Similar to AttributeMappingsList, this is essentially an immutable Map of AttributeMapping(s), allowing iteration of all mappings but also retrieval by name (a String key). Exposing a custom interface is more suitable to our needs than reusing the stock Map API; it expresses the immutable nature of this structure, and might allow us to extend it with additional convenience methods such as needs evolve. And additional reason for the custom interface is to allow custom implementations which can be highly optimised as necessary for our specific needs; for example the implementation ImmutableAttributeMappingsMap is able to avoid caching problems related to JDK-8180450, which would not have been possible with a standard generic container.