Java 8从Map中的匹配值中提取所有键 [英] Java 8 extract all keys from matching values in a Map

问题描述

我对Java8还是比较陌生，我有一个场景，我需要从Map中检索与对象匹配的所有键.

I'm relatively new to Java8 and I have a scenario where I need to retrieve all the keys from the Map which matched with the objects.

想知道是否有一种方法可以获取所有密钥，而无需再次从列表中进行迭代.

Wanted to know if there is a way to get all keys without iterating them from the list again.

Person.java
private String firstName;
private String lastName;
//setters and getters & constructor


MAIN Class.

String inputCriteriaFirstName = "john";   

Map<String, Person> inputMap = new HashMap<>();
Collection<Person> personCollection = inputMap.values();
List<Person> personList = new ArrayList<>(personCollection);
List<Person> personOutputList = personList.stream()
.filter(p -> p.getFirstName().contains(inputCriteriaFirstName ))
.collect(Collectors.toList());


//IS There a BETTER way to DO Below ??

Set<String> keys = new HashSet<>();
for(Person person : personOutputList) {
    keys.addAll(inputMap.entrySet().stream().filter(entry -> Objects.equals(entry.getValue(), person))
        .map(Map.Entry::getKey).collect(Collectors.toSet()));
}

推荐答案

inputMap.entrySet() 
        .stream()
        .filter(entry -> personOutputList.contains(entry.getValue()))
        .map(Entry::getKey)
        .collect(Collectors.toCollection(HashSet::new))

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