Here is some reading that might help explain: Please welcome ImmutableArray Here’s an excerpt: Reasons to use immutable array: Updating the data is rare or the number of elements is quite small (<16) you need to be able to iterate over the data in performance critical sections you have many instances of immutable collections and … Read more
A ReadOnlyDictionary can be initialized once via constructor, then you can’t add or remove items from it (they throw NotSupportedExceptions). It’s useful if you want to ensure that it won’t be modified while it’s sent across multiple layers of your application. An ImmutableDictionary has methods to modify it like Add or Remove, but they will … Read more
If you are writing a public-facing API and that immutability is an important aspect of your design, I would definitely make it explicit either by having the name of the method clearly denotes that the returned map will be immutable or by returning the concrete type of the map. Mentioning it in the javadoc is … Read more
With a ReadOnlyCollection: A collection that is read-only is simply a collection with a wrapper that prevents modifying the collection; therefore, if changes are made to the underlying collection, the read-only collection reflects those changes. This can’t happen with an ImmutableList.