求各位大佬帮忙写下java程序，初学多态真的搞不懂研究一天了

public abstract class Automatic {
// 省略属性
public abstract int pay();
}

public class Car extends Automatic {
public int pay() {
return 5;
}
}

public class Truck {
private int weight;
public Truck(int weight) {
this.weight = weight;
}

    public int pay() {
        return weight < 20 ? 10 : weight >= 20 && weight <= 30 ? 15 : weight > 30 ? 20 : -1;
    }

}

public static void main(String[] args) {
Automatic a = new Truck(27);
shoufei(a);
}

public static void shoufei(Automatic a) {
System.out.println(a.pay());
}

 public abstract class Automatic {
    private String name;

    private String color;

    abstract int pay();

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

}
class Car extends Automatic{

    @Override
    public int pay() {
        return 5;
    }
}

class Truck extends Automatic{

    private int weight;

    @Override
   public int pay() {
        if(this.weight<20){
            return 10;
        }else if(this.weight>=20 && this.weight<=30){
            return 15;
        }else if(this.weight>30){
            return 20;
        }
        return 0;
    }

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

    public int getWeight() {
        return weight;
    }

    public void setWeight(int weight) {
        this.weight = weight;
    }
}

class Bus extends Automatic{

    private int num;

    @Override
    int pay() {
        if(this.num<15){
            return 10;
        }else if(this.num>=15 && this.num<=30){
            return 20;
        }else if(this.num>30){
            return 25;
        }
        return 0;
    }

    public Bus(int num) {
        this.num = num;
    }

    public int getNum() {
        return num;
    }

    public void setNum(int num) {
        this.num = num;
    }
}
class Test{
    public static void main(String[] args) {
        Automatic carAutomatic = new Car();
        shoufei(carAutomatic);
        Automatic truck = new Truck(27);
        shoufei(truck);
        Automatic busAutomatic = new Bus(20);
        shoufei(busAutomatic);
    }
    public static void shoufei(Automatic automatic){
        System.out.println("本次收费:"+automatic.pay());
    }
}