接口中的Java转换 [英] Java casting in interfaces

问题描述

有人可以向我解释一下，编译器在第一次强制转换中如何不抱怨，而在第二次强制转换中又不抱怨吗?

Can someone please explain to me how the compiler does not complain in the first casting, but does complain in the second?

interface I1 { }
interface I2 { }
class C1 implements I1 { }
class C2 implements I2 { }

public class Test{
     public static void main(){
        C1 o1 = new C1();
        C2 o2 = new C2();
        Integer o3 = new Integer(4);

        I2 x = (I2)o1; //compiler does not complain
        I2 y = (I2)o3; //compiler complains here !!
     }
}

推荐答案

使用(I2)转换o1和o3时，您告诉编译器该对象的类实际上是其声明类型的子类. ，并且该子类实现了I2.

When you cast o1 and o3 with (I2), you tell the compiler that the class of the object is actually a subclass of its declared type, and that this subclass implements I2.

Integer类是 final ，因此o3不能是Integer子类的实例:编译器知道您在说谎. C1并不是最终的，因此o1 可以是实现I2的C1子类型的实例.

The Integer class is final, so o3 cannot be an instance of a subclass of Integer: the compiler knows that you're lying. C1 however is not final, so o1 could be an instance of a subtype of C1 that implements I2.

如果将C1定为最终版本，编译器也会抱怨:

If you make C1 final, the compiler will complain too:

interface I1 { }
interface I2 { }
final class C1 implements I1 { }
class C2 implements I2 { }

public class Test{
     public static void main(){
        C1 o1 = new C1();
        C2 o2 = new C2();
        Integer o3 = new Integer(4);

        I2 y = (I2)o3; //compiler complains here !!
        I2 x = (I2)o1; //compiler complains too
     }
}

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