示例Map<String, List<String>>的inputMap：

{"product":["132377","2123232","312335678","423432","5215566"],"order":["3174252","1468453","1264543","35723112","235775645"]}

1、使用forEach实现

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

2、使用collect()实现

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

或

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

3、Collect的使用

以stream的元素转变成一种不同的结果，可以是一个List，Set或Map。

例如：

List<Person> filtered = persons .stream() .filter(p -> p.name.startsWith("P")) .collect(Collectors.toList()); System.out.println(filtered);

输出：

 [Peter, Pamela]

Stream文档：https://docs.oracle.com/javase/8/docs/api/java/util/stream/Stream.html