是否可以在有条件的情况下在 Java 中声明变量? [英] Is it possible to declare a variable within a Java while conditional?

问题描述

在 Java 中，可以在 for 循环的初始化部分声明一个变量:

In Java it is possible to declare a variable in the initialization part of a for-loop:

for ( int i=0; i < 10; i++) {
  // do something (with i)
}

但是对于 while 语句，这似乎是不可能的.

But with the while statement this seems not to be possible.

我经常看到这样的代码，当每次迭代后都需要更新 while 循环的条件时:

Quite often I see code like this, when the conditional for the while loop needs to be updated after every iteration:

List<Object> processables = retrieveProcessableItems(..); // initial fetch
while (processables.size() > 0) {
    // process items
    processables = retrieveProcessableItems(..); // update
}

在 stackoverflow 上，我至少找到了一个解决方案来防止获取可处理项目的重复代码:

Here on stackoverflow I found at least a solution to prevent the duplicate code of fetching the processable items:

List<Object> processables;
while ((processables = retrieveProcessableItems(..)).size() > 0) {
    // process items
}

但是变量仍然需要在while循环之外声明.

But the variable still has to be declared outside the while-loop.

所以，由于我想保持我的变量范围干净，是否可以在 while 条件中声明一个变量，或者对于这种情况是否有其他解决方案?

So, as I want to keep my variable scopes clean, is it possible to declare a variable within the while conditional, or is there some other solution for such cases?

推荐答案

您可以使用 for 循环编写 while 循环:

You can write a while loop using a for loop:

while (condition) { ... }

与

for (; condition; ) { ... }

由于 基本语句声明 是可选的:

BasicForStatement:
    for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement

同样，您可以将 while 循环重写为 for 循环:

Similarly, you can just rewrite your while loop as a for loop:

for (List<Object> processables;
     (processables = retrieveProcessableItems(..)).size() > 0;) {
  // ... Process items.
}

请注意，某些静态分析工具(例如 eclipse 4.5)可能要求将初始值分配给 processables，例如<代码>列表<对象>processables = null.根据 JLS 的说法，这是不正确的；如果变量最初未赋值，我的 javac 版本不会抱怨.

Note that some static analysis tools (e.g. eclipse 4.5) might demand that an initial value is assigned to processables, e.g. List<Object> processables = null. This is incorrect, according to JLS; my version of javac does not complain if the variable is left initially unassigned.

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