Using streams, how can I map the values in a HashMap?

Question

Given a Map<String, Person> where Person has a String getName() (etc) method on it, how can I turn the Map<String, Person> into a Map<String, String> where the String is obtained from calling Person::getName()?

Pre-Java 8 I'd use

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

for (Map.Entry<String, Person> person : people.entrySet()) {
    byNameMap.put(person.getKey(), person.getValue().getName());
}

but I'd like to do it using streams and lambdas.

I can't see how to do this in a functional style: Map/HashMap don't implement Stream.

people.entrySet() returns a Set<Entry<String, Person>> which I can stream over, but how can I add a new Entry<String, String> to the destination map?

Answer 1

With Java 8 you can do:

Map<String, String> byNameMap = new HashMap<>();
people.forEach((k, v) -> byNameMap.put(k, v.getName());

Though you'd be better off using Guava's Maps.transformValues, which wraps the original Map and does the conversion when you do the get, meaning you only pay the conversion cost when you actually consume the value.

Using Guava would look like this:

Map<String, String> byNameMap = Maps.transformValues(people, Person::getName);

EDIT:

Following @Eelco's comment (and for completeness), the conversion to a map is better done with Collectors.toMap like this:

Map<String, String> byNameMap = people.entrySet()
  .stream()
  .collect(Collectors.toMap(Map.Entry::getKey, (entry) -> entry.getValue().getName());

Answer 2

One way is to use a toMap collector:

import static java.util.stream.Collectors.toMap;

Map<String, String> byNameMap = people.entrySet().stream()
                                     .collect(toMap(Entry::getKey, 
                                                    e -> e.getValue().getName()));

Answer 3

Using a bit of generic code that I've sadly not found in the libraries I had at hand

public static <K, V1, V2> Map<K, V2> remap(Map<K, V1> map,
        Function<? super V1, ? extends V2> function) {

    return map.entrySet()
            .stream() // or parallel
            .collect(Collectors.toMap(
                    Map.Entry::getKey, 
                    e -> function.apply(e.getValue())
                ));
}

This becomes essentially the same as Guavas Maps.transformValues minus the downsides mentioned by others.

Map<String, Person> persons = ...;
Map<String, String> byNameMap = remap(persons, Person::getName);

And in case you need the key as well as the value in your remapping function, this second version makes that possible

public static <K, V1, V2> Map<K, V2> remap(Map<K, V1> map,
        BiFunction<? super K, ? super V1, ? extends V2> function) {

    return map.entrySet()
            .stream() // or parallel
            .collect(Collectors.toMap(
                    Map.Entry::getKey,
                    e -> function.apply(e.getKey(), e.getValue())
                ));
}

It can be used for example like

Map<String, String> byNameMap = remap(persons, (key, val) -> key + ":" + val.getName());

Answer 4

Since Java 9 you can also do:

Entry<String, String> entry = Map.entry("a", "b");

In your Map it would be used like this:

Map<String, String> byNameMap = people.entrySet()
  .stream()
  .map(entry -> Map.entry(entry.getKey(), entry.getValue().getName()))
  .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue);