If you use a boundary interface like Map, keep it inside the class, or close family of classes, where it is used. Avoid returning it from, or accepting it as an argument to, public APIs.
Learning the third-party code is hard. Integrating the third-party code is hard too. Doing both at the same time is doubly hard.
Instead of experimenting and trying out the new stuff in our production code, we should write some tests to explore our understanding of the third-party code.
In these learning tests we call the third-party API, as we expect to use it in our application. We’re essentially doing controlled experiments that check our understanding of that API. The tests focus on what we want out of the API.
Not only are learning tests free, they have a positive return on investment. When there are new releases of the third-party package, we run the learning tests to see whether there are behavioral differences.
Whether you need the learning provided by the learning tests or not, a clean boundary should be supported by a set of outbound tests that exercise the interface the same way the production code does. Without these boundary tests to ease the migration, we might be tempted to stay with the old version longer than we should.
There is another kind of boundary, one that separates the known from the unknown. There are often places in the code where our knowledge seems to drop off the edge. Sometimes what is on the other side of the boundary is unknowable (at least right now). i.e.: and API that we will use, but its specification is not ready yet.
Good software designs accommodate change without huge investments and rework. When we use code that is out of our control, special care must be taken to protect our investment and make sure future change is not too costly.
Code at the boundaries needs clear separation and tests that define expectations. We should avoid letting too much of our code know about the third-party particulars. It’s better to depend on something you control than on something you don’t control, lest it end up controlling you.