使用 java 8 中的列表值迭代 map

Question

Iterate over a Map<Integer, List<String>> and convert to type List<KeyPair> . Any better way to do it in java 8 (Using streams).

Naive way:

final List<KeyPair> keyPairs = Lists.newArrayList();
 for (final Map.Entry<Integer, List<String>> entry : map.entrySet()) {
            for (final String value : entry.getValue()) {
                keyPairs.add(new KeyPair()
                .withHashKey(value)
                .withRangeKey(entry.getKey()));
            }
   }

Answer 1

First, iterate over the map entrySet and then you can use flatMap for list of value for the map key and create KeyPair with entry key and every value of list and collect as a list.

List<KeyPair> keyPairs = map.entrySet()
       .stream()
       .flatMap(entry -> entry.getValue()
                              .stream()
                              .map(value -> new KeyPair()
                                               .withHashKey(value)
                                               .withRangeKey(entry.getKey())))
       .collect(Collectors.toList());