是否可以在有条件的情况下在 Java 中声明变量？
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Is it possible to declare a variable within a Java while conditional?
提问by Markus Mitterauer
In Java it is possible to declare a variable in the initialization part of a for-loop:
在 Java 中，可以在for-loop的初始化部分声明一个变量：
for ( int i=0; i < 10; i++) {
// do something (with i)
}
But with the whilestatement this seems not to be possible.
但是根据while声明，这似乎是不可能的。
Quite often I see code like this, when the conditional for the while loop needs to be updated after every iteration:
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：
List<Object> processables = retrieveProcessableItems(..); // initial fetch
while (processables.size() > 0) {
// process items
processables = retrieveProcessableItems(..); // update
}
Here on stackoverflow I found at least a solutionto prevent the duplicate code of fetching the processable items:
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：
List<Object> processables;
while ((processables = retrieveProcessableItems(..)).size() > 0) {
// process items
}
But the variable still has to be declared outside the while-loop.
但是变量仍然必须在while循环之外声明。
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?
所以，因为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？
回答by Andy Turner
You can write a whileloop using a forloop:
您可以while使用for循环编写循环：
while (condition) { ... }
is the same as
是相同的
for (; condition; ) { ... }
since all three bits in the brackets of the basic for statement declarationare optional:
因为基本 for 语句声明的括号中的所有三位都是可选的：
BasicForStatement:
for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement
Similarly, you can just rewrite your whileloop as a forloop:
同样，您可以将while循环重写为for循环：
for (List<Object> processables;
(processables = retrieveProcessableItems(..)).size() > 0;) {
// ... Process items.
}
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 javacdoes not complain if the variable is left initially unassigned.
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List<Object> processables = null。根据 JLS 的说法，这是不正确的；javac如果变量最初未分配，我的版本不会抱怨。
回答by Bathsheba
No it's not possible.
不，这是不可能的。
It doesn't really make too much sense either: unlike a forloop where you can set up the initial state of the "looping variable", in a whileloop you test the value of an existingvariable, akin to the conditional checkof the forloop.
它并没有真正太大的意义之一：不像一个for循环，你可以设置“循环变量”的初始状态，在一个while循环中，您测试的价值存在变数，类似的条件检查的的for循环。
Of course, if you're concerned about variables "leaking" into other parts of your code, you could enclose the whole thing in an extra scope block:
当然，如果您担心变量“泄漏”到代码的其他部分，您可以将整个内容包含在一个额外的作用域块中：
{
/*declare variable here*/
while(...){...}
}
Alternatively, convert the whileloop into a forloop.
或者，将while循环转换为for循环。
