Restore independent LinkedMultiValueMap implementation (without base class)

Closes gh-25960

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

@@ -17,10 +17,14 @@
 package org.springframework.util;
 
 import java.io.Serializable;
+import java.util.Collection;
 import java.util.LinkedHashMap;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
+
+import org.springframework.lang.Nullable;
 
 /**
  * Simple implementation of {@link MultiValueMap} that wraps a {@link LinkedHashMap},
@@ -35,16 +39,18 @@ import java.util.Map;
  * @param <K> the key type
  * @param <V> the value element type
  */
-public class LinkedMultiValueMap<K, V> extends MultiValueMapAdapter<K, V> implements Serializable, Cloneable {
+public class LinkedMultiValueMap<K, V> implements MultiValueMap<K, V>, Serializable, Cloneable {
 
 	private static final long serialVersionUID = 3801124242820219131L;
 
+	private final Map<K, List<V>> targetMap;
+
 
 	/**
 	 * Create a new LinkedMultiValueMap that wraps a {@link LinkedHashMap}.
 	 */
 	public LinkedMultiValueMap() {
-		super(new LinkedHashMap<>());
+		this.targetMap = new LinkedHashMap<>();
 	}
 
 	/**
@@ -53,7 +59,7 @@ public class LinkedMultiValueMap<K, V> extends MultiValueMapAdapter<K, V> implem
 	 * @param initialCapacity the initial capacity
 	 */
 	public LinkedMultiValueMap(int initialCapacity) {
-		super(new LinkedHashMap<>(initialCapacity));
+		this.targetMap = new LinkedHashMap<>(initialCapacity);
 	}
 
 	/**
@@ -65,7 +71,140 @@ public class LinkedMultiValueMap<K, V> extends MultiValueMapAdapter<K, V> implem
 	 * @see #deepCopy()
 	 */
 	public LinkedMultiValueMap(Map<K, List<V>> otherMap) {
-		super(new LinkedHashMap<>(otherMap));
+		this.targetMap = new LinkedHashMap<>(otherMap);
+	}
+
+
+	// MultiValueMap implementation
+
+	@Override
+	@Nullable
+	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, @Nullable V value) {
+		List<V> values = this.targetMap.computeIfAbsent(key, k -> new LinkedList<>());
+		values.add(value);
+	}
+
+	@Override
+	public void addAll(K key, List<? extends V> values) {
+		List<V> currentValues = this.targetMap.computeIfAbsent(key, k -> new LinkedList<>());
+		currentValues.addAll(values);
+	}
+
+	@Override
+	public void addAll(MultiValueMap<K, V> values) {
+		for (Entry<K, List<V>> entry : values.entrySet()) {
+			addAll(entry.getKey(), entry.getValue());
+		}
+	}
+
+	@Override
+	public void set(K key, @Nullable V value) {
+		List<V> values = new LinkedList<>();
+		values.add(value);
+		this.targetMap.put(key, values);
+	}
+
+	@Override
+	public void setAll(Map<K, V> values) {
+		values.forEach(this::set);
+	}
+
+	@Override
+	public Map<K, V> toSingleValueMap() {
+		Map<K, V> singleValueMap = new LinkedHashMap<>(this.targetMap.size());
+		this.targetMap.forEach((key, values) -> {
+			if (values != null && !values.isEmpty()) {
+				singleValueMap.put(key, values.get(0));
+			}
+		});
+		return singleValueMap;
+	}
+
+
+	// Map implementation
+
+	@Override
+	public int size() {
+		return this.targetMap.size();
+	}
+
+	@Override
+	public boolean isEmpty() {
+		return this.targetMap.isEmpty();
+	}
+
+	@Override
+	public boolean containsKey(Object key) {
+		return this.targetMap.containsKey(key);
+	}
+
+	@Override
+	public boolean containsValue(Object value) {
+		return this.targetMap.containsValue(value);
+	}
+
+	@Override
+	@Nullable
+	public List<V> get(Object key) {
+		return this.targetMap.get(key);
+	}
+
+	@Override
+	@Nullable
+	public List<V> put(K key, List<V> value) {
+		return this.targetMap.put(key, value);
+	}
+
+	@Override
+	@Nullable
+	public List<V> remove(Object key) {
+		return this.targetMap.remove(key);
+	}
+
+	@Override
+	public void putAll(Map<? extends K, ? extends List<V>> map) {
+		this.targetMap.putAll(map);
+	}
+
+	@Override
+	public void clear() {
+		this.targetMap.clear();
+	}
+
+	@Override
+	public Set<K> keySet() {
+		return this.targetMap.keySet();
+	}
+
+	@Override
+	public Collection<List<V>> values() {
+		return this.targetMap.values();
+	}
+
+	@Override
+	public Set<Entry<K, List<V>>> entrySet() {
+		return this.targetMap.entrySet();
+	}
+
+	@Override
+	public boolean equals(@Nullable Object other) {
+		return (this == other || this.targetMap.equals(other));
+	}
+
+	@Override
+	public int hashCode() {
+		return this.targetMap.hashCode();
+	}
+
+	@Override
+	public String toString() {
+		return this.targetMap.toString();
 	}
 
 

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

@@ -48,6 +48,8 @@ class MultiValueMapAdapter<K, V> implements MultiValueMap<K, V>, Serializable {
 	}
 
 
+	// MultiValueMap implementation
+
 	@Override
 	@Nullable
 	public V getFirst(K key) {
@@ -97,6 +99,9 @@ class MultiValueMapAdapter<K, V> implements MultiValueMap<K, V>, Serializable {
 		return singleValueMap;
 	}
 
+
+	// Map implementation
+
 	@Override
 	public int size() {
 		return this.targetMap.size();