将任意大括号作为陈述 - 它们是什么意思？

Question

The following is a valid Java program:

public class _ {

    public static void main(String[] args) {
        {
            {
                {
                    {

                    }
                }
            }
        }
    }

}

Do those curly braces add extra semantics? Or are they treated just like whitespace?

Answer 1

The curly braces just create a local block scope inside your main method. The variables declared inside a block will not be visible outside.

This way of coding is used to minimize the scope of local variables.

public void someMethod() {
    {
        int x = 10;
        System.out.println(x); // Ok
    }
    System.out.println(x); // Error, x not visible here
}

It is a good practice to minimize the scope of the local variables you create, specially when you know they won't be used anywhere else in your program. So, it doesn't make sense to let it in the larger scope till it ends. Just create a local block scope, so that it will be eligible for Garbage Collection as soon as the block ends.

Also see @Bohemian's answer below that also quotes one more benefit.

Answer 2

They are a code block. They can be handy when copy-pasting code that contains local variable declarations:

{
    int x = foo * bar;
    someMethod(x);
}
{
    int x = foo * y; // won't get "already defined variable" compile error
    someMethod(x);
}

I've also seen them used for code generation, so you can safely add some code to a method and define a variable without having to worry if it's been defined earlier in the method.

Answer 3

The curly braces are meant to define and limit scope. Since they do not enclose any statements or declarations, they would eliminated by the compiler.

Answer 4

The only purpose of the extra braces is to provide scope-limit. The variable a

void ad()
    {
    int a;
    }

will only exist within those braces, and will have no scope outside of them.