[sevensor]

I think the first sentence is key, "reusing code through inheritance" is the fragile thing that doesn't work. Pure interfaces do better, but then you're not reusing much code by inheritance. Programming to interfaces lets you reuse the code that uses the interface, but not the code that implements it. That's my take, anyhow.

[throwawayjava]

More importantly, this forces you to create an explicit interface for any code you'd like to reuse between implementations.

Inheritance is fragile for a lot of reasons (see e.g., fragile base class). But the most common failure modes all have the same thing in common: the error happens because a developer fails to realize that there's an implicit interface between two things in the object heirarchy.

[sevensor]

> there's an implicit interface between two things in the object heirarchy

I think the tendency to miss this is compounded by fuzzy thinking about class hierarchies. I've lost count of the times I've heard somebody talk about an object of a child class "talking to" an object of the parent class, when calling an inherited method. They think there's actually another object -- their mental model for inheritance is composition. Using pure interfaces helps to quash this kind of thinking.