LinkedCaseInsensitiveMap explicitly implements put/computeIfAbsent

Issue: SPR-16926

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -23,6 +23,7 @@ import java.util.LinkedHashMap;
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.Function;
 
 import org.springframework.lang.Nullable;
 
@@ -151,6 +152,7 @@ public class LinkedCaseInsensitiveMap<V> implements Map<String, V>, Serializable
 	}
 
 	@Override
+	@Nullable
 	public V getOrDefault(Object key, V defaultValue) {
 		if (key instanceof String) {
 			String caseInsensitiveKey = this.caseInsensitiveKeys.get(convertKey((String) key));
@@ -162,12 +164,15 @@ public class LinkedCaseInsensitiveMap<V> implements Map<String, V>, Serializable
 	}
 
 	@Override
+	@Nullable
 	public V put(String key, @Nullable V value) {
 		String oldKey = this.caseInsensitiveKeys.put(convertKey(key), key);
+		V oldKeyValue = null;
 		if (oldKey != null && !oldKey.equals(key)) {
-			this.targetMap.remove(oldKey);
+			oldKeyValue = this.targetMap.remove(oldKey);
 		}
-		return this.targetMap.put(key, value);
+		V oldValue = this.targetMap.put(key, value);
+		return (oldKeyValue != null ? oldKeyValue : oldValue);
 	}
 
 	@Override
@@ -178,6 +183,26 @@ public class LinkedCaseInsensitiveMap<V> implements Map<String, V>, Serializable
 		map.forEach(this::put);
 	}
 
+	@Override
+	@Nullable
+	public V putIfAbsent(String key, @Nullable V value) {
+		String oldKey = this.caseInsensitiveKeys.putIfAbsent(convertKey(key), key);
+		if (oldKey != null) {
+			return this.targetMap.get(oldKey);
+		}
+		return this.targetMap.putIfAbsent(key, value);
+	}
+
+	@Override
+	@Nullable
+	public V computeIfAbsent(String key, Function<? super String, ? extends V> mappingFunction) {
+		String oldKey = this.caseInsensitiveKeys.putIfAbsent(convertKey(key), key);
+		if (oldKey != null) {
+			return this.targetMap.get(oldKey);
+		}
+		return this.targetMap.computeIfAbsent(key, mappingFunction);
+	}
+
 	@Override
 	@Nullable
 	public V remove(Object key) {