In Java 10, following new methods have been added to java.util.stream.Collectors which input elements into an unmodifiable List/Set/Map. The resultant unmodifiable collections are created with the same characteristic as if they are created via copyOf() methods (last tutorial)

Followings are the new methods:

Collector<T, ?, List<T>> toUnmodifiableList()

Collector<T, ?, Set<T>> toUnmodifiableSet()

Collector<T, ?, Map<K,U>> toUnmodifiableMap(Function<? super T, ? extends K> keyMapper,
                                            Function<? super T, ? extends U> valueMapper)

Collector<T, ?, Map<K,U>> toUnmodifiableMap(Function<? super T, ? extends K> keyMapper,
                                            Function<? super T, ? extends U> valueMapper,
                                            BinaryOperator<U> mergeFunction)

Examples

Collectors.toUnmodifiableList()

public class ToUnmodifiableListExample {
  public static void main(String[] args) {
      List<Integer> list = IntStream.range(1, 5)
                                    .boxed()
                                    .collect(Collectors.toUnmodifiableList());
      System.out.println(list);
      System.out.println(list.getClass().getName());
  }
}

[1, 2, 3, 4]
java.util.ImmutableCollections$ListN

Collectors.toUnmodifiableSet()

public class ToUnmodifiableSetExample {
  public static void main(String[] args) {
      Set<Integer> set = IntStream.range(1, 5)
                                   .boxed()
                                   .collect(Collectors.toUnmodifiableSet());
      System.out.println(set);
      System.out.println(set.getClass().getTypeName());
  }
}

[4, 1, 2, 3]
java.util.ImmutableCollections$SetN

Collectors.toUnmodifiableMap()

There are two methods:

Collector<T, ?, Map<K,U>> toUnmodifiableMap(Function<? super T, ? extends K> keyMapper,
                                            Function<? super T, ? extends U> valueMapper)

public class ToUnmodifiableMapExample {
  public static void main(String[] args) {
      Map<Integer, Double> map =
              IntStream.range(1, 5)
                       .boxed()
                       .collect(Collectors.toUnmodifiableMap(
                               i -> i,
                               i -> Math.pow(i, 3))
                       );
      System.out.println(map);
      System.out.println(map.getClass().getTypeName());
  }
}

{4=64.0, 1=1.0, 3=27.0, 2=8.0}
java.util.ImmutableCollections$MapN

Here's the second method:

Collector<T, ?, Map<K,U>> toUnmodifiableMap(Function<? super T, ? extends K> keyMapper,
                                            Function<? super T, ? extends U> valueMapper,
                                            BinaryOperator<U> mergeFunction)

The mergeFunction is used to resolve collisions between values associated with the same key. Here's an example

public class ToUnmodifiableMapMergeExample {
  public static void main(String[] args) {
      Map<Integer, List<String>> map =
              Stream.of("rover", "joyful", "depth", "hunter")
                    .collect(Collectors.toUnmodifiableMap(
                            String::length,
                            List::of,
                            ToUnmodifiableMapMergeExample::mergeFunction)
                    );
      System.out.println(map);
  }

  private static List<String> mergeFunction(List<String> l1, List<String> l2) {
      List<String> list = new ArrayList<>();
      list.addAll(l1);
      list.addAll(l2);
      return list;
  }
}

{6=[joyful, hunter], 5=[rover, depth]}

Example Project

Dependencies and Technologies Used:

• JDK 9.0.1