java - 为什么这两个代码示例会产生不同的输出？

Question

示例 1:

 class Animal {
     public static void saySomething() { System.out.print(" Gurrr!"); 
   }
 }
 class Cow extends Animal {
    public static void saySomething() { 
     System.out.print(" Moo!"); 
    }
    public static void main(String [] args) {
         Animal [] animals = {new Animal(), new Cow()};
         for( Animal a : animals) {
           a.saySomething();
         }
         new Cow().saySomething();
    }
 }

输出是:

 Gurrr! Gurrr! Moo!

示例 2:

 class Animal {
     public void saySomething() { System.out.print(" Gurrr!"); 
   }
 }
 class Cow extends Animal {
    public void saySomething() { 
     System.out.print(" Moo!"); 
    }
    public static void main(String [] args) {
         Animal [] animals = {new Animal(), new Cow()};
         for( Animal a : animals) {
           a.saySomething();
         }
         new Cow().saySomething();
    }
 }

输出:

 Gurrr! Moo! Moo!

我只是不明白为什么将 saySomething 设为非静态会导致对 saySomething 的第二次调用调用 Cow 版本而不是 Animal 版本。我的理解是 Gurrr!哞! Moo! 将是两种情况下的输出。

Answer 1

当您对动物调用 saySomething() 时，动物的实际类型不算数，因为 saySomething() 是静态的。

Animal cow = new Cow();
cow.saySomething(); 

与

相同

Animal.saySomething();

---

一个 JLS 例子:

When a target reference is computed and then discarded because the invocation mode is static, the reference is not examined to see whether it is null:

class Test {
  static void mountain() {
      System.out.println("Monadnock");
  }
  static Test favorite(){
       System.out.print("Mount ");
       return null;
   }
   public static void main(String[] args) {
       favorite().mountain();
   }

}

which prints:
Mount Monadnock
Here favorite returns null, yet no NullPointerException is thrown.

---

资源:

• JLS - Is the Chosen Method Appropriate?

关于同一主题:

• Java Static confusion