It's the opposite - you can spend less time worrying about the details because you know they're opaque. It's like how in Java it can actually be easier to work on a generic datastructure (e.g. a tree) than a more concrete implementation (e.g. a tree of integers) - the generics force you to cleanly separate the structure-specific parts and the value-specific parts and not get them mixed up.

You can achieve the same kind of hiding in Java by using an existential type. E.g. you could have a library that looked like:

    interface Cursor<T> {
      T start();
      T nextStep(T currentState, int move);
      void finish(T);
    }
    class DataStructure {
      Cursor<?> traverse();
    }

And then you don't know what the internal state of the cursor is because the T type is hidden from you, but the compiler checks that you always call nextStep() with the same state you got back from start(), and you can't possibly mix up the state from two different cursors.