Convert List to Map Using Java

Java

In this article, let’s check how we can convert a given List to a Java Map.

Suppose we have a User class having name and age fields, and we create three User objects and keep them in a list.

We want a map with user.name as map key, and user object as map value.

  @Data
  @NoArgsConstructor
  @AllArgsConstructor
  public class User {
      private String name;
      private Integer age;
  }
  // ...

  User user1 = new User("Jack", 25);
  User user2 = new User("Tom", 30);
  User user3 = new User("Tom", 33);
  List<User> list = Arrays.asList(user1, user2, user3);

There might be duplicates in our list, so we should be carefully while writing converting codes.

Plain Java For Loop

  Map<String, User> map = new HashMap<>();
  for (User u : list) {
      if (!map.containsKey(u.getName())) {
          map.put(u.getName(), u);
      }
  }
  log.info("map: {}", map);

Java 8 Stream

  Map<String, User> map = list.stream()
      .collect(Collectors.toMap(User::getName, Function.identity());
  log.info("map: {}", map);

Collectors.toMap() will throw IllegalStateException when the mapped keys contain duplicates, If the mapped keys might have duplicates, we can pass an aditional function parameter to resolve collisions between values.

  Map<String, User> map = list.stream()
      .collect(Collectors.toMap(User::getName, Function.identity(), (o1, o2) -> o2));
  log.info("map: {}", map);

Apache Commons MapUtils.populateMap()

If you have Apache Commons Collections library in your classpath, you can also use MapUtils.populateMap(), which populates a Map using the supplied Transformers to transform the elements into keys and values.

  
    MapUtils.populateMap(map, list, User::getName, o -> o);
    log.info("map: {}", map);

Besides, this method helps us deal with duplicate data. From the source codes we can know that when there is duplicate data, only the last occurrence would be retained.

  public static <K, V, E> void populateMap(final Map<K, V> map, final Iterable<? extends E> elements,
                                            final Transformer<E, K> keyTransformer,
                                            final Transformer<E, V> valueTransformer) {
      final Iterator<? extends E> iter = elements.iterator();
      while (iter.hasNext()) {
          final E temp = iter.next();
          map.put(keyTransformer.transform(temp), valueTransformer.transform(temp));
      }
  }

Google Guava Maps.uniqueIndex()

If you have Google Guava library in your classpath, you can also use Maps.uniqueIndex().

But it is not a recomended method because it will also throw IllegalArgumentException when there are multiple entries with same key.

  try {
        map = Maps.uniqueIndex(list, User::getName);
        log.info("map: {}", map);
    } catch (Exception e) {
        log.error("", e);
    }

Read More