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/test/java/org/springframework/util/MultiValueMapCollectorTests.java

@@ -0,0 +1,41 @@
+/*
+ * 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();
+	}
+}