Introduce MultiValueMapCollector for use with streams

See https://github.com/spring-projects/spring-data-commons/issues/3420
See gh-35958

Signed-off-by: Florian Hof <florian.hof@sbb.ch>

spring-core/src/main/java/org/springframework/util/MultiValueMapCollector.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright 2002-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.util;
+
+import java.util.EnumSet;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map.Entry;
+import java.util.Set;
+import java.util.function.BiConsumer;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.Supplier;
+import java.util.stream.Collector;
+
+/**
+ * A {@link Collector} for building a {@link MultiValueMap} from a {@link java.util.stream.Stream}.
+ * <br/>
+ * Moved from {@code org.springframework.data.util.MultiValueMapCollector}.
+ *
+ * @author Jens Schauder
+ *
+ * @param <T> – the type of input elements to the reduction operation
+ * @param <K> – the type of the key elements
+ * @param <V> – the type of the value elements
+ */
+public class MultiValueMapCollector<T, K, V> implements Collector<T, MultiValueMap<K, V>, MultiValueMap<K, V>> {
+	private final Function<T, K> keyFunction;
+	private final Function<T, V> valueFunction;
+
+	public MultiValueMapCollector(Function<T, K> keyFunction, Function<T, V> valueFunction) {
+		this.keyFunction = keyFunction;
+		this.valueFunction = valueFunction;
+	}
+
+	public static <K, V> MultiValueMapCollector<V, K, V> indexingBy(Function<V, K> indexer) {
+		return new MultiValueMapCollector<>(indexer, Function.identity());
+	}
+
+	@Override
+	public Supplier<MultiValueMap<K, V>> supplier() {
+		return () -> CollectionUtils.toMultiValueMap(new HashMap<K, List<V>>());
+	}
+
+	@Override
+	public BiConsumer<MultiValueMap<K, V>, T> accumulator() {
+		return (map, t) -> map.add(this.keyFunction.apply(t), this.valueFunction.apply(t));
+	}
+
+	@Override
+	public BinaryOperator<MultiValueMap<K, V>> combiner() {
+		return (map1, map2) -> {
+			for (Entry<K, List<V>> entry : map2.entrySet()) {
+				map1.addAll(entry.getKey(), entry.getValue());
+			}
+			return map1;
+		};
+	}
+
+	@Override
+	public Function<MultiValueMap<K, V>, MultiValueMap<K, V>> finisher() {
+		return Function.identity();
+	}
+
+	@Override
+	public Set<Characteristics> characteristics() {
+		return EnumSet.of(Characteristics.IDENTITY_FINISH, Characteristics.UNORDERED);
+	}
+}

spring-core/src/test/java/org/springframework/util/MultiValueMapCollectorTests.java

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+/*
+ * Copyright 2002-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.util;
+
+import java.util.stream.Stream;
+
+import org.junit.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link MultiValueMapCollector}.
+ *
+ * @author Florian Hof
+ */
+class MultiValueMapCollectorTests {
+
+	@Test
+	void indexingBy() {
+		MultiValueMapCollector<String, Integer, String> collector = MultiValueMapCollector.indexingBy(String::length);
+		MultiValueMap<Integer, String> content = Stream.of("abc", "ABC", "123", "1234", "abcdef", "ABCDEF").collect(collector);
+		assertThat(content.get(3)).containsOnly("abc", "ABC", "123");
+		assertThat(content.get(4)).containsOnly("abcdef", "ABCDEF");
+		assertThat(content.get(6)).containsOnly("1234");
+		assertThat(content.get(1)).isNull();
+	}
+}