Java中的转换（接口和类） [英] Casting in Java(interface and class)

问题描述

if：

 接口I {} 
 
类A实现I {} 
 
 class B extends A {} 
 
 class C extends B {} 
 
 A a = new A（）; 
 B b = new B（）; 
 
为什么a =（B）（I）b;是正确的
，但b =（B）（I）a;是假的？ 
  

我发现转换是很混乱，什么是最好的方式来了解， up cast一个对象？

解决方案

与代码示例中回答问题相关的唯一事情如下：

  B类扩展A {} 
  

这意味着B是A的子类。子类可以转换为超类类型，但是超类不能转换为子类类型。

$ b 因此，A不能转换为类型B.

以这种方式思考逻辑：

是 Programming_language 的类型，但 Programming_language 不是

if:

interface I{}

class A implements I{}

class B extends A{}

class C extends B{}

A a = new A();
B b = new B();

Why a = (B)(I)b; is correct
but b = (B)(I)a; is false?

I find casting to be very confusing, what is the best way to understand if I can down cast or up cast an object?

解决方案

The only thing of relevance to answering the question in your coding sample is the following:

class B extends A{}

This means that B is a subclass of A. Subclasses can be cast to super class types, but super class cannot be cast to subclass types.

Therefore, A cannot be cast to type B.

Why? Think about the logic this way:

is a type of Programming_language, but Programming_language is not a type of

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