For a project I'm working on (in Java), I wanted to store pairs of items in a key-value map. The catch is that I need the items sorted according to the values, not the keys. Java provides an efficient, presorted mapping in the TreeMap class, but it sorts on the keys, not the values. Revering the roles of keys and values is not an option for me, as I might have several keys mapped to the same value.
A Google search wasn't particularly helpful, other than to confirm that I was screwed. The most practical suggestions I saw revolved around the theme of using a conventional mapping (HashMap, TreeMap, whatever) and then, as needed, dumping the entries into a list or stream and sorting that using a comparator based on the values. For my application, though, this would mean modifying the map contents, exporting it and sorting it thousands of times, which would put a big drag on the application. So I really wanted something moderately efficient that would maintain the map contents in sorted order. Eventually, I gave up looking for a solution and wrote one myself. I gave my class the not entirely compact name ValueOrderedMap. It's generic, meaning you can specify any types for keys and values (as long as both types are comparable).
I've implemented most of the commonly used methods associated with map classes (but not all known map methods), and I've tried to make it thread-safe (but as yet have not tested that). Anyone who wants to take it for a spin is welcome to. It's released under a Creative Commons license. There are only two classes, and only one (ValueOrderedMap) is essential; the other is just a small demo program. You can find the source code in my campus GitLab repository. There's an issue tracker (requires registration) where you can report bugs or ask for new features. If you just want a binary jar file (along with the Javadoc, README and license files), I've parked a zip archive on my faculty website.