Types permitted in for loop variable declarations? [closed]

问题

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?

回答1:

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() );

回答2:

Second one also will work fine. You can use any type for for loop

 for(String str="a";str.length()<10;str+="a")
    {
        System.out.println(str);
    }

I just tried for your scenario and result is

a
aa
aaa
aaaa
aaaaa
aaaaaa
aaaaaaa
aaaaaaaa
aaaaaaaaa

来源：https://stackoverflow.com/questions/19341216/types-permitted-in-for-loop-variable-declarations

标签

java

for-loop

variable-declaration