Java Generic Method basics (reflection)

Question

I am trying to correctly understand how to use generics. I have been searching on it all morning but I get confused when the tutorials start adding multiple generic values, or using very abstract terms which I am still wrestling with.

I am still learning so any general advice is welcome, but I would like to specifically figure out the syntax for the method returning the generic class.

For example consider:

public class GenericsExample4 {

    public static void main(String args[]) {

        Car car;
        Truck truck;

        car = buy(Car.class, 95);
        truck = buy(Truck.class, 45);
    }

    // HELP HERE!
    public static <T extends Vehicle> T buy(Class<T> type, int topSpeed) {

        // create a new dynamic class T . . . I am lost on syntax

        return null; // return the new class T. I am lost on the syntax here :(
    }
}

interface Vehicle {
    public void floorIt();
}

class Car implements Vehicle {

    int topSpeed;

    public Car(int topSpeed) {
        this.topSpeed = topSpeed;
    }

    @Override
    public void floorIt() {
        System.out.println("Vroom! I am going " + topSpeed + " miles per hour");
    }
}

class Truck implements Vehicle {
    int topSpeed;

    public Truck(int topSpeed) {
        this.topSpeed = topSpeed;
    }

    @Override
    public void floorIt() {
        System.out.println("I can only go " + topSpeed + " miles per hour");
    }
}

Can someone point out how to tie together this generic method?

Answer 1

You can't generically call the new operator. What you can do is use reflection, under the assumption you know the constructor's parameters. For example, assuming every vehicle has a constructor which takes an int top speed:

public static <T extends Vehicle> T buy(Class<T> type, int topSpeed) {
    try {
        return type.getConstructor(Integer.TYPE).newInstance(topSpeed);
    } catch (Exception e) { // or something more specific
        System.err.println("Can't create an instance");
        System.err.println(e);
        return null;
    }
}