It's not really about replacing the implementation. Depending on interfaces means you (by definition) can't depend on the implementation. It's a decoupling thi

It's not really about replacing the implementation. Depending on interfaces means you (by definition) can't depend on the implementation. It's a decoupling thing more than anything else. You get some benefits for mocking during testing and so on but it's mostly just to get something like a C++-style class declaration/definition (or .h/.cpp)-split. It's admittedly taken a bit too far in some codebases and often done without much thought to why, but it's also a bit of an artifact of pre Jigsaw-era Java where it was very hard to enforce boundaries between modules. In that light it makes at least some sense.