java 子类重写函数的几个测试

public class Test {
       public static void main (String[] args) {
           Animal h = new Horse();
           h.eat();  
       }
   }

   class Animal {
       public void eat(){
           System.out.println ("Animal is eating.");
       }
   }
  
   class Horse extends Animal{
       public void eat(){
           System.out.println ("Horse is eating.");
       }
       public void buck(){
       }
   }

　　一个原则是：使用了什么引用，编译器就会只调用引用类所拥有的方法。如果调用子类特有的方法，如上例的h.buck(); 编译器会抱怨的（编译错误）。也就是说，编译器只看引用类型，而不是对象类型。

下面是有关继承中重写的几个测试

1.函数同等权限，父类引用，子类实例；

public class HelloWorld{

     public static void main(String []args){
        Animal A = new Horse();
        A.eat();
     }
}
class Animal
{
    public void eat()
    {
        System.out.println("A is eating");
    }
}
class Horse extends Animal
{
    public void eat()
    {
        System.out.println("B is eating");
    }
}

　　编译执行

sh-4.3$ javac HelloWorld.java                                                                                                            

sh-4.3$ javac HelloWorld.java                                                                                                            

sh-4.3$ java -Xmx128M -Xms16M HelloWorld                                                                                                 

B is eating 

没有问题

2.函数同等权限，子类引用，父类实例

public class HelloWorld{

     public static void main(String []args){
        //Animal A = new Horse();
        Horse A = new Animal();
        A.eat();
     }
}
class Animal
{
    public void eat()
    {
        System.out.println("A is eating");
    }
}
class Horse extends Animal
{
    public void eat()
    {
        System.out.println("B is eating");
    }
}

　　编译报错：

sh-4.3$ javac HelloWorld.java                                                                                                            

HelloWorld.java:5: error: incompatible types: Animal cannot be converted to Horse                                                        

        Horse A = new Animal();                                                                                                          

                  ^                                                                                                                      

1 error  

向下转型不安全，子类引用不能指向父类对象；

3.函数不同权限，父类引用，子类实例

public class HelloWorld{

     public static void main(String []args){
        Animal A = new Horse();
        //Horse A = new Animal();
        A.eat();
     }
}
class Animal
{
    protected void eat()
    {
        System.out.println("A is eating");
    }
}
class Horse extends Animal
{
    public void eat()
    {
        System.out.println("B is eating");
    }
}

　　

　　                                                                                 

编译执行

sh-4.3$ javac HelloWorld.java                                                                                                            

sh-4.3$ java -Xmx128M -Xms16M HelloWorld                                                                                                 

B is eating

没有问题，重写的权限要比父类函数的权限相等或更宽。