Consistent handling of empty List entries in LinkedMultiValueMap

Closes gh-22912

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 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.
@@ -34,6 +34,8 @@ import java.util.Set;
  * @author Arjen Poutsma
  * @author Juergen Hoeller
  * @since 3.0
+ * @param <K> the key type
+ * @param <V> the value element type
  */
 public class LinkedMultiValueMap<K, V> implements MultiValueMap<K, V>, Serializable, Cloneable {
 
@@ -73,6 +75,12 @@ public class LinkedMultiValueMap<K, V> implements MultiValueMap<K, V>, Serializa
 
 	// MultiValueMap implementation
 
+	@Override
+	public V getFirst(K key) {
+		List<V> values = this.targetMap.get(key);
+		return (values != null && !values.isEmpty() ? values.get(0) : null);
+	}
+
 	@Override
 	public void add(K key, V value) {
 		List<V> values = this.targetMap.get(key);
@@ -83,12 +91,6 @@ public class LinkedMultiValueMap<K, V> implements MultiValueMap<K, V>, Serializa
 		values.add(value);
 	}
 
-	@Override
-	public V getFirst(K key) {
-		List<V> values = this.targetMap.get(key);
-		return (values != null ? values.get(0) : null);
-	}
-
 	@Override
 	public void set(K key, V value) {
 		List<V> values = new LinkedList<V>();
@@ -107,7 +109,10 @@ public class LinkedMultiValueMap<K, V> implements MultiValueMap<K, V>, Serializa
 	public Map<K, V> toSingleValueMap() {
 		LinkedHashMap<K, V> singleValueMap = new LinkedHashMap<K,V>(this.targetMap.size());
 		for (Entry<K, List<V>> entry : this.targetMap.entrySet()) {
-			singleValueMap.put(entry.getKey(), entry.getValue().get(0));
+			List<V> values = entry.getValue();
+			if (values != null && !values.isEmpty()) {
+				singleValueMap.put(entry.getKey(), values.get(0));
+			}
 		}
 		return singleValueMap;
 	}

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

@@ -1,5 +1,5 @@
 /*
- * 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.
@@ -28,10 +28,11 @@ 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
@@ -39,15 +40,23 @@ public class LinkedMultiValueMapTests {
 		map.add("key", "value1");
 		map.add("key", "value2");
 		assertEquals(1, map.size());
-		List<String> expected = new ArrayList<String>(2);
+		List<String> expected = new ArrayList<>(2);
 		expected.add("value1");
 		expected.add("value2");
 		assertEquals(expected, map.get("key"));
 	}
 
+	@Test
+	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 getFirst() {
-		List<String> values = new ArrayList<String>(2);
+		List<String> values = new ArrayList<>(2);
 		values.add("value1");
 		values.add("value2");
 		map.put("key", values);
@@ -56,22 +65,40 @@ 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
 	public void equals() {
 		map.set("key1", "value1");
 		assertEquals(map, map);
-		MultiValueMap<String, String> o1 = new LinkedMultiValueMap<String, String>();
+		MultiValueMap<String, String> o1 = new LinkedMultiValueMap<>();
 		o1.set("key1", "value1");
 		assertEquals(map, o1);
 		assertEquals(o1, map);
-		Map<String, List<String>> o2 = new HashMap<String, List<String>>();
+		Map<String, List<String>> o2 = new HashMap<>();
 		o2.put("key1", Collections.singletonList("value1"));
 		assertEquals(map, o2);
 		assertEquals(o2, map);