Java中泛型类的泛型容器

Question

What should I write for "self" in Java? What I want "self" to mean here is "whatever class this concrete implementation of Vehicle is". Is that possible in Java?

import java.util.ArrayList;
import java.util.List;

abstract class Vehicle {
    private Garage<self> garage;

    void setGarage(Garage<self> g) {
        this.garage = g;
        g.vehicles.add(this);
    }
}

class Car extends Vehicle {
}

class Garage<T extends Vehicle> {
    List<T> vehicles = new ArrayList<>();

    void addVehicle(T t) {
        vehicles.add(t);
    }
}

public class GenericFoo {
    public static void main(String[] args) {
        Garage<Car> carGarage = new Garage<>();

        Car c = new Car();
        carGarage.addVehicle(c);
        c.setGarage(carGarage);
    }
}

Answer 1

It's possible to create something equivalent but requires some additional effort. Hava a look at below code:

import java.util.ArrayList;
import java.util.List;

public class GenericsSample {

    public static void main(String[] args) {
        Garage<Car> carGarage = new Garage<>();

        Car c = new Car();
        carGarage.addVehicle(c);
        c.setGarage(carGarage);
    }

    abstract static class Vehicle<T extends Vehicle<T>> {

        private Garage<T> garage;

        void setGarage(Garage<T> g) {
            this.garage = g;
            g.vehicles.add(self());
        }

        abstract T self();
    }

    static class Car extends Vehicle<Car> {

        @Override
        Car self() {
            return this;
        }
    }

    static class Garage<T extends Vehicle<T>> {

        List<T> vehicles = new ArrayList<>();

        void addVehicle(T t) {
            vehicles.add(t);
        }
    }
}

What's important here:

• Vehicle class parametrization - in fact, T type is Vehicle itself. We need it to be able to get actual type from parent level. In our case, T represents Car (and can represent any subclass actually). Vehicle<T extends Vehicle<T>> may look a bit tricky but, in fact, it's quite easy to understand - T is a "reference" to subclass from superclass.
• self() method - I added self() method to Vehicle class. Please notice that self() returns T , not Vehicle , what means that type of returned object is this concrete, implementation class. Each implementation has to just return this .

Hope it solves your problem.