为什么在Java中一个接口不能实现另一个接口？

interface MainInterface {
   void mainMethod();
}
interface SubInterface extends MainInterface { // If we put implements keyword in place of extends,                                               // compiler throws an error.
   void subMethod();
}
class MainClass implements MainInterface {
   public void mainMethod() {
      System.out.println("Main Interface Method");
   }
   public void subMethod() {
      System.out.println("Sub Interface Method");
   }
}
public class Test {
   public static void main(String args[]) {
      MainClass main = new MainClass();
      main.mainMethod();
      main.subMethod();
   }
}

Main Interface Method
Sub Interface Method