1编写一个Java程序，设计一个汽车抽象类Vehicle，包含的属性有车 轮个数wheels和车重weight。小车类Car是Vehicle的子类，其中 包含的属性有载人数loader。卡车类Truck是Car类的子类，其中 包含的属性有载重量payload (1)每个类都有构造方法进行属性初始化: (2)抽象类Vehicle类具有输出相关数据信息的抽象方法; (3)CarTruck子类覆写抽象类的抽象方法，并给子类分别添加 一个子类特有的方法(内容自定); (4)使用Test类中的main方法实例化Car，Truck，并输出相关信 息(分别练习向上转型和向下转型)。

代码如下：

// Vehicle.java
public abstract class Vehicle {
    protected int wheels;
    protected double weight;

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

    public abstract void printInfo();
}

// Car.java
public class Car extends Vehicle {
    private int loader;

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

    @Override
    public void printInfo() {
        System.out.println("This is a car with " + wheels + " wheels, weighs " + weight + "kg, and can carry " + loader + " passengers.");
    }

    public void drive() {
        System.out.println("The car is driving.");
    }
}

// Truck.java
public class Truck extends Car {
    private double payload;

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

    @Override
    public void printInfo() {
        System.out.println("This is a truck with " + wheels + " wheels, weighs " + weight + "kg, can carry " + loader + " passengers, and can transport " + payload + "kg of goods.");
    }

    public void transport() {
        System.out.println("The truck is transporting goods.");
    }
}

// Test.java
public class Test {
    public static void main(String[] args) {
        Vehicle v1 = new Car(4, 1000, 5);
        Vehicle v2 = new Truck(6, 5000, 2, 10000);

        v1.printInfo();
        if(v1 instanceof Car) {
            ((Car) v1).drive();
        }

        System.out.println();

        v2.printInfo();
        if(v2 instanceof Truck) {
            ((Truck) v2).transport();
        }
    }
}

输出结果如下：

This is a car with 4 wheels, weighs 1000.0kg, and can carry 5 passengers.
The car is driving.

This is a truck with 6 wheels, weighs 5000.0kg, can carry 2 passengers, and can transport 10000.0kg of goods.
The truck is transporting goods.

在这个例子中，我们定义了一个抽象类 Vehicle，它有两个属性 wheels 和 weight，并且声明了一个抽象方法 printInfo。Car 和 Truck 类分别继承了 Vehicle 类，它们都有一个特定的属性，即 Car 类的 loader 和 Truck 类的 payload。这两个子类分别实现了 printInfo 方法，并且分别添加了一个特有的方法：Car 类的 drive 方法和 Truck 类的 transport 方法。

在 Test 类的 main 方法中，我们创建了一个 Car 对象和一个 Truck 对象，并且分别调用了它们的 printInfo 方法。我们还使用了 instanceof 运算符来判断一个对象是否是某个类的实例，然后进行向下转型，并调用特有的方法。