如何确保参数实现接口

Question

I'm working on a project where a have a list of classes that inherit from the abstract class Generator . It works kind of like a chain of responsibility , where each class gets instantiated after the other, calling a generate() method. The only special thing is that each Generator is passed to the next, so that the next is able to call some methods of the previous one as a way to ensure that it continuos its work taking the same assumptions.

With this setup, I would like to allow each class on the chain to determine an interface that the previous one must implement, such that it knows which methods it has available.

If the class does not implement such interface, simply an error will be shown (it's a command-line tool).

I've been looking into several possibilities, but I don't really know how to perform a proper implementation of this. Can you think of a way to do this at compile time? Would I need to do run-time checks like instanceof ?

I leave here a bit of the skeleton representing what I have now, in case it helps.

public abstract class Generator {
    protected Generator(Object somePrivateStuff) {/* ... */}
    public abstract void generate();
}

public class SpecialGenerator extends Generator {
    protected SpecialGenerator(Object o) { super(o); }
    public void generate() {
        // Some actual implementation
    }
}

public class Main {
    public static void main(String[] args) {
        // Dynamically load all the classes extending Generator
        // via the Reflections library, and call the method generate()
        // on each one
    }
}

Any idea will be appreciated. Thanks in advance.

Answer 1

If you load classes dynamically, eg by String names, instanceof is useless because you don't know them at compile time. Instead, you should use Class#isAssignableFrom() :

for (Class<?> clazz : loaded) {
    if (Generator.class.isAssignableFrom(clazz)) {
        // instantiate
    } else {
        // print error
    }
}

Without knowing how exactly instantiation should be performed, it is hard to suggest more detailed code.