java – Java 8 NullPointerException in Collectors.toMap

Yes, a belated answer from me, but I think it may help figure out what’s going on under the hood in case someone wants to code some other logic Collector-.

I tried to solve the problem by coding a more native and direct approach. I think it’s as direct as possible:

public class LambdaUtilities {

  /**
   * In contrast to {@link Collectors#toMap(Function, Function)} the result map
   * may have null values.
   */
  public static > Collector toMapWithNullValues(Function keyMapper, Function valueMapper) {
    return toMapWithNullValues(keyMapper, valueMapper, HashMap::new);
  }

  /**
   * In contrast to {@link Collectors#toMap(Function, Function, BinaryOperator, Supplier)}
   * the result map may have null values.
   */
  public static > Collector toMapWithNullValues(Function keyMapper, Function valueMapper, Supplier> supplier) {
    return new Collector() {

      @Override
      public Supplier supplier() {
        return () -> {
          @SuppressWarnings("unchecked")
          M map = (M) supplier.get();
          return map;
        };
      }

      @Override
      public BiConsumer accumulator() {
        return (map, element) -> {
          K key = keyMapper.apply(element);
          if (map.containsKey(key)) {
            throw new IllegalStateException("Duplicate key " + key);
          }
          map.put(key, valueMapper.apply(element));
        };
      }

      @Override
      public BinaryOperator combiner() {
        return (map1, map2) -> {
          map1.putAll(map2);
          return map1;
        };
      }

      @Override
      public Function finisher() {
        return Function.identity();
      }

      @Override
      public Set characteristics() {
        return Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));
      }

    };
  }

}

And the tests using JUnit and assertj:

  @Test
  public void testToMapWithNullValues() throws Exception {
    Map result = Stream.of(1, 2, 3)
        .collect(LambdaUtilities.toMapWithNullValues(Function.identity(), x -> x % 2 == 1 ? x : null));

    assertThat(result)
        .isExactlyInstanceOf(HashMap.class)
        .hasSize(3)
        .containsEntry(1, 1)
        .containsEntry(2, null)
        .containsEntry(3, 3);
  }

  @Test
  public void testToMapWithNullValuesWithSupplier() throws Exception {
    Map result = Stream.of(1, 2, 3)
        .collect(LambdaUtilities.toMapWithNullValues(Function.identity(), x -> x % 2 == 1 ? x : null, LinkedHashMap::new));

    assertThat(result)
        .isExactlyInstanceOf(LinkedHashMap.class)
        .hasSize(3)
        .containsEntry(1, 1)
        .containsEntry(2, null)
        .containsEntry(3, 3);
  }

  @Test
  public void testToMapWithNullValuesDuplicate() throws Exception {
    assertThatThrownBy(() -> Stream.of(1, 2, 3, 1)
        .collect(LambdaUtilities.toMapWithNullValues(Function.identity(), x -> x % 2 == 1 ? x : null)))
            .isExactlyInstanceOf(IllegalStateException.class)
            .hasMessage("Duplicate key 1");
  }

  @Test
  public void testToMapWithNullValuesParallel() throws Exception {
    Map result = Stream.of(1, 2, 3)
        .parallel() // this causes .combiner() to be called
        .collect(LambdaUtilities.toMapWithNullValues(Function.identity(), x -> x % 2 == 1 ? x : null));

    assertThat(result)
        .isExactlyInstanceOf(HashMap.class)
        .hasSize(3)
        .containsEntry(1, 1)
        .containsEntry(2, null)
        .containsEntry(3, 3);
  }

And how do you use it? Well, just use it instead of toMap() as the tests show. This makes the calling code as clean as possible.

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