建一个抽象类Car，它有wheelNum和seatNum两个成员变量以及抽象方法display（）。类Bus和Motorcycle继承自Car，实现输出成员变量的display（）方法。在主方法中，用向上转型对象调用display（）方法。

以下是Java代码实现：

//抽象类 Car
abstract class Car {
    protected int wheelNum;
    protected int seatNum;

    public abstract void display();
}

//类 Bus
class Bus extends Car {
    public Bus(int wheelNum, int seatNum) {
        this.wheelNum = wheelNum;
        this.seatNum = seatNum;
    }

    @Override
    public void display() {
        System.out.println("This is a bus with " + wheelNum + " wheels and " + seatNum + " seats.");
    }
}

//类 Motorcycle
class Motorcycle extends Car {
    public Motorcycle(int wheelNum, int seatNum) {
        this.wheelNum = wheelNum;
        this.seatNum = seatNum;
    }

    @Override
    public void display() {
        System.out.println("This is a motorcycle with " + wheelNum + " wheels and " + seatNum + " seat.");
    }
}

//主方法
public class Main {
    public static void main(String[] args) {
        Car car1 = new Bus(4, 40);
        Car car2 = new Motorcycle(2, 1);

        car1.display(); //输出：This is a bus with 4 wheels and 40 seats.
        car2.display(); //输出：This is a motorcycle with 2 wheels and 1 seat.
    }
}

在主方法中，分别用向上转型对象car1和car2调用display()方法，输出结果是Bus类和Motorcycle类中实现的内容。