java类多态

现在要实现乘坐交通工具到一个景点旅游，距离5000公里。交通工具有飞机、轮船、火车、汽车。

要求实现一个父类Transport，有速度域，有计算时间的方法，参数是里程；其子类有Ship、Plane、Train、Car；子类要重写从父类继承的计算时间的方法。利用类型兼容赋值的形式（也即多态的形式）实现。

代码：

import java.util.*;
public class Main {

 public static void main(String[] args) {
  Scanner input = new Scanner(System.in);
  while(input.hasNext())
  {   
            String str=input.next();
            Transport transport;
            int speed=input.nextInt();
            char tool=str.charAt(0);

             if(tool=='P')
            {
              transport=new Plane(speed);
              transport.Time();
            }
            else if(tool=='S')
            {
             transport=new Ship(speed);
             transport.Time();
            }
            else if(tool=='C')
            {
            transport=new Car(speed);
            transport.Time();
            }
             else if(tool=='T')
            {
             transport=new Train(speed);
             transport.Time();
            }
   }
 }
}

class Transport
{
     int speed;
 public Transport(int speed)
 {
    this.speed=speed;
 }
 public void Time()
 {
  double time=(double)5000/speed;
  System.out.printf("%.2f",time);
  System.out.println();
 }
 
}
class Plane extends Transport
{
 public Plane(int speed)
 {
  super(speed);
 }
}

class Ship extends Transport
{
 public Ship(int speed)
 {
  super(speed);
 }
}

class Car extends Transport
{
 public Car(int speed)
 {
  super(speed);
 }
}

class Train extends Transport
{
 public Train(int speed)
 {
  super(speed);
 }
}