有人可以解释这个输出吗？ （JAVA代码） [英] Can someone explain this output? (JAVA CODE)

问题描述

public class Foo {
 private int[] arr;
 public Foo(int i) {
 arr = new int[i];
 for(int j = 0; j < i; j++) {
 arr[j] = j*3;
 }

 }
 public void h() {
 for(int j = arr.length-2; j >= 0; j--) {
 arr[j] = arr[j+1] + 3;
 }
 }

 public static void main(String[] args) {
 Foo f = new Foo(5);
 f.h();
 for(int j = 0; j < 5; j++) {
 System.out.println( f.arr[j] );
 }
 }
}

以下代码的输出是什么？ （答案应该是12,15,18,21,24）



我尝试过：



我尝试在纸上解决这个问题，虽然我得到了3的倍数，而不是（12,15,18,21,24）我得到了（3,9,12， 15,18）？我做错了什么？

What is the output of the following code? (The answer should be 12,15,18,21,24)

What I have tried:

I tried solving this code on paper and although i am getting the multiple of 3, as opposed to (12,15,18,21,24) i am getting (3,9,12,15,18)? What am i doing wrong?

推荐答案

编译并不意味着你的代码是对的！ ：笑：

将开发过程想象成编写电子邮件：成功编译意味着您使用正确的语言编写电子邮件 - 例如英语而不是德语 - 而不是电子邮件包含您的邮件想发送。



所以现在你进入第二阶段的发展（实际上它是第四或第五阶段，但你将在之后的阶段进入）：测试和调试。



首先查看它的作用，以及它与你想要的有何不同。这很重要，因为它可以为您提供有关其原因的信息。例如，如果程序旨在让用户输入一个数字并将其翻倍并打印答案，那么如果输入/输出是这样的：

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

然后很明显问题出在将它加倍的位 - 它不会将自身加到自身上，或者将它乘以2，它会将它自身相乘并返回输入的平方。

所以，你可以查看代码和很明显，它在某处：

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

int Double(int value)
   {
   return value * value;
   }

一旦你知道可能出现的问题，就开始使用teh调试器找出原因。在你的行上设一个断点：

Once you have an idea what might be going wrong, start using teh debugger to find out why. Put a breakpoint on your line:

Foo f = new Foo(5);

并运行你的应用程序。在执行代码之前，请考虑代码中的每一行应该做什么，并将其与使用Step over按钮依次执行每一行时实际执行的操作进行比较。它符合您的期望吗？如果是这样，请转到下一行。

如果没有，为什么不呢？它有何不同？



这是一项非常值得开发的技能，因为它可以帮助你在现实世界和发展中。和所有技能一样，它只能通过使用来改进！

and run your app. Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?

This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Quote：

我试过解决这个代码虽然我得到3的倍数，而不是（12,15,18,21,24）我得到（3,9,12,15,18）？我究竟做错了什么？

I tried solving this code on paper and although i am getting the multiple of 3, as opposed to (12,15,18,21,24) i am getting (3,9,12,15,18)? What am i doing wrong?

使用调试器，您将看到阵列是如何构建的。



有一个工具可以让你看到你的代码在做什么，它的名字是调试器。它也是一个很好的学习工具，因为它向你展示了现实，你可以看到哪种期望与现实相符。

当你不明白你的代码在做什么或为什么它做它做的时候，答案就是答案是调试器。

使用调试器查看代码正在执行的操作。只需设置断点并查看代码执行情况，调试器允许您逐行执行第1行并在执行时检查变量。



调试器 - 维基百科，免费的百科全书 [ ^ ]

http://docs.oracle.com/javase/7/docs/technotes/tools/windows/jdb.html [ ^ ]

https://www.jetbrains.com/idea/help/debugging-your-first-java-application.html [ ^ ]

调试器在这里向您展示你的代码正在做，你的任务是与它应该做的事情进行比较。

调试器中没有魔法，它没有找到错误，它只是帮助你。当代码没有达到预期的效果时，你就接近了一个bug。

Use the debugger and you will see how the array is build.

There is a tool that allow you to see what your code is doing, its name is debugger. It is also a great learning tool because it show you reality and you can see which expectation match reality.
When you don't understand what your code is doing or why it does what it does, the answer is debugger.
Use the debugger to see what your code is doing. Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]
http://docs.oracle.com/javase/7/docs/technotes/tools/windows/jdb.html[^]
https://www.jetbrains.com/idea/help/debugging-your-first-java-application.html[^]
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't find bugs, it just help you to. When the code don't do what is expected, you are close to a bug.

即使我们试着解释你也不会理解。你最好试试Debugging。

您使用的编辑器是什么？

几乎所有标准API都支持轻松调试。我可以建议你Eclipse;还有一些其他编辑器支持简单方式调试。您也可以使用jdb从命令行进行调试，但这有点困难;



这里是如何处理函数h

You will not understand even if we try to explain. You better try Debugging.
What Editor are you using?
Almost all standard API support easy debugging. I Can suggest you Eclipse; there are also few other Editor that support easy way debugging. You can also debug from command line with "jdb" but it is slightly difficult;

here how your function h is being processed

0 3 6 9 12
when j=3
 0   3  6 15 12              -- since you are adding arr[3+1]+3 and putting at arr[3]
when j=2
 0   3 18 15 12              -- since you are adding arr[2+1]+3 and putting at arr[2]
when j=1
 0  21 18 15 12              -- since you are adding arr[1+1]+3 and putting at arr[1]
when j=0
24  21 18 15 12              -- since you are adding arr[0+1]+3 and putting at arr[0]

这有意义吗？

Does this make sense?

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