Why can't you assign an int to an Integer in a loop without curly braces?

Question

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Why this is not compiling in Java?

In java, curly braces are optional for one line for loops, but I've found a case where it isn't allowed. For instance, this code:

for(int i = 0; i < 10; i++)
    Integer a = i;

won't compile, but if you add curly braces, like so:

for(int i = 0; i < 10; i++){
    Integer a = i;
}

it will. Why won't this code compile?

Answer 1

I wonder if it has anything to do with the fact that the following statement is compound?

for(int i = 0; i < 10; i++)
    Integer a;
    a = i;

Answer 2

Because it wouldn't make sense to declare a new variable in a single line statement as it goes out of scope immediately.

If you look at the definition of a statement in Java, it doesn't include a LocalVariableDeclarationStatement, whereas a block does.

Answer 3

If you don't have curly braces, you can't put in a variable declaration. (The variable would be useless)

See the specification. (A LocalVariableDeclarationStatement can only occur directly inside of a Block.)