Consistent handling of empty List entries in LinkedMultiValueMap

Closes gh-22912

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

 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -81,7 +81,7 @@ public class LinkedMultiValueMap<K, V> implements MultiValueMap<K, V>, Serializa
 	@Nullable
 	public V getFirst(K key) {
 		List<V> values = this.targetMap.get(key);
-		return (values != null ? values.get(0) : null);
+		return (values != null && !values.isEmpty() ? values.get(0) : null);
 	}
 
 	@Override
@@ -118,7 +118,11 @@ public class LinkedMultiValueMap<K, V> implements MultiValueMap<K, V>, Serializa
 	@Override
 	public Map<K, V> toSingleValueMap() {
 		LinkedHashMap<K, V> singleValueMap = new LinkedHashMap<>(this.targetMap.size());
-		this.targetMap.forEach((key, value) -> singleValueMap.put(key, value.get(0)));
+		this.targetMap.forEach((key, values) -> {
+			if (values != null && !values.isEmpty()) {
+				singleValueMap.put(key, values.get(0));
+			}
+		});
 		return singleValueMap;
 	}
 

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

 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -29,10 +29,11 @@ import static org.junit.Assert.*;
 
 /**
  * @author Arjen Poutsma
+ * @author Juergen Hoeller
  */
 public class LinkedMultiValueMapTests {
 
-	private final LinkedMultiValueMap<String, String> map = new LinkedMultiValueMap<String, String>();
+	private final LinkedMultiValueMap<String, String> map = new LinkedMultiValueMap<>();
 
 
 	@Test
@@ -47,7 +48,15 @@ public class LinkedMultiValueMapTests {
 	}
 
 	@Test
-	public void addAll() throws Exception {
+	public void set() {
+		map.set("key", "value1");
+		map.set("key", "value2");
+		assertEquals(1, map.size());
+		assertEquals(Collections.singletonList("value2"), map.get("key"));
+	}
+
+	@Test
+	public void addAll() {
 		map.add("key", "value1");
 		map.addAll("key", Arrays.asList("value2", "value3"));
 		assertEquals(1, map.size());
@@ -58,6 +67,14 @@ public class LinkedMultiValueMapTests {
 		assertEquals(expected, map.get("key"));
 	}
 
+	@Test
+	public void addAllWithEmptyList() {
+		map.addAll("key", Collections.emptyList());
+		assertEquals(1, map.size());
+		assertEquals(Collections.emptyList(), map.get("key"));
+		assertNull(map.getFirst("key"));
+	}
+
 	@Test
 	public void getFirst() {
 		List<String> values = new ArrayList<>(2);
@@ -69,11 +86,29 @@ public class LinkedMultiValueMapTests {
 	}
 
 	@Test
-	public void set() {
-		map.set("key", "value1");
-		map.set("key", "value2");
-		assertEquals(1, map.size());
-		assertEquals(Collections.singletonList("value2"), map.get("key"));
+	public void getFirstWithEmptyList() {
+		map.put("key", Collections.emptyList());
+		assertNull(map.getFirst("key"));
+		assertNull(map.getFirst("other"));
+	}
+
+	@Test
+	public void toSingleValueMap() {
+		List<String> values = new ArrayList<>(2);
+		values.add("value1");
+		values.add("value2");
+		map.put("key", values);
+		Map<String, String> svm = map.toSingleValueMap();
+		assertEquals(1, svm.size());
+		assertEquals("value1", svm.get("key"));
+	}
+
+	@Test
+	public void toSingleValueMapWithEmptyList() {
+		map.put("key", Collections.emptyList());
+		Map<String, String> svm = map.toSingleValueMap();
+		assertEquals(0, svm.size());
+		assertNull(svm.get("key"));
 	}
 
 	@Test