Question

As you know the ‘program to an interface’ design principle broadly prefers supertypes instead of concrete types or implementations.

Is it consistent with the principle to use instanceof in a Java program to derive a concrete type from a supertype?

In my application, Storehouse is an abstract supertype class with a couple of private variables and public getters and setters.

ConcreteStorehouseA inherits from Storehouse and has a lot of concrete methods and variables. ConcreteStorehouseB is similar but different.

My application receives a Storehouse. However, Storehouse is not a useful type to operate on. Because the only really useful methods are contained in the concrete types, I use instanceof as follows:

if (storehouse instanceof ConcreteStorehouseA) {
    ConcreteStorehouseA concreteStorehouseA = (ConcreteStorehouseA) storehouse;
    // perform operations on the concrete type's useful methods and variables

Is using instanceof compatible with the principle?

Edit:

In essence the application is a dice simulator for a table top RPG, Shadowrun. The concrete types are the different test types – Success Test, Opposed Test, Extended Test – which all have very different factors and parameters for their successful operation. The supertype essentially contains the dice pool!

Answer 1

As a rule of thumb, that “program to interfaces” principle that you mentioned can be translated into: import only the interface type, don’t have any compile-time dependency on subclasses.

Therefore, the answer to your question would be definitely no. You are not programming to interfaces since you cast to concrete types.