如果你想在循环外使用STR;在外面宣布。否则，第二版就可以了。

根据谷歌Android开发指南，变量范围应该是有限的。请查看此链接:

极限变量范围

局部变量的作用域应该总是尽可能的小。

在你的例子中，我假设str没有在while循环之外使用，否则你就不会问这个问题，因为在while循环内部声明它不是一个选项，因为它不会编译。

因此，由于str不在循环之外使用，因此str的最小作用域是在while循环内。

因此，答案强调str绝对应该在while循环中声明。没有如果，没有并且，没有但是。

The only case where this rule might be violated is if for some reason it is of vital importance that every clock cycle must be squeezed out of the code, in which case you might want to consider instantiating something in an outer scope and reusing it instead of re-instantiating it on every iteration of an inner scope. However, this does not apply to your example, due to the immutability of strings in java: a new instance of str will always be created in the beginning of your loop and it will have to be thrown away at the end of it, so there is no possibility to optimize there.

编辑:(在答案下面注入我的评论)

In any case, the right way to do things is to write all your code properly, establish a performance requirement for your product, measure your final product against this requirement, and if it does not satisfy it, then go optimize things. And what usually ends up happening is that you find ways to provide some nice and formal algorithmic optimizations in just a couple of places which make our program meet its performance requirements instead of having to go all over your entire code base and tweak and hack things in order to squeeze clock cycles here and there.

在while循环之外声明String str允许它在while循环内外被引用。在while循环中声明String str只允许它在while循环中被引用。

str变量将可用，并在内存中保留一些空间，即使在执行下面的代码之后。

 String str;
    while(condition){
        str = calculateStr();
        .....
    }

str变量将不可用，内存也将被释放，这是在下面的代码中为str变量分配的。

while(condition){
    String str = calculateStr();
    .....
}

如果我们采用第二种方法，肯定会减少系统内存，提高性能。

这个问题的一个解决方案是提供一个变量作用域来封装while循环:

{
  // all tmp loop variables here ....
  // ....
  String str;
  while(condition){
      str = calculateStr();
      .....
  }
}

当外部作用域结束时，它们将自动取消引用。