for循环变量声明中允许的类型？ [英] Types permitted in for loop variable declarations?

问题描述

我做了一些搜索，找不到在for循环初始化语句中使用的有效类型列表。 循环变量声明中是否有可以在中使用的固定列表类型？例如，考虑下面的代码：



  for（int i = 0; i <5; i ++）// 。
 for（String str =a; str.length（）< 10; str + =a）// ... 
  

第一部作品，但我不认为第二部作品。有没有一个在for循环初始化中允许的所有类型的列表？

解决方案

查看Java语言规范对于 为语句。你可以在中为循环声明和初始化任何类型的变量，甚至可以声明多个变量，只要它们是相同的类型。语法中的相关产品是：

$ p $ $ $ $ $ $ $ $ Basic $语句
for（ForInitopt; Expressionopt; ForUpdateopt）

ForInit：
StatementExpressionList
LocalVariableDeclaration
$ b LocalVariableDeclaration：
VariableModifiersopt类型VariableDeclarators

VariableDeclarators：
VariableDeclarator
VariableDeclarators，VariableDeclarator


这意味着您可以执行以下任何操作，例如< （$; $; $; $; $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ /变量声明没有初始值
for（int i = 0; ...; ...）//变量声明与初始值
for（int i = 0，j = 1; ...; ...）// （final Iterator< T> it = ...; ...; ...）//最终变量


这里的第一个例子显示你根本不需要任何变量，并且作为在评论中指出，你不必有 ForUpdate 。唯一的约束是你必须在中间有一个表达式，它需要是一个布尔值表达式。另外， ForInit 也可以是 StatementExpressionList ，这意味着不用声明和初始化变量，也可以执行一些语句。例如，你可以做到这一点（但这不是一个特别有用的例子）：

  for （System.out.println（begin loop; ...; ...）
  

这可能很有用，我想，在模拟 do / while 循环（如果你想这样做），如果body是一个简单的函数调用：

  for（method（）; condition; method（））; 
  

I've done some searches and couldn't find a list of valid types to use in for loop initialization statements. Is there a fixed list of types that can be used in for loop variable declarations? For instance, consider the following code:

for (int i = 0; i < 5; i++) // ...
for (String str = "a"; str.length() < 10; str+="a") // ...

The first works, but I don't think the second will. Is there a list of all the types which are permitted in a for loop initialization?

解决方案

Have a look at the Java language specification for the for statement. You can declare and initialize any type of variable in a for loop, and can even declare multiple variables, so long as they're all the same type. The relevant productions in the grammar are:

BasicForStatement:
    for ( ForInitopt ; Expressionopt ; ForUpdateopt ) Statement

ForInit:
    StatementExpressionList
    LocalVariableDeclaration

LocalVariableDeclaration:
    VariableModifiersopt Type VariableDeclarators

VariableDeclarators:
    VariableDeclarator
    VariableDeclarators , VariableDeclarator

This means that you can do any of the following, e.g.,

for ( ; … ; … )                         // no variable declaration at all
for ( int i; … ; … )                    // variable declaration with no initial value
for ( int i=0; … ; … )                  // variable declaration with initial value
for ( int i=0, j=1; … ; … )             // multiple variables
for ( final Iterator<T> it = …; … ; … ) // final variable

The first example there shows that you don't need any variables at all, and as pointed out in the comments, you don't have to have a ForUpdate either. The only constraint is that you must have an expression in the middle, and it needs to be a boolean valued expression.

As an aside, the ForInit can also be a StatementExpressionList, which means that instead of declaring and initializing variables, you can also just execute some statements. E.g, you could do this (but this isn't a particularly useful example):

for ( System.out.println( "beginning loop" ; … ; … )

This could be useful, I suppose, in simulating a do/while loop (if you wanted to do that), if the body is a simple function call:

for ( method() ; condition ; method() );

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