The problem is that all the items you've listed are basically "just in case we'll need to do something later". And this basically never happens, but you still

The problem is that all the items you've listed are basically "just in case we'll need to do something later". And this basically never happens, but you still have to carry that extra overhead of interfaces. Java also supports private/public method visibility, and this can be used to clearly show the contract. No need for interfaces.