为了方便理解，画了个UML图，可以参考一下

定义Car类型：载人+载货

public class Car {
    int id;
    String name;
    int seat;
    double load;
    double price;
    public Car(int id,String name,int seat,double load,double price){
        this.id = id;
        this.name = name;
        this.seat = seat;
        this.load = load;
        this.price = price;
    }
    public String toString(){
        return id+"t"+name+"t"+seat+"座t"+load+"吨t"+price+"元";
    }
}

定义LoadCar继承Car，重写构造方法（载货，则座位=0）

public class LoadCar extends Car {
    public LoadCar(int id,String name,double load,double price){
        super(id,name,0,load,price);
    }
    public String toString(){
        return id+"t"+name+"t"+seat+"t"+load+"t"+price;
    }
}

定义SeatCar继承Car,重写构造方法（载人，则载货=0）

public class SeatCar extends Car {
    public SeatCar(int id,String name,int seat,double price){
        super(id,name,seat,0,price);
    }
    public String toString(){
        return id+"t"+name+"t"+seat+"t"+load+"t"+price;
    }
}

实现（主要就是业务流程）：

public class Initail {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        System.out.println("您是否要租车？1、是；2、否");
        Scanner in = new Scanner(System.in);
        if(in.nextInt()==1){
            Car[] cars = new Car[5];
            cars[0]=new SeatCar(1,"奥迪A8",4,400);
            cars[1]=new SeatCar(2,"特斯拉",4,500);
            cars[2]=new Car(3,"皮卡丘",4,2,200);
            cars[3]=new LoadCar(4,"松花江",10,200);
            cars[4]=new LoadCar(5,"黑龙江",20,300);
            System.out.println("现有5种车型可供您选择:");
            System.out.println("序号"+"t"+"车型"+"t"+"座位(座)"+"t"+"载重(吨)"+"t"+"租金/天");
            for(Car c : cars){
                System.out.println(c);
            }
            int totalSeat = 0;
            double totalLoad = 0;
            double totalPrice = 0;
            String loadCarList = "";
            String seatCarList = "";
            System.out.println("请输入您要租车的数量");
            int rentNum = in.nextInt();
            for(int i=0;i