Java 8 extract first key from matching value in a Map

问题

Suppose I have a map of given name, surname pairs and I want to find the given name of the first entry in that map that has the surname matching a certain value. How would we do this in a java 8 fashion.

In my test case example below I put two ways that would do it.

However the first one (looking for the given name of the first person with a surname of "Donkey") will throw java.util.NoSuchElementException: No value present so it is not safe.

The second one works but it is not only harder to read but it it is a bit not quite functional.

Just wondering if someone here would suggest me an easier clearer way of achieving this using either stream() or forEach() or both.

@Test
public void shouldBeAbleToReturnTheKeyOfTheFirstMatchingValue() throws Exception {
    Map<String, String> names = new LinkedHashMap<>();
    names.put("John", "Doe");
    names.put("Fred", "Flintstone");
    names.put("Jane", "Doe");
    String keyOfTheFirst = names.entrySet().stream().filter(e -> e.getValue().equals("Doe")).findFirst().get().getKey();
    assertEquals("John", keyOfTheFirst);

    try {
        names.entrySet().stream().filter(e -> e.getValue().equals("Donkey")).findFirst().get();
    } catch (NoSuchElementException e){
        // Expected
    }

    Optional<Map.Entry<String, String>> optionalEntry = names.entrySet().stream().filter(e -> e.getValue().equals("Donkey")).findFirst();
    keyOfTheFirst = optionalEntry.isPresent() ? optionalEntry.get().getKey() : null;

    assertNull(keyOfTheFirst);
}

Thank you in advance.

回答1:

To return a default value if there is no match, use Optional#orElse

names.entrySet().stream()
  .filter(e -> e.getValue().equals("Donkey"))
  .map(Map.Entry::getKey)
  .findFirst()
  .orElse(null);

回答2:

The solution provided by @Misha is the best one if you don't want to use the third-party code. My library has the special shortcut method ofKeys for such cases as I discovered that it's quite common task:

StreamEx.ofKeys(names, "Donkey"::equals).findFirst().orElse(null);

回答3:

From a similar question:

public static <T, E> Set<T> getKeysByValue(Map<T, E> map, E value) {
    return map.entrySet()
              .stream()
              .filter(entry -> Objects.equals(entry.getValue(), value))
              .map(Map.Entry::getKey)
              .collect(Collectors.toSet());
}

Then you can select the first, if you want to. Remember that the key is unique, the value is not.

Edit: The whole code (thanks @Peter Lawrey)

package test;

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Objects;
import java.util.Optional;

public class Main {

    public static void main(String[] args) {
        Map<String, String> names = new LinkedHashMap<>();
        names.put("John", "Doe");
        names.put("Fred", "Flintstone");
        names.put("Jane", "Doe");

        Optional<String> firstKey = names.entrySet().stream()
                .filter(entry -> Objects.equals(entry.getValue(), "Doe"))
                .map(Map.Entry::getKey).findFirst();

        if (firstKey.isPresent()) {
            System.out.println(firstKey.get());
        }
    }
}

回答4:

I like old fashioned:

static <K, V> K findFirstKeyByValue(Map<K, V> map, String value) {
    for (Entry<K, V> e : map.entrySet())
        if (e.getValue().equals(value))
            return e.getKey();
    return null;
}

回答5:

Below is my code snippet to get key from map,

Map<String,String> pageDetails = new HashMap<String,String>();

public String getAssociatedKey(){
pageDetails.entrySet().stream().filter( e -> e.getValue().contains("John").findFirst().get().getKey();
}

回答6:

In order to avoid null pointer exception if map entry exist with null value:

private String getValueByKey(Map<String, String> map, String key)
{
        return map.entrySet().stream().filter(e -> 
        StringUtils.equalsIgnoreCase(e.getKey(), key)).findFirst().get()
                .getValue();
}

来源：https://stackoverflow.com/questions/30608360/java-8-extract-first-key-from-matching-value-in-a-map