BAEL-4589: Update Guide to Java 8's Collectors (#10009)
* BAEL-4589: Move Guide to Java 8’s Collectors to core-java-11-2 * BAEL-4589: Add Java 10 unmodifiable* collectors examples
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kwoyke
GitHub
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package com.baeldung.streams.collectors;
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import com.google.common.collect.ImmutableList;
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import com.google.common.collect.ImmutableSet;
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import com.google.common.collect.Sets;
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import org.junit.Test;
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import java.util.Arrays;
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import java.util.Comparator;
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import java.util.DoubleSummaryStatistics;
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import java.util.LinkedList;
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import java.util.List;
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import java.util.Map;
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import java.util.Optional;
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import java.util.Set;
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import java.util.function.BiConsumer;
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import java.util.function.BinaryOperator;
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import java.util.function.Function;
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import java.util.function.Supplier;
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import java.util.stream.Collector;
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import static com.google.common.collect.Sets.newHashSet;
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import static java.util.stream.Collectors.averagingDouble;
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import static java.util.stream.Collectors.collectingAndThen;
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import static java.util.stream.Collectors.counting;
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import static java.util.stream.Collectors.groupingBy;
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import static java.util.stream.Collectors.joining;
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import static java.util.stream.Collectors.maxBy;
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import static java.util.stream.Collectors.partitioningBy;
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import static java.util.stream.Collectors.summarizingDouble;
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import static java.util.stream.Collectors.summingDouble;
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import static java.util.stream.Collectors.toCollection;
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import static java.util.stream.Collectors.toList;
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import static java.util.stream.Collectors.toMap;
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import static java.util.stream.Collectors.toSet;
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import static org.assertj.core.api.Assertions.assertThat;
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import static org.assertj.core.api.Assertions.assertThatThrownBy;
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public class Java8CollectorsUnitTest {
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private final List<String> givenList = Arrays.asList("a", "bb", "ccc", "dd");
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private final List<String> listWithDuplicates = Arrays.asList("a", "bb", "c", "d", "bb");
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@Test
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public void whenCollectingToList_shouldCollectToList() throws Exception {
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final List<String> result = givenList.stream().collect(toList());
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assertThat(result).containsAll(givenList);
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}
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@Test
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public void whenCollectingToSet_shouldCollectToSet() throws Exception {
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final Set<String> result = givenList.stream().collect(toSet());
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assertThat(result).containsAll(givenList);
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}
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@Test
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public void givenContainsDuplicateElements_whenCollectingToSet_shouldAddDuplicateElementsOnlyOnce() throws Exception {
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final Set<String> result = listWithDuplicates.stream().collect(toSet());
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assertThat(result).hasSize(4);
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}
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@Test
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public void whenCollectingToCollection_shouldCollectToCollection() throws Exception {
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final List<String> result = givenList.stream().collect(toCollection(LinkedList::new));
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assertThat(result).containsAll(givenList).isInstanceOf(LinkedList.class);
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}
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@Test
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public void whenCollectingToImmutableCollection_shouldThrowException() throws Exception {
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assertThatThrownBy(() -> {
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givenList.stream().collect(toCollection(ImmutableList::of));
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}).isInstanceOf(UnsupportedOperationException.class);
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}
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@Test
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public void whenCollectingToMap_shouldCollectToMap() throws Exception {
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final Map<String, Integer> result = givenList.stream().collect(toMap(Function.identity(), String::length));
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assertThat(result).containsEntry("a", 1).containsEntry("bb", 2).containsEntry("ccc", 3).containsEntry("dd", 2);
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}
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@Test
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public void whenCollectingToMapwWithDuplicates_shouldCollectToMapMergingTheIdenticalItems() throws Exception {
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final Map<String, Integer> result = listWithDuplicates.stream().collect(
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toMap(
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Function.identity(),
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String::length,
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(item, identicalItem) -> item
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)
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);
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assertThat(result).containsEntry("a", 1).containsEntry("bb", 2).containsEntry("c", 1).containsEntry("d", 1);
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}
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@Test
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public void givenContainsDuplicateElements_whenCollectingToMap_shouldThrowException() throws Exception {
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assertThatThrownBy(() -> {
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listWithDuplicates.stream().collect(toMap(Function.identity(), String::length));
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}).isInstanceOf(IllegalStateException.class);
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}
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@Test
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public void whenCollectingAndThen_shouldCollect() throws Exception {
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final List<String> result = givenList.stream().collect(collectingAndThen(toList(), ImmutableList::copyOf));
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assertThat(result).containsAll(givenList).isInstanceOf(ImmutableList.class);
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}
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@Test
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public void whenJoining_shouldJoin() throws Exception {
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final String result = givenList.stream().collect(joining());
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assertThat(result).isEqualTo("abbcccdd");
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}
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@Test
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public void whenJoiningWithSeparator_shouldJoinWithSeparator() throws Exception {
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final String result = givenList.stream().collect(joining(" "));
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assertThat(result).isEqualTo("a bb ccc dd");
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}
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@Test
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public void whenJoiningWithSeparatorAndPrefixAndPostfix_shouldJoinWithSeparatorPrePost() throws Exception {
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final String result = givenList.stream().collect(joining(" ", "PRE-", "-POST"));
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assertThat(result).isEqualTo("PRE-a bb ccc dd-POST");
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}
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@Test
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public void whenPartitioningBy_shouldPartition() throws Exception {
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final Map<Boolean, List<String>> result = givenList.stream().collect(partitioningBy(s -> s.length() > 2));
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assertThat(result).containsKeys(true, false).satisfies(booleanListMap -> {
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assertThat(booleanListMap.get(true)).contains("ccc");
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assertThat(booleanListMap.get(false)).contains("a", "bb", "dd");
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});
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}
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@Test
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public void whenCounting_shouldCount() throws Exception {
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final Long result = givenList.stream().collect(counting());
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assertThat(result).isEqualTo(4);
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}
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@Test
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public void whenSummarizing_shouldSummarize() throws Exception {
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final DoubleSummaryStatistics result = givenList.stream().collect(summarizingDouble(String::length));
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assertThat(result.getAverage()).isEqualTo(2);
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assertThat(result.getCount()).isEqualTo(4);
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assertThat(result.getMax()).isEqualTo(3);
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assertThat(result.getMin()).isEqualTo(1);
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assertThat(result.getSum()).isEqualTo(8);
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}
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@Test
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public void whenAveraging_shouldAverage() throws Exception {
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final Double result = givenList.stream().collect(averagingDouble(String::length));
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assertThat(result).isEqualTo(2);
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}
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@Test
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public void whenSumming_shouldSum() throws Exception {
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final Double result = givenList.stream().filter(i -> true).collect(summingDouble(String::length));
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assertThat(result).isEqualTo(8);
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}
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@Test
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public void whenMaxingBy_shouldMaxBy() throws Exception {
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final Optional<String> result = givenList.stream().collect(maxBy(Comparator.naturalOrder()));
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assertThat(result).isPresent().hasValue("dd");
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}
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@Test
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public void whenGroupingBy_shouldGroupBy() throws Exception {
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final Map<Integer, Set<String>> result = givenList.stream().collect(groupingBy(String::length, toSet()));
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assertThat(result).containsEntry(1, newHashSet("a")).containsEntry(2, newHashSet("bb", "dd")).containsEntry(3, newHashSet("ccc"));
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}
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@Test
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public void whenCreatingCustomCollector_shouldCollect() throws Exception {
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final ImmutableSet<String> result = givenList.stream().collect(toImmutableSet());
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assertThat(result).isInstanceOf(ImmutableSet.class).contains("a", "bb", "ccc", "dd");
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}
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private static <T> ImmutableSetCollector<T> toImmutableSet() {
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return new ImmutableSetCollector<>();
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}
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private static class ImmutableSetCollector<T> implements Collector<T, ImmutableSet.Builder<T>, ImmutableSet<T>> {
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@Override
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public Supplier<ImmutableSet.Builder<T>> supplier() {
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return ImmutableSet::builder;
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}
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@Override
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public BiConsumer<ImmutableSet.Builder<T>, T> accumulator() {
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return ImmutableSet.Builder::add;
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}
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@Override
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public BinaryOperator<ImmutableSet.Builder<T>> combiner() {
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return (left, right) -> left.addAll(right.build());
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}
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@Override
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public Function<ImmutableSet.Builder<T>, ImmutableSet<T>> finisher() {
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return ImmutableSet.Builder::build;
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}
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@Override
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public Set<Characteristics> characteristics() {
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return Sets.immutableEnumSet(Characteristics.UNORDERED);
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}
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}
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}
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