java - 多态性和对象类

Question

我们知道每个类都是从 Object 扩展的，这意味着我们可以使用 Object 的所有方法在任何类(class)。我的问题如下:

   interface B{

       }

       public class A implements B{
          public static void main(String[] args){

             B i = new A();
             i.display();//we can't do this : because the interface B doesn't define such a method

             System.out.println(i.toString());// we can do this although the interface doesn't extend from Object                           

          }
          public void display(){         

          }

      }

所以我认为问题很清楚，为什么我可以调用 toString 方法，尽管接口(interface) B 不能从 Object 扩展？

Answer 1

这在 JLS §9.2. Interface Members 中有详细说明:

• If an interface has no direct superinterfaces, then the interface implicitly declares a public abstract member method m with signature s, return type r, and throws clause t corresponding to each public instance method m with signature s, return type r, and throws clause t declared in Object, unless an abstract method with the same signature, same return type, and a compatible throws clause is explicitly declared by the interface.

这确保可以通过任何接口(interface)调用 Object 方法。