Hanging Out with Linked Hash Maps

LinkedHashMap is a surprising useful data structure, and missing from many standard libraries.

In my case I have class that represents a message and contains a set of headers. Imagining an HTTP message might be useful. Historically these sets have been mapped using a configuration file that dictates the order of field keys and values (primarily it’s done this way to support ordered value-only fields):

So, to support messages for which the index has no special significance, we’re basically implementing a dictionary inside a list. The LinkedHashMap has a few features I don’t need, for instance I don’t care about access time so I don’t particularly need a hash map (but it gives me a set cheaply), and I don’t need to iterate in both directions (but I do want to pull the fields out in insertion order).

The LinkedHashMap like HashMap is a collection of buckets. Duplicates aren’t permitted so we get set behaviour, and there’s a performance benefit for look-ups. Additionally, LinkedHashMap maintains a bi-directional link between elements, so iterators return elements in insertion order and can be navigated forwards and backwards.

There’s a moderately helpful SO answer to this question that gives a bit of detail on the different children of Map. My Java adventure continues….