error on if condition java

Question

public class SomeClass{

    public static void main (String[] args){
        if(true) int a = 0;// this is being considered as an error 
        if(true){
            int b =0;
        }//this works pretty fine
    }
}//end class

In the above class first if statement is showing an compilation error

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
    Syntax error on token "int", delete this token
    a cannot be resolved to a variable

However second if statement works fine. I just cannot figure it out myself. I know it is of no use declaring a variable in a single statement if. How those two statements are different can some one please explain me. Excuse me if the question is really simple.

Answer 1

To define the scope of int a you need curly braces. That's why you get compiler error for

if(true) int a = 0;

while this works :

if(true){
    int b =0;
}

See JLS §14.9 for if statements,

IfThenStatement:
    if ( Expression ) Statement

While in if(true) int a = 0;, int a = 0 is LocalVariableDeclarationStatement

Answer 2

It is specified in the java language specification. The IfThenStatement is specified as

if ( Expression ) Statement

int a = 0; is not a statement, but a LocalVariableDeclarationStatement (which is not a subtype of Statement). But a Block is a Statement and a LocalVariableDeclarationStatement is legal content of a block.

 if (true) int a = 0;
           ^--------^    LocalVariableDeclarationStatement, not allowed here!

 if (true) {int a = 0;}
           ^----------^  Statement (Block), allowed.

Reference

• Local Variable Declaration Statements
• Statements
• The If Statement