对接口的对象进行显式转换。括号的用法是什么 [英] Explicit casting on objects with interface. What's the use of parentheses

问题描述

我发现了一个类似的问题，但它并没有真正关注界面问题 - 问题关于投射对象括号

I have found a similar question, but it didn't really concern the interface issue - Issue about casting object brackets

public class Speak { /* Line 1 */

    public static void main(String[] args) { /* Line 2 */

        Speak speakIT = new Tell(); /* Line 3 */ 
        Tell tellIt = new Tell(); /* Line 4 */ 

        speakIT.tellItLikeItIs(); /* Line 5 */ 
        (Truth)speakIt.tellItLikeItIs(); /*Line 6 */ 
        ((Truth)speakIt).tellItLikeItIs(); /* Line 7 */ 
        tellIt.tellItLikeItIs(); /* Line 8 */ 
        (Truth)tellIt.tellItLikeItIs(); /* Line 9 */ 
        ((Truth)tellIt).tellItLikeItIs(); /* Line 10 */
    } 
}

class Tell extends Speak implements Truth {
    public void tellItLikeItIs() { 
        System.out.println("Right on!"); 
    }
}

interface Truth {     
    public void tellItLikeItIs()
}

第7,8和10行是正确的。我得到数字7，但为什么不是数字6和9正确，而不是8和10？我们如何才能显式地转换到一个接口？

Line 7, 8 and 10 are the correct ones. I get number 7, but why isn't number 6 and 9 correct instead of 8 and 10? And how come we can explicitly cast to an interface? what logic is behind the use of parentheses of answer 8 and 10?

推荐答案

(Truth) tellIt.tellItLikeItIs()

调用 tellItLikeItIs（）在 tellIt 对象上，并将此方法返回的值转换为 Truth 。

calls tellItLikeItIs() on the tellIt object, and casts the value returned by this method to Truth.

((Truth) tellIt).tellItLikeItIs()

casts tellIt 到 Truth ，然后调用 tellItLikeItIs 。

casts tellIt to Truth, and then calls tellItLikeItIs() on the object.

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