为什么一个报不兼容，另一个文件不报错

39.p440集韩老师讲的第10章的homework06题，在
package com.hspedu.homework06Try02;

/**
 * @author code
 * @version 1.0
 */
public class Homework06 {
    public static void main(String[] args){
       Person person= new Person("唐僧", new Boat());
       person.common();
       person.passRiver();
       person.passRiver();
       person.passRiver();
       person.common();
    }
}

这里的Person里面的参数 new Boat()改为null或者new Horse()也可以不报错并运行出结果来,这是为什么！

为什么D:\idea_java_projects\chapter11\src\com\hspedu\homework\Person.java中写成
Boat boat=VehiclesFactory.getBoat();会报错；
报的错误：
D:\idea_java_projects\chapter11\src\com\hspedu\homework\Person.java:16:42
java: 不兼容的类型: com.hspedu.homework.Vehicles无法转换为com.hspedu.homework.Boat

public void passRiver(){
    Boat boat=VehiclesFactory.getBoat();
    boat.work();
}
public void common(){
    vehicles=VehiclesFactory.getHorse();
    vehicles.work();
}

而在D:\idea_java_projects\chapter11\src\com\hspedu\homework06Try02\Person.java中写成
Boat boat=VehiclesFactory.getBoat();却没有报错！

public void passRiver(){
    Boat boat=VehiclesFactory.getBoat();
    boat.work();
}
public void common(){
    Horse horse=VehiclesFactory.getHorse();
    horse.work();
}