Validate value objects (inheritance) in java

Question

I want to validate my domain objects before I pass them on to a other part of the system. All the objects that I want to validate share the same interface. The problem is that I can't figure out how to write this in a good way. I don't want to move the validation inside my value object. But I don't want to be forced to do a instanceOf-check either.

An example:

public interface Vehicle {}

public class Car implements Vehicle {}

public class MotorBike implements Vehicle {}

public interface VehicleValidator {
    void validate();

}

public class CarValidator implements VehicleValidator {

    @Override
    public void validate() {}
}

public class MotorBikeValidator implements VehicleValidator {

    @Override
    public void validate() {}
}

public void process(Vehicle vehicle) {
    //TODO: validate vehicle
    doSomething(vehicle);
}

In Scala I would have done something similar to http://debasishg.blogspot.se/2010/06/scala-implicits-type-classes-here-i.html But those language constructs is not possible in Java.

Answer 1

This is a classic case for the Double Dispatch design pattern.

You need to add a tiny bit of call-back code in the vehicle, which will be dynamically bound to the appropriate method of the validator at runtime:

public interface Vehicle {
    void validate(Validator validator);
}

public class Car implements Vehicle {
    public void validate(Validator validator) {
        validator.validateCar(this);
    }
}

public class MotorBike implements Vehicle {
    public void validate(Validator validator) {
        validator.validateMotorBike(this);
    }
}

public class Validator {
    public void validateCar(Car car) {
       // validate a car
    }

    public void validateMotorBike(MotorBike motorBike) {
        // validate a motorbike
    }
}

public void process(Vehicle vehicle) {
    Validator validator = new Validator();
    vehicle.validate(validator);
    doSomething(vehicle);
}