编译时与运行时多态性(或方法签名)的行为

我用Java编写了以下说明性代码。它显示了 introduceYourself() 不同自行车的方法。

public class Bicycle{
    public void introduceYourself(){
        System.out.println("Hello I am just a bicycle.");
    }
}

public class MountainBike extends Bicycle{
    public void introduceYourself(){
        System.out.println("Hello I am a mountain bike and I love going outdoors.");
    }
}

public class CityBike extends Bicycle{
    public void introduceYourself(){
        System.out.println("My name is city bike and I prefer calm trips.");
    }
}

正如我所期待的, 以下代码调用 介绍自己() 每个运行时对象的方法 ,尽管变量被声明为基 Bicycle 班如果我要将Bicycle或Bicycle子类型对象添加到数组并在循环中调用该方法,这将非常有用。

public class HelloWorld{

     public static void main(String []args){
        Bicycle b1 = new Bicycle();
        Bicycle b2 = new MountainBike();
        Bicycle b3 = new CityBike();

        b1.introduceYourself(); // Output: Hello I am just a bicycle.       
        b2.introduceYourself(); // Output: Hello I am a mountain bike and I love going outdoors.
        b3.introduceYourself(); // Output: My name is city bike and I prefer calm trips.  
     }
}

然而,我很难理解其他代码的行为。我有以下类,它们再次显示继承,但方法具有不同的签名(重载):

public class A{
    public int calc (double num){
    return (int)(num + 1);
    }
}

public class B extends A{
    public int calc (long num){
        return (int)(num + 2);
    }
}

public class C extends B{
    public int calc (int num){
        return (num + 3);
    }
}

public class D extends C{
    public int calc (float num){
        return (int)(num + 4);
    }
}

以及main方法中的以下代码:

public class HelloWorld{
 public static void main(String []args){
    int num1 = 10;
    long num2 = 10;

    A a1 = new D();
    D d1 = new D();

    System.out.println(a1.calc(num1)); // Output: 11
    System.out.println(a1.calc(num2)); // Output: 11

    System.out.println(d1.calc(num1)); // Output: 13
    System.out.println(d1.calc(num2)); // Output: 12
 }

}

为什么引用的对象 a1 (具有声明的类型 A 和运行时类型 D )调用中声明的方法 A. 而不是类的运行时对象已知的最合适的(通过签名) D ? (另外,我认为有一个自动转换,因为参数类型不同。)为什么它的行为看起来与Bicycle示例如此不同?谢谢