[英]Java: create copy of ArrayList of HashMaps but include only certain keys
考虑以下数据结构:
ArrayList<HashMap<String, String>> entries = new ArrayList<>();
ArrayList<String> keyNamesToInclude = new ArrayList<>();
此代码创建条目的副本,但哈希映射仅包含 keyNamesToInclude 中的键:
ArrayList<HashMap<String, String>> copies = new ArrayList<>();
for (HashMap<String, String> entry: entries) {
HashMap<String, String> copy = new HashMap<>();
for (String keyName: keyNamesToInclude) {
copy.put(keyName, entry.get(keyName));
}
copies.add(copy);
}
如何以一种功能性的方式使用 Streams 来创建它?
最好将keyNamesToInclude
转换为Set
以方便查找键。
然后使用List::stream
获取Stream<HashMap>
并为每个映射获取其条目的过滤流,重新收集到新映射中并相应地列出。
Set<String> keys = new HashSet<>(keyNamesToInclude); // removes possible duplicates
List<Map<String, String>> copies = entries.stream() // Stream<HashMap>
.map(m -> m.entrySet()
.stream()
.filter(e -> keys.contains(e.getKey()))
.collect(Collectors.toMap(
Map.Entry::getKey, Map.Entry::getValue
))
)
.collect(Collectors.toList());
如果在copies
具有List
和Map
具体实现非常重要,即使Collectors.toList()
返回ArrayList
和Collectors.toMap
返回HashMap
,也可能需要转换或特殊形式的收集Collectors.toMap
:
// casting
ArrayList<HashMap<String, String>> copies2 = (ArrayList) entries.stream() // Stream<HashMap>
.map(m -> (HashMap<String, String>) m.entrySet()
.stream()
.filter(e -> keys.contains(e.getKey()))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue))
)
.collect(Collectors.toList());
// special collectors
// toMap(keyMapper, valueMapper, mergeFunction, mapFactory)
// toList -> toCollection(ArrayList::new)
ArrayList<HashMap<String, String>> copies3 = entries.stream() // Stream<HashMap>
.map(m -> m.entrySet()
.stream()
.filter(e -> keys.contains(e.getKey()))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (a, b) -> a, HashMap::new))
)
.collect(Collectors.toCollection(ArrayList::new));
您可以执行以下操作:
Set<String> setNamesToInclude = new HashSet<>(keyNamesToInclude);
List<Map<String, String>> copies = entries.stream()
.map(hm -> hm.keySet()
.stream()
.filter(setNamesToInclude::contains)
.collect(Collectors.toMap(Function.identity(), hm::get))
)
.collect(Collectors.toList());
也许使用 hashmap 而不是 list contains 的更有效的解决方案。
List<HashMap<String, String>> copies = maps.stream().map(e -> {
HashMap<String, String> copy = new HashMap<>();
keys.forEach(k -> {
String value = e.get(k);
if (value != null)
copy.put(k, e.get(k));
});
return copy;
}).collect(Collectors.toList());
您可以执行以下操作(伪代码):
return entries.stream()
.map(hashmap -> {
return hashmap.stream()
.filter((k,v) -> keyNamesToInclude.contains(k))
.collect(toMap());
})
.collect(toList());
我建议使用Set
来包含keysToBeIncluded
因为这样效率更高一些。 这也会从最终列表中过滤掉空地图。
List<Map<String, String>> entries = List.of(
Map.of("1", "one", "2", "two", "3", "three"),
Map.of("2", "two", "3", "three"), Map.of("1", "one"),
Map.of("6", "six", "7", "seven"));
entries.forEach(System.out::println);
Set<String> keyNamesToInclude = Set.of("2", "3", "6");
List<Map<String, String>> copies = entries.stream()
.map(m -> m.entrySet().stream().filter(
e -> keyNamesToInclude.contains(e.getKey()))
.collect(Collectors.toMap(Entry::getKey,
Entry::getValue))).filter(m->!m.isEmpty())
.toList(); // java 16 - can be replaced with a collector
copies.stream().forEach(System.out::println);
打印四行源代码和三行结果
{3=three, 2=two, 1=one}
{3=three, 2=two}
{1=one}
{7=seven, 6=six}
{2=two, 3=three}
{2=two, 3=three}
{6=six}
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