    private <T> Map<T, Double> computeA(
        Map<T, ? extends Collection<String>> inputMap)
    {
        Map<T, Double> finalResult = new HashMap<>();
        for (Entry<T, ? extends Collection<String>> entry : inputMap.entrySet())
        {
            double score = computeScore(entry.getValue());
            finalResult.put(entry.getKey(), score);
        }
        return finalResult;
    }

    private <T> Map<T, Double> computeB(
        Map<T, ? extends Collection<String>> inputMap)
    {
        return inputMap.entrySet().stream().collect(
            Collectors.toMap(Entry::getKey, e -> computeScore(e.getValue()))); 
    }

    private double computeScore(Collection<String> strings) 
    {
        return strings.stream().mapToDouble(this::computeScore).sum();
    }

    private double computeScore(String a)
    {
        return Double.parseDouble(a);
    }

}

Answer 5

我发现它有点短：

value = startDates.entrySet().stream().mapToDouble(Entry::getValue).sum();