设计一个汽车类 Car 类，包含的属性有车轮的个数 wheels 和车重 weight。 小汽车类 SmallCar 是 Car 类的子类。包含的属性有载人数 loader。卡车 类 Truck 类是 SmallCar 类的子类，其中包含的属性有载重量 payload，每 个类都有构造方法和输出相关数据的方法。编写程序测试类。java编写

好的，这是一个简单的实现：

public class Car {
    private int wheels;
    private double weight;

    public Car(int wheels, double weight) {
        this.wheels = wheels;
        this.weight = weight;
    }

    public int getWheels() {
        return wheels;
    }

    public double getWeight() {
        return weight;
    }

    public void display() {
        System.out.println("Wheels: " + wheels);
        System.out.println("Weight: " + weight);
    }
}

public class SmallCar extends Car {
    private int loader;

    public SmallCar(int wheels, double weight, int loader) {
        super(wheels, weight);
        this.loader = loader;
    }

    public int getLoader() {
        return loader;
    }

    public void display() {
        super.display();
        System.out.println("Loader: " + loader);
    }
}

public class Truck extends SmallCar {
    private double payload;

    public Truck(int wheels, double weight, int loader, double payload) {
        super(wheels, weight, loader);
        this.payload = payload;
    }

    public double getPayload() {
        return payload;
    }

    public void display() {
        super.display();
        System.out.println("Payload: " + payload);
    }
}

public class Main {
    public static void main(String[] args) {
        Car car = new Car(4, 1200);
        car.display();

        SmallCar smallCar = new SmallCar(4, 800, 5);
        smallCar.display();

        Truck truck = new Truck(6, 5000, 2, 10000);
        truck.display();
    }
}

输出结果为：

Wheels: 4
Weight: 1200.0
Wheels: 4
Weight: 800.0
Loader: 5
Wheels: 6
Weight: 5000.0
Loader: 2
Payload: 10000.0