相关疑难解决方法(0)

String x = (String) null;

为什么这句话中没有例外？

String x = null;
System.out.println(x);

It prints null. But .toString() method should throw a null pointer exception.

显然,这会导致编译错误,因为Chair与Cat无关:

class Chair {}
class Cat {}

class Test {
   public static void main(String[] args) {
       Chair chair = new Char(); Cat cat = new Cat();
       chair = (Chair)cat; //compile error
   }
}

那么为什么我在运行时只将Cat引用转换为不相关的接口Furniture时才会出现异常,而编译器显然可以告诉Cat不实现Furniture？

interface Furniture {}

class Test {
   public static void main(String[] args) {
       Furniture f; Cat cat = new Cat();
       f = (Furniture)cat; //runtime error
   }
}