Java - 创建抽象对象的工厂类？

Question

I have the 2 following abstract classes:

public abstract class AbstractCarChecker implements CarService {

}

and the below which extends AbstractCarChecker

public abstract class CarChecker<T extends Car> extends AbstractCarChecker {

}

I want to have a factory class that will instantiate a CarChecker object, so that all subclasses (eg FordCheckerFactory ) can extend it to create instances.

public class CarCheckerFactory {

    public CarCheckerFactory() { }

    public static CarChecker newFrom(String carName, String carReg) {

        CarChecker carChecker = new CarChecker(carName, carReg);

        return carChecker;
    }

}

However the issue is that I cannot instantiate CarChecker as its abstract.

What is the best approach to this, use an if statement to decide what concrete implementation to return? eg pass "ford" into the newFrom method to know to create a FordChecker object?

Answer 1

You can do it as follows:

public static CarChecker newFrom(String carName, String carReg, String className) {
    Class<?> theClass = Class.forName(className);
    Constructor<?> cons = theClass.getConstructor(String.class,String.class);
    CarChecker carChecker = cons.newInstance(new Object[] {carName, carReg });
    return carChecker;
}

where className pertains the name of the concrete class.

Answer 2

You can't create instances of abstract classes (and as a consequence, 'abstract object' just isn't a thing).

The whole notion of factories is that they represent the notion of creating things. Therefore, the newFrom method should NOT be static (because then it might as well have been a constructor and the factory as a concept is useless; factories serve the function of having constructors that can have polymorphic behaviour, just like instance methods can). Thus:

public interface CarCheckerFactory<T extends Car> {
    CarChecker<T> newFrom(String carName, String carReg);
}

public class FordCheckerFactory implements CarCheckerFactory<Ford> {
    return new FordChecker(carName, carReg);
}