CollectorTester<T, A, R> of(
          Collector<T, A, R> collector, BiPredicate<? super R, ? super R> equivalence) {
    return new CollectorTester<>(collector, equivalence);
  }

  private final Collector<T, A, R> collector;
  private final BiPredicate<? super R, ? super R> equivalence;

  private CollectorTester(
      Collector<T, A, R> collector, BiPredicate<? super R, ? super R> equivalence) {

   */
  @SuppressWarnings("unchecked")
  public static <T> Collector<T, ?, Optional<T>> toOptional() {
    return (Collector) TO_OPTIONAL;
  }

  private static final Object NULL_PLACEHOLDER = new Object();

  private static final Collector<@Nullable Object, ?, @Nullable Object> ONLY_ELEMENT =
      Collector.<@Nullable Object, ToOptionalState, @Nullable Object>of(
          ToOptionalState::new,

                .append(": ")
                .append(problem.getMessage()));
        return msg.toString();
    }

    /**
     * Returns the problem collector associated with this exception.
     *
     * @return the problem collector containing all problems related to this exception
     */
    public ProblemCollector<BuilderProblem> getProblemCollector() {
        return problems;
    }

   * collector without breaking j2cl?
   */
  private static final Collector<Object, ?, Optional<Object>> TO_OPTIONAL =
      Collector.of(
          ToOptionalState::new,
          ToOptionalState::add,
          ToOptionalState::combine,
          ToOptionalState::getOptional,
          Collector.Characteristics.UNORDERED);

  /**
   * A collector that converts a stream of zero or one elements to an {@code Optional}.
   *

  private final BiPredicate<? super R, ? super R> equivalence;

  private CollectorTester(
      Collector<T, A, R> collector, BiPredicate<? super R, ? super R> equivalence) {
    this.collector = checkNotNull(collector);
    this.equivalence = checkNotNull(equivalence);
  }

  /**
   * Different orderings for combining the elements of an input array, which must all produce the
   * same result.
   */

        ProblemCollector collector = ProblemCollectorFactory.newInstance(null);
        collector.setSource("pom.xml");
        collector.add(Problem.Severity.ERROR, "Error message", 10, 5, null);
        collector.add(Problem.Severity.WARNING, "Warning message", 15, 3, null);

        List<Problem> problems = collector.getProblems();
        assertEquals(2, problems.size(), "Should collect both problems");

import java.util.function.Supplier;
import java.util.stream.Collector;
import org.jspecify.annotations.Nullable;

/** Collectors utilities for {@code common.collect.Table} internals. */
@GwtCompatible
@IgnoreJRERequirement // used only from APIs with Java 8 types in them
final class TableCollectors {

  static <T extends @Nullable Object, R, C, V>
      Collector<T, ?, ImmutableTable<R, C, V>> toImmutableTable(

        collector.setSource("SOURCE_PROBLEM2");
        collector.add(null, "PROBLEM2", -1, -1, null);

        collector.setSource("SOURCE_PROBLEM3");
        collector.add(null, "PROBLEM3", -1, -1, null);

        assertEquals("", collector.getProblems().get(0).getSource());
        assertEquals("SOURCE_PROBLEM2", collector.getProblems().get(1).getSource());

    Collector<Entry<AnEnum, Integer>, ?, ImmutableMap<AnEnum, Integer>> collector =
        toImmutableEnumMap(Entry::getKey, Entry::getValue);
    assertThrows(
        IllegalArgumentException.class,
        () -> Stream.of(mapEntry(AnEnum.A, 1), mapEntry(AnEnum.A, 11)).collect(collector));
  }

  public void testToImmutableMapMerging() {

import java.util.function.Supplier;
import java.util.stream.Collector;
import org.jspecify.annotations.Nullable;

/** Collectors utilities for {@code common.collect.Table} internals. */
@GwtCompatible
final class TableCollectors {

  static <T extends @Nullable Object, R, C, V>
      Collector<T, ?, ImmutableTable<R, C, V>> toImmutableTable(
          Function<? super T, ? extends R> rowFunction,