Enhancement JDK-8176894 inadvertently introduced erroneous behavior in the `TreeMap.computeIfAbsent` method. The other `TreeMap` methods that were modified by this enhancement are unaffected. The erroneous behavior is that, if the map contains an existing mapping whose value is null, the `computeIfAbsent` method immediately returns null. To conform with the specification, `computeIfAbsent` should instead call the mapping function and update the map with the function's result.