Java将一个对象转换为未实现的接口 [英] Java casting an object to an interface which is not implemented

问题描述

我在研究书中发现了以下问题，有点困惑：

给定以下代码，如果用于替换 * INSERT CODE
HERE * / 将启用 Roamable 类型的引用变量来引用
的对象电话课？ （选择1个选项。）

  interface Roamable {} 
 class Phone {} 
 class Tablet extended Phone implements Roamable {
 // INSERT CODE HERE 
} 
  

选项包括： / p>

1. Roamable var = new Phone（）;


2. Roamable var =（Roamable）Phone（）;


3. 电话（）;


4. 因为介面 Roamable 和类 c $ c>不相关， Roamable 类型的引用变量
不能引用 Phone $ c>。



我认为正确的选项是4，但是它是3。

但是， Phone 不实现 Roamable 界面，因此您无法投射，你可以吗？

解决方案

正确的答案是 3 只有电话正在投放到 Roamable 和电话不是最终的，所以它认为被转换的对象，尽管被称为 Phone 可以是 Phone 实施 Roamable ，因此不会发出编译时错误或警告。

根据 JLS第5章 p>

5.5.1。参考类型铸造

给定编译时引用类型S（源）和编译时引用类型T ），如果由于以下规则没有发生编译时错误，则从S到T存在转换转换。
如果T是接口类型：

如果S不是最终类（§8.1.1），则如果存在T的超类型X ，以及S的超类型Y，使得X和Y都是可区别的参数化类型，并且X和Y的擦除是相同的，则会发生编译时错误。

否则，转换在编译时总是合法的（因为即使S不实现T，S的子类也可能）。

如果S是final class（§8.1.1），那么S必须实现T，否则会出现编译时错误。

---

以下代码编译：

  interface Roamable {} 
 class Phone {} 
 class Tablet extensions电话实现Roamable {
 Roamable var =（Roamable）new Phone（）; //编译
} 
  

I found following question in my study book and a bit confused:

Given the following code, which option, if used to replace /* INSERT CODE HERE */, will enable a reference variable of type Roamable to refer to an object of the Phone class? (Select 1 option.)

interface Roamable{}
class Phone {}
class Tablet extends Phone implements Roamable {
    //INSERT CODE HERE
}

Options are:

1. Roamable var = new Phone();
2. Roamable var = (Roamable)Phone();
3. Roamable var = (Roamable)new Phone();
4. Because interface Roamable and class Phone are unrelated, a reference variable of type Roamable can’t refer to an object of class Phone.

I thought the correct option is 4, however it says it is 3.

But, Phone doesn't implement Roamable interface, so you can't cast, can you?

解决方案

The correct answer is 3: The compiler sees only that a Phone is being cast to a Roamable and that Phone is not final, so it thinks that the object being cast, although referred to as Phone may be a subclass of Phone that does implement Roamable, so no compile-time error or warning is issued.

According to JLS chapter 5

5.5.1. Reference Type Casting

Given a compile-time reference type S (source) and a compile-time reference type T (target), a casting conversion exists from S to T if no compile-time errors occur due to the following rules. If T is an interface type:

If S is not a final class (§8.1.1), then, if there exists a supertype X of T, and a supertype Y of S, such that both X and Y are provably distinct parameterized types, and that the erasures of X and Y are the same, a compile-time error occurs.

Otherwise, the cast is always legal at compile time (because even if S does not implement T, a subclass of S might).

If S is a final class (§8.1.1), then S must implement T, or a compile-time error occurs.

---

The following code compiles:

interface Roamable{}
class Phone {}
class Tablet extends Phone implements Roamable {
    Roamable var = (Roamable)new Phone(); // Compiles
}

这篇关于Java将一个对象转换为未实现的接口的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！