Java - 访问for循环内外的变量 [英] Java - Access variable inside and outside of for-loop

问题描述

  String playerlist; 
 
（Player p：allplayers）{
 playerlist + = p.getDisplayName（）+，; 
} 
 
 sendMessage（playerlist）; 
  

我该如何做到这一点？我需要在我的for循环中有一个变量，然后在循环之外访问它。

谢谢，如果这是一个非常不好的问题，非常抱歉。我只是想不出来。

解决方案

你很近。您需要将第一行更改为

  String playerlist =; 
  

在Java中，在初始化之前使用变量是非法的，而行

  playerlist + = p.getDisplayName（）+，; 
  

desugars（*）to

  String temp = playerlist; 
 playerlist = temp +（p.getDisplayName（）+，）; 
  

初始化 playerlist 将其内容读入临时变量，然后更新 playerlist 。

有两个更高级别的建议如果你有一个可用的连接方法，就像一些库提供的那样（就像 ）。 /msdn.microsoft.com/en-us/library/system.string.join.aspxrel =nofollow>。NET 和

然而，如果if你没有可用的连接方法（AFAIK，Java标准库没有提供这样的东西），你应该使用 StringBuilder ，符合@ anubhava的建议。你会在大量的字符串中获得更好的性能，代码更容易理解，这是一个双赢。

---

编辑：正如@edalorzo在评论中正确地提到，你只会得到一个错误，不会初始化一个本地变量，并且静态和实例字段都是自动的初始化。具体而言，数字类型（ int ， long ， double ）等等）被初始化为0，布尔值被初始化为 false ，并且引用类型被初始化为 null 该规范明确描述了字段的自动初始化，所以你可以依靠它，但它很少是你想要的。通常情况下，稍后您将需要使用该字段的非默认值，因为 null 不是非常有用（许多语言，特别是标准ML ，完全没有 null ）。所以我建议在构造函数中初始化所有对象的字段。


$ b

（*）：desugaringI上面显示的几乎是准确的：实际上， playerlist 只计算一次，不影响这个特定程序的行为。

String playerlist;

for(Player p : allplayers){
    playerlist += p.getDisplayName() + ", ";
}

sendMessage(playerlist);

How can I accomplish this? I need to have a variable inside my for-loop, and then access it outside of the loop.

Thanks, and sorry if this is a very nooby question. I just can't figure it out.

解决方案

You're close. You need to change the first line to

String playerlist = "";

In Java it's illegal to use a variable before it's initialized, and the line

playerlist += p.getDisplayName() + ", ";

desugars(*) to

String temp = playerlist;
playerlist = temp + (p.getDisplayName() + ", ");

Once you initialize playerlist, you are able to read off its contents into the temporary variable, and then update playerlist.

There are two higher-level suggestions, though.

1. If you have a join method available, as some libraries provide (as do the the .NET and Python platforms), you can just use a join, which is easier to read.
2. However, if you don't have a join method available (AFAIK, the Java standard library provides no such thing), you should use a StringBuilder, consistent with @anubhava's suggestion. You'll get better performance on large numbers of strings, and the code is easier to understand, which is a win-win.

---

EDIT: As @edalorzo rightly mentions in the comments, you will only get an error for not initializing a local variable, and both static and instance fields are automatically initialized. Specifically, numeric types (int, long, double, etc.) are initialized to 0, booleans are initialized to false, and reference types are initialized to null.

The spec explicitly describes the automatic initialization of fields, so you can rely on it, but it's rarely what you want. Normally, you will want a non-default value of the field when you use it later, since null isn't terribly useful (and many languages, particularly functional languages like Standard ML, go without null altogether). So I would recommend initializing all the fields of your objects in the constructor.

---

(*): The "desugaring" I show above is almost accurate: in truth, playerlist is evaluated only once, which doesn't affect the behavior of this particular program.

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