A single-line loop with a mandatory pair of braces in Java

Question

The code in the following snippet works just fine. It counts the number of objects created using a static field of type int which is cnt.

public class Main {     private static int cnt;      public Main()     {         ++cnt;     }      public static void main(String[] args)     {         for (int a=0;a<10;a++)         {             Main main=new Main();         }          /*for (int a=0;a<10;a++)             Main main=new Main();*/          System.out.println("Number of objects created : "+cnt+"\n\n");     } } 

It displays the following output.

Number of objects created : 10 

The only question is that when I remove the pair of curly braces from the above for loop (see the commented for loop), a compile-time error is issued indicating

not a statement.

Why in this particular situation, a pair of braces is mandatory even though the loop contains only a single statement?

Answer 1

When you declare a variable (main in this case):

Main main = new Main(); 

it doesn't count as a statement, even if it has side-effects. For it to be a proper statement, you should just do

new Main(); 

---

So why is

... {     Main main = new Main(); } 

allowed? { ... } is a block of code, and does count as a statement. In this case the main variable could be used after the declaration, but before the closing brace. Some compilers ignore the fact that it's indeed not used, other compilers emit warnings regarding this.