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

java, java-8, java-stream

Solution

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());

Problem

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?

Original source