Go语言视频零基础入门到精通

zhezhe

别的不多说了，本题的答案是A，A选项没什么问题~唯一不理解的是选项C，为什么是错的？我思考了好久，求各位朋友帮忙看看

Given:
5. class Payload {
6. private int weight;
7. public Payload (int w) { weight = w; }
8. public void setWeight(int w) { weight = w; }
9. public String toString() { return Integer.toString(weight); }
10. }
11. public class TestPayload {
12. static void changePayload(Payload p) { /* insert code */ }
13. public static void main(String[] args) {

14. Payload p = new Payload(200);
15. p.setWeight(1024);
16. changePayload(p);
17. System.out.println("p is " + p);
18. } }
Which code fragment, inserted at the end of line 12, produces the output p is 420?


A. p.setWeight(420);
B. p.changePayload(420);
C. p = new Payload(420);
D. Payload.setWeight(420);
E. p = Payload.setWeight(420);

Answer: A
Explanation:


----------------------------------------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------------------------------------------------

hbh820326

A. p.setWeight(420);//修改对象P中的weight值为420
B. p.changePayload(420);//changePayload(Payload)这个函数不属于P对象呀
C. p = new Payload(420);//给P重新赋值并初始化weight的值为420
D. Payload.setWeight(420);//非静态函数不能这么调用
E. p = Payload.setWeight(420);//setWeight(int)这个函数是无返回的怎么给P赋值？而且也不是静态的，这样是调不到的

taiga_

静态方法传入的参数p跟main方法中创建的类p不是同一个,但都指向类p,答案C将参数p指向新创建的类,这没有改变main方法中创建类p的weight值.

1. static void changePayload(Payload p)
   
2.     {
   
3.         System.out.println("pp1 is " + p);
   
4.         p = new Payload(420);
   
5.         System.out.println("pp2 is " + p);
   
6.     }
   
7.    
   
8.     public static void main(String[] args)
   
9.     {
   
10.         Payload p = new Payload(200);
    
11.         p.setWeight(1024);
    
12.         System.out.println("p1 is " + p);
    
13.         changePayload(p);
    
14.         System.out.println("p2 is " + p);
    
15.         System.out.println("p is " + p);
    
16.     }

复制代码

若将Payload中的toString方法注释掉,添加打印方法,就可以看出:

1. p1 is zc.Payload@6406c7e
   
2. pp1 is zc.Payload@6406c7e
   
3. pp2 is zc.Payload@6f92c766
   
4. p2 is zc.Payload@6406c7e
   
5. p is zc.Payload@6406c7e

复制代码